The document at https://tc39.es/ecma262/ is the most accurate and up-to-date ECMAScript specification. It contains the content of the most recent yearly snapshot plus any finished proposals (those that have reached Stage 4 in the proposal process and thus are implemented in several implementations and will be in the next practical revision) since that snapshot was taken.
This specification is developed on GitHub with the help of the ECMAScript community. There are a number of ways to contribute to the development of this specification:
Refer to the colophon for more information on how this document is created.
Introduction
This Ecma Standard defines the ECMAScript 2025 Language. It is the sixteenth edition of the ECMAScript Language Specification. Since publication of the first edition in 1997, ECMAScript has grown to be one of the world's most widely used general-purpose programming languages. It is best known as the language embedded in web browsers but has also been widely adopted for server and embedded applications.
ECMAScript is based on several originating technologies, the most well-known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company's Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with Internet Explorer 3.0.
The development of the ECMAScript Language Specification started in November 1996. The first edition of this Ecma Standard was adopted by the Ecma General Assembly of June 1997.
That Ecma Standard was submitted to ISO/IEC JTC 1 for adoption under the fast-track procedure, and approved as international standard ISO/IEC 16262, in April 1998. The Ecma General Assembly of June 1998 approved the second edition of ECMA-262 to keep it fully aligned with ISO/IEC 16262. Changes between the first and the second edition are editorial in nature.
The third edition of the Standard introduced powerful regular expressions, better string handling, new control statements, try/catch exception handling, tighter definition of errors, formatting for numeric output and minor changes in anticipation of future language growth. The third edition of the ECMAScript standard was adopted by the Ecma General Assembly of December 1999 and published as ISO/IEC 16262:2002 in June 2002.
After publication of the third edition, ECMAScript achieved massive adoption in conjunction with the World Wide Web where it has become the programming language that is supported by essentially all web browsers. Significant work was done to develop a fourth edition of ECMAScript. However, that work was not completed and not published as the fourth edition of ECMAScript but some of it was incorporated into the development of the sixth edition.
The fifth edition of ECMAScript (published as ECMA-262 5th edition) codified de facto interpretations of the language specification that have become common among browser implementations and added support for new features that had emerged since the publication of the third edition. Such features include accessor properties, reflective creation and inspection of objects, program control of property attributes, additional array manipulation functions, support for the JSON object encoding format, and a strict mode that provides enhanced error checking and program security. The fifth edition was adopted by the Ecma General Assembly of December 2009.
The fifth edition was submitted to ISO/IEC JTC 1 for adoption under the fast-track procedure, and approved as international standard ISO/IEC 16262:2011. Edition 5.1 of the ECMAScript Standard incorporated minor corrections and is the same text as ISO/IEC 16262:2011. The 5.1 Edition was adopted by the Ecma General Assembly of June 2011.
Focused development of the sixth edition started in 2009, as the fifth edition was being prepared for publication. However, this was preceded by significant experimentation and language enhancement design efforts dating to the publication of the third edition in 1999. In a very real sense, the completion of the sixth edition is the culmination of a fifteen year effort. The goals for this edition included providing better support for large applications, library creation, and for use of ECMAScript as a compilation target for other languages. Some of its major enhancements included modules, class declarations, lexical block scoping, iterators and generators, promises for asynchronous programming, destructuring patterns, and proper tail calls. The ECMAScript library of built-ins was expanded to support additional data abstractions including maps, sets, and arrays of binary numeric values as well as additional support for Unicode supplementary characters in strings and regular expressions. The built-ins were also made extensible via subclassing. The sixth edition provides the foundation for regular, incremental language and library enhancements. The sixth edition was adopted by the General Assembly of June 2015.
ECMAScript 2016 was the first ECMAScript edition released under Ecma TC39's new yearly release cadence and open development process. A plain-text source document was built from the ECMAScript 2015 source document to serve as the base for further development entirely on GitHub. Over the year of this standard's development, hundreds of pull requests and issues were filed representing thousands of bug fixes, editorial fixes and other improvements. Additionally, numerous software tools were developed to aid in this effort including Ecmarkup, Ecmarkdown, and Grammarkdown. ES2016 also included support for a new exponentiation operator and adds a new method to Array.prototype called includes.
ECMAScript 2017 introduced Async Functions, Shared Memory, and Atomics along with smaller language and library enhancements, bug fixes, and editorial updates. Async functions improve the asynchronous programming experience by providing syntax for promise-returning functions. Shared Memory and Atomics introduce a new memory model that allows multi-agent programs to communicate using atomic operations that ensure a well-defined execution order even on parallel CPUs. It also included new static methods on Object: Object.values, Object.entries, and Object.getOwnPropertyDescriptors.
ECMAScript 2018 introduced support for asynchronous iteration via the AsyncIterator protocol and async generators. It also included four new regular expression features: the dotAll flag, named capture groups, Unicode property escapes, and look-behind assertions. Lastly it included object rest and spread properties.
ECMAScript 2019 introduced a few new built-in functions: flat and flatMap on Array.prototype for flattening arrays, Object.fromEntries for directly turning the return value of Object.entries into a new Object, and trimStart and trimEnd on String.prototype as better-named alternatives to the widely implemented but non-standard String.prototype.trimLeft and trimRight built-ins. In addition, it included a few minor updates to syntax and semantics. Updated syntax included optional catch binding parameters and allowing U+2028 (LINE SEPARATOR) and U+2029 (PARAGRAPH SEPARATOR) in string literals to align with JSON. Other updates included requiring that Array.prototype.sort be a stable sort, requiring that JSON.stringify return well-formed UTF-8 regardless of input, and clarifying Function.prototype.toString by requiring that it either return the corresponding original source text or a standard placeholder.
ECMAScript 2020, the 11th edition, introduced the matchAll method for Strings, to produce an iterator for all match objects generated by a global regular expression; import(), a syntax to asynchronously import Modules with a dynamic specifier; BigInt, a new number primitive for working with arbitrary precision integers; Promise.allSettled, a new Promise combinator that does not short-circuit; globalThis, a universal way to access the global this value; dedicated export * as ns from 'module' syntax for use within modules; increased standardization of for-in enumeration order; import.meta, a host-populated object available in Modules that may contain contextual information about the Module; as well as adding two new syntax features to improve working with “nullish” values (undefined or null): nullish coalescing, a value selection operator; and optional chaining, a property access and function invocation operator that short-circuits if the value to access/invoke is nullish.
ECMAScript 2021, the 12th edition, introduced the replaceAll method for Strings; Promise.any, a Promise combinator that short-circuits when an input value is fulfilled; AggregateError, a new Error type to represent multiple errors at once; logical assignment operators (??=, &&=, ||=); WeakRef, for referring to a target object without preserving it from garbage collection, and FinalizationRegistry, to manage registration and unregistration of cleanup operations performed when target objects are garbage collected; separators for numeric literals (1_000); and Array.prototype.sort was made more precise, reducing the amount of cases that result in an implementation-definedsort order.
ECMAScript 2022, the 13th edition, introduced top-level await, allowing the keyword to be used at the top level of modules; new class elements: public and private instance fields, public and private static fields, private instance methods and accessors, and private static methods and accessors; static blocks inside classes, to perform per-class evaluation initialization; the #x in obj syntax, to test for presence of private fields on objects; regular expression match indices via the /d flag, which provides start and end indices for matched substrings; the cause property on Error objects, which can be used to record a causation chain in errors; the at method for Strings, Arrays, and TypedArrays, which allows relative indexing; and Object.hasOwn, a convenient alternative to Object.prototype.hasOwnProperty.
ECMAScript 2023, the 14th edition, introduced the toSorted, toReversed, with, findLast, and findLastIndex methods on Array.prototype and TypedArray.prototype, as well as the toSpliced method on Array.prototype; added support for #! comments at the beginning of files to better facilitate executable ECMAScript files; and allowed the use of most Symbols as keys in weak collections.
ECMAScript 2024, the 15th edition, added facilities for resizing and transferring ArrayBuffers and SharedArrayBuffers; added a new RegExp /v flag for creating RegExps with more advanced features for working with sets of strings; and introduced the Promise.withResolvers convenience method for constructing Promises, the Object.groupBy and Map.groupBy methods for aggregating data, the Atomics.waitAsync method for asynchronously waiting for a change to shared memory, and the String.prototype.isWellFormed and String.prototype.toWellFormed methods for checking and ensuring that strings contain only well-formed Unicode.
Dozens of individuals representing many organizations have made very significant contributions within Ecma TC39 to the development of this edition and to the prior editions. In addition, a vibrant community has emerged supporting TC39's ECMAScript efforts. This community has reviewed numerous drafts, filed thousands of bug reports, performed implementation experiments, contributed test suites, and educated the world-wide developer community about ECMAScript. Unfortunately, it is impossible to identify and acknowledge every person and organization who has contributed to this effort.
Allen Wirfs-Brock
ECMA-262, Project Editor, 6th Edition
Brian Terlson
ECMA-262, Project Editor, 7th through 10th Editions
Jordan Harband
ECMA-262, Project Editor, 10th through 12th Editions
Shu-yu Guo
ECMA-262, Project Editor, 12th through 15th Editions
Michael Ficarra
ECMA-262, Project Editor, 12th through 15th Editions
Kevin Gibbons
ECMA-262, Project Editor, 12th through 15th Editions
1 Scope
This Standard defines the ECMAScript 2025 general-purpose programming language.
2 Conformance
A conforming implementation of ECMAScript must provide and support all the types, values, objects, properties, functions, and program syntax and semantics described in this specification.
A conforming implementation of ECMAScript must interpret source text input in conformance with the latest version of the Unicode Standard and ISO/IEC 10646.
A conforming implementation of ECMAScript that provides an application programming interface (API) that supports programs that need to adapt to the linguistic and cultural conventions used by different human languages and countries must implement the interface defined by the most recent edition of ECMA-402 that is compatible with this specification.
A conforming implementation of ECMAScript may provide additional types, values, objects, properties, and functions beyond those described in this specification. In particular, a conforming implementation of ECMAScript may provide properties not described in this specification, and values for those properties, for objects that are described in this specification.
A conforming implementation of ECMAScript may support program and regular expression syntax not described in this specification. In particular, a conforming implementation of ECMAScript may support program syntax that makes use of any “future reserved words” noted in subclause 12.7.2 of this specification.
A conforming implementation of ECMAScript must not implement any extension that is listed as a Forbidden Extension in subclause 17.1.
A conforming implementation of ECMAScript may choose to implement or not implement Normative Optional subclauses. If any Normative Optional behaviour is implemented, all of the behaviour in the containing Normative Optional clause must be implemented. A Normative Optional clause is denoted in this specification with the words "Normative Optional" in a coloured box, as shown below.
A conforming implementation of ECMAScript must implement Legacy subclauses, unless they are also marked as Normative Optional. All of the language features and behaviours specified within Legacy subclauses have one or more undesirable characteristics. However, their continued usage in existing applications prevents their removal from this specification. These features are not considered part of the core ECMAScript language. Programmers should not use or assume the existence of these features and behaviours when writing new ECMAScript code.
2.3 Example Legacy Normative Optional Clause Heading
Example clause contents.
3 Normative References
The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
IEEE 754-2019, IEEE Standard for Floating-Point Arithmetic.
ISO/IEC 10646, Information Technology — Universal Multiple-Octet Coded Character Set (UCS) plus Amendment 1:2005, Amendment 2:2006, Amendment 3:2008, Amendment 4:2008, and additional amendments and corrigenda, or successor.
This section contains a non-normative overview of the ECMAScript language.
ECMAScript is an object-oriented programming language for performing computations and manipulating computational objects within a host environment. ECMAScript as defined here is not intended to be computationally self-sufficient; indeed, there are no provisions in this specification for input of external data or output of computed results. Instead, it is expected that the computational environment of an ECMAScript program will provide not only the objects and other facilities described in this specification but also certain environment-specific objects, whose description and behaviour are beyond the scope of this specification except to indicate that they may provide certain properties that can be accessed and certain functions that can be called from an ECMAScript program.
ECMAScript was originally designed to be used as a scripting language, but has become widely used as a general-purpose programming language. A scripting language is a programming language that is used to manipulate, customize, and automate the facilities of an existing system. In such systems, useful functionality is already available through a user interface, and the scripting language is a mechanism for exposing that functionality to program control. In this way, the existing system is said to provide a host environment of objects and facilities, which completes the capabilities of the scripting language. A scripting language is intended for use by both professional and non-professional programmers.
ECMAScript was originally designed to be a Web scripting language, providing a mechanism to enliven Web pages in browsers and to perform server computation as part of a Web-based client-server architecture. ECMAScript is now used to provide core scripting capabilities for a variety of host environments. Therefore the core language is specified in this document apart from any particular host environment.
ECMAScript usage has moved beyond simple scripting and it is now used for the full spectrum of programming tasks in many different environments and scales. As the usage of ECMAScript has expanded, so have the features and facilities it provides. ECMAScript is now a fully featured general-purpose programming language.
4.1 Web Scripting
A web browser provides an ECMAScript host environment for client-side computation including, for instance, objects that represent windows, menus, pop-ups, dialog boxes, text areas, anchors, frames, history, cookies, and input/output. Further, the host environment provides a means to attach scripting code to events such as change of focus, page and image loading, unloading, error and abort, selection, form submission, and mouse actions. Scripting code appears within the HTML and the displayed page is a combination of user interface elements and fixed and computed text and images. The scripting code is reactive to user interaction, and there is no need for a main program.
A web server provides a different host environment for server-side computation including objects representing requests, clients, and files; and mechanisms to lock and share data. By using browser-side and server-side scripting together, it is possible to distribute computation between the client and server while providing a customized user interface for a Web-based application.
Each Web browser and server that supports ECMAScript supplies its own host environment, completing the ECMAScript execution environment.
4.2 Hosts and Implementations
To aid integrating ECMAScript into host environments, this specification defers the definition of certain facilities (e.g., abstract operations), either in whole or in part, to a source outside of this specification. Editorially, this specification distinguishes the following kinds of deferrals.
An implementation is an external source that further defines facilities enumerated in Annex D or those that are marked as implementation-defined or implementation-approximated. In informal use, an implementation refers to a concrete artefact, such as a particular web browser.
An implementation-defined facility is one that defers its definition to an external source without further qualification. This specification does not make any recommendations for particular behaviours, and conforming implementations are free to choose any behaviour within the constraints put forth by this specification.
An implementation-approximated facility is one that defers its definition to an external source while recommending an ideal behaviour. While conforming implementations are free to choose any behaviour within the constraints put forth by this specification, they are encouraged to strive to approximate the ideal. Some mathematical operations, such as Math.exp, are implementation-approximated.
A host is an external source that further defines facilities listed in Annex D but does not further define other implementation-defined or implementation-approximated facilities. In informal use, a host refers to the set of all implementations, such as the set of all web browsers, that interface with this specification in the same way via Annex D. A host is often an external specification, such as WHATWG HTML (https://html.spec.whatwg.org/). In other words, facilities that are host-defined are often further defined in external specifications.
A host hook is an abstract operation that is defined in whole or in part by an external source. All host hooks must be listed in Annex D. A host hook must conform to at least the following requirements:
A host-defined facility is one that defers its definition to an external source without further qualification and is listed in Annex D. Implementations that are not hosts may also provide definitions for host-defined facilities.
A host environment is a particular choice of definition for all host-defined facilities. A host environment typically includes objects or functions which allow obtaining input and providing output as host-defined properties of the global object.
This specification follows the editorial convention of always using the most specific term. For example, if a facility is host-defined, it should not be referred to as implementation-defined.
Both hosts and implementations may interface with this specification via the language types, specification types, abstract operations, grammar productions, intrinsic objects, and intrinsic symbols defined herein.
4.3 ECMAScript Overview
The following is an informal overview of ECMAScript—not all parts of the language are described. This overview is not part of the standard proper.
ECMAScript is object-based: basic language and host facilities are provided by objects, and an ECMAScript program is a cluster of communicating objects. In ECMAScript, an object is a collection of zero or more properties each with attributes that determine how each property can be used—for example, when the Writable attribute for a property is set to false, any attempt by executed ECMAScript code to assign a different value to the property fails. Properties are containers that hold other objects, primitive values, or functions. A primitive value is a member of one of the following built-in types: Undefined, Null, Boolean, Number, BigInt, String, and Symbol; an object is a member of the built-in type Object; and a function is a callable object. A function that is associated with an object via a property is called a method.
ECMAScript defines a collection of built-in objects that round out the definition of ECMAScript entities. These built-in objects include the global object; objects that are fundamental to the runtime semantics of the language including Object, Function, Boolean, Symbol, and various Error objects; objects that represent and manipulate numeric values including Math, Number, and Date; the text processing objects String and RegExp; objects that are indexed collections of values including Array and nine different kinds of Typed Arrays whose elements all have a specific numeric data representation; keyed collections including Map and Set objects; objects supporting structured data including the JSON object, ArrayBuffer, SharedArrayBuffer, and DataView; objects supporting control abstractions including generator functions and Promise objects; and reflection objects including Proxy and Reflect.
ECMAScript also defines a set of built-in operators. ECMAScript operators include various unary operations, multiplicative operators, additive operators, bitwise shift operators, relational operators, equality operators, binary bitwise operators, binary logical operators, assignment operators, and the comma operator.
Large ECMAScript programs are supported by modules which allow a program to be divided into multiple sequences of statements and declarations. Each module explicitly identifies declarations it uses that need to be provided by other modules and which of its declarations are available for use by other modules.
ECMAScript syntax intentionally resembles Java syntax. ECMAScript syntax is relaxed to enable it to serve as an easy-to-use scripting language. For example, a variable is not required to have its type declared nor are types associated with properties, and defined functions are not required to have their declarations appear textually before calls to them.
4.3.1 Objects
Even though ECMAScript includes syntax for class definitions, ECMAScript objects are not fundamentally class-based such as those in C++, Smalltalk, or Java. Instead objects may be created in various ways including via a literal notation or via constructors which create objects and then execute code that initializes all or part of them by assigning initial values to their properties. Each constructor is a function that has a property named "prototype" that is used to implement prototype-based inheritance and shared properties. Objects are created by using constructors in new expressions; for example, new Date(2009, 11) creates a new Date object. Invoking a constructor without using new has consequences that depend on the constructor. For example, Date() produces a string representation of the current date and time rather than an object.
Every object created by a constructor has an implicit reference (called the object's prototype) to the value of its constructor's "prototype" property. Furthermore, a prototype may have a non-null implicit reference to its prototype, and so on; this is called the prototype chain. When a reference is made to a property in an object, that reference is to the property of that name in the first object in the prototype chain that contains a property of that name. In other words, first the object mentioned directly is examined for such a property; if that object contains the named property, that is the property to which the reference refers; if that object does not contain the named property, the prototype for that object is examined next; and so on.
Figure 1: Object/Prototype Relationships
In a class-based object-oriented language, in general, state is carried by instances, methods are carried by classes, and inheritance is only of structure and behaviour. In ECMAScript, the state and methods are carried by objects, while structure, behaviour, and state are all inherited.
All objects that do not directly contain a particular property that their prototype contains share that property and its value. Figure 1 illustrates this:
CF is a constructor (and also an object). Five objects have been created by using new expressions: cf1, cf2, cf3, cf4, and cf5. Each of these objects contains properties named "q1" and "q2". The dashed lines represent the implicit prototype relationship; so, for example, cf3's prototype is CFp. The constructor, CF, has two properties itself, named "P1" and "P2", which are not visible to CFp, cf1, cf2, cf3, cf4, or cf5. The property named "CFP1" in CFp is shared by cf1, cf2, cf3, cf4, and cf5 (but not by CF), as are any properties found in CFp's implicit prototype chain that are not named "q1", "q2", or "CFP1". Notice that there is no implicit prototype link between CF and CFp.
Unlike most class-based object languages, properties can be added to objects dynamically by assigning values to them. That is, constructors are not required to name or assign values to all or any of the constructed object's properties. In the above diagram, one could add a new shared property for cf1, cf2, cf3, cf4, and cf5 by assigning a new value to the property in CFp.
Although ECMAScript objects are not inherently class-based, it is often convenient to define class-like abstractions based upon a common pattern of constructor functions, prototype objects, and methods. The ECMAScript built-in objects themselves follow such a class-like pattern. Beginning with ECMAScript 2015, the ECMAScript language includes syntactic class definitions that permit programmers to concisely define objects that conform to the same class-like abstraction pattern used by the built-in objects.
4.3.2 The Strict Variant of ECMAScript
The ECMAScript Language recognizes the possibility that some users of the language may wish to restrict their usage of some features available in the language. They might do so in the interests of security, to avoid what they consider to be error-prone features, to get enhanced error checking, or for other reasons of their choosing. In support of this possibility, ECMAScript defines a strict variant of the language. The strict variant of the language excludes some specific syntactic and semantic features of the regular ECMAScript language and modifies the detailed semantics of some features. The strict variant also specifies additional error conditions that must be reported by throwing error exceptions in situations that are not specified as errors by the non-strict form of the language.
The strict variant of ECMAScript is commonly referred to as the strict mode of the language. Strict mode selection and use of the strict mode syntax and semantics of ECMAScript is explicitly made at the level of individual ECMAScript source text units as described in 11.2.2. Because strict mode is selected at the level of a syntactic source text unit, strict mode only imposes restrictions that have local effect within such a source text unit. Strict mode does not restrict or modify any aspect of the ECMAScript semantics that must operate consistently across multiple source text units. A complete ECMAScript program may be composed of both strict mode and non-strict mode ECMAScript source text units. In this case, strict mode only applies when actually executing code that is defined within a strict mode source text unit.
In order to conform to this specification, an ECMAScript implementation must implement both the full unrestricted ECMAScript language and the strict variant of the ECMAScript language as defined by this specification. In addition, an implementation must support the combination of unrestricted and strict mode source text units into a single composite program.
4.4 Terms and Definitions
For the purposes of this document, the following terms and definitions apply.
4.4.1 implementation-approximated
an implementation-approximated facility is defined in whole or in part by an external source but has a recommended, ideal behaviour in this specification
4.4.2 implementation-defined
an implementation-defined facility is defined in whole or in part by an external source to this specification
The value of a constructor's "prototype" property is a prototype object that is used to implement inheritance and shared properties.
4.4.8 prototype
object that provides shared properties for other objects
Note
When a constructor creates an object, that object implicitly references the constructor's "prototype" property for the purpose of resolving property references. The constructor's "prototype" property can be referenced by the program expression constructor.prototype, and properties added to an object's prototype are shared, through inheritance, by all objects sharing the prototype. Alternatively, a new object may be created with an explicitly specified prototype by using the Object.create built-in function.
4.4.9 ordinary object
object that has the default behaviour for the essential internal methods that must be supported by all objects
4.4.10 exotic object
object that does not have the default behaviour for one or more of the essential internal methods
There are only two Boolean values, true and false.
4.4.18 Boolean type
type consisting of the primitive values true and false
4.4.19 Boolean object
member of the Object type that is an instance of the standard built-in Boolean constructor
Note
A Boolean object is created by using the Boolean constructor in a new expression, supplying a Boolean value as an argument. The resulting object has an internal slot whose value is the Boolean value. A Boolean object can be coerced to a Boolean value.
4.4.20 String value
primitive value that is a finite ordered sequence of zero or more 16-bit unsigned integer values
Note
A String value is a member of the String type. Each integer value in the sequence usually represents a single 16-bit unit of UTF-16 text. However, ECMAScript does not place any restrictions or requirements on the values except that they must be 16-bit unsigned integers.
4.4.21 String type
set of all possible String values
4.4.22 String object
member of the Object type that is an instance of the standard built-in String constructor
Note
A String object is created by using the String constructor in a new expression, supplying a String value as an argument. The resulting object has an internal slot whose value is the String value. A String object can be coerced to a String value by calling the String constructor as a function (22.1.1.1).
4.4.23 Number value
primitive value corresponding to a double-precision 64-bit binary format IEEE 754-2019 value
Note
A Number value is a member of the Number type and is a direct representation of a number.
4.4.24 Number type
set of all possible Number values including the special “Not-a-Number” (NaN) value, positive infinity, and negative infinity
4.4.25 Number object
member of the Object type that is an instance of the standard built-in Number constructor
Note
A Number object is created by using the Number constructor in a new expression, supplying a Number value as an argument. The resulting object has an internal slot whose value is the Number value. A Number object can be coerced to a Number value by calling the Number constructor as a function (21.1.1.1).
4.4.26 Infinity
Number value that is the positive infinite Number value
4.4.27 NaN
Number value that is an IEEE 754-2019 “Not-a-Number” value
4.4.28 BigInt value
primitive value corresponding to an arbitrary-precision integer value
4.4.29 BigInt type
set of all possible BigInt values
4.4.30 BigInt object
member of the Object type that is an instance of the standard built-in BigInt constructor
4.4.31 Symbol value
primitive value that represents a unique, non-String Object property key
4.4.32 Symbol type
set of all possible Symbol values
4.4.33 Symbol object
member of the Object type that is an instance of the standard built-in Symbol constructor
4.4.34 function
member of the Object type that may be invoked as a subroutine
Note
In addition to its properties, a function contains executable code and state that determine how it behaves when invoked. A function's code may or may not be written in ECMAScript.
4.4.35 built-in function
built-in object that is a function
Note
Examples of built-in functions include parseInt and Math.exp. A host or implementation may provide additional built-in functions that are not described in this specification.
Examples of built-in constructors include Object and Function. A host or implementation may provide additional built-in constructors that are not described in this specification.
4.4.37 property
part of an object that associates a key (either a String value or a Symbol value) and a value
Note
Depending upon the form of the property the value may be represented either directly as a data value (a primitive value, an object, or a function object) or indirectly by a pair of accessor functions.
4.4.38 method
function that is the value of a property
Note
When a function is called as a method of an object, the object is passed to the function as its this value.
4.4.39 built-in method
method that is a built-in function
Note
Standard built-in methods are defined in this specification. A host or implementation may provide additional built-in methods that are not described in this specification.
4.4.40 attribute
internal value that defines some characteristic of a property
4.4.41 own property
property that is directly contained by its object
4.4.42 inherited property
property of an object that is not an own property but is a property (either own or inherited) of the object's prototype
4.5 Organization of This Specification
The remainder of this specification is organized as follows:
Clause 5 defines the notational conventions used throughout the specification.
Clauses 6 through 10 define the execution environment within which ECMAScript programs operate.
Clauses 11 through 17 define the actual ECMAScript programming language including its syntactic encoding and the execution semantics of all language features.
Clauses 18 through 28 define the ECMAScript standard library. They include the definitions of all of the standard objects that are available for use by ECMAScript programs as they execute.
Clause 29 describes the memory consistency model of accesses on SharedArrayBuffer-backed memory and methods of the Atomics object.
5 Notational Conventions
5.1 Syntactic and Lexical Grammars
5.1.1 Context-Free Grammars
A context-free grammar consists of a number of productions. Each production has an abstract symbol called a nonterminal as its left-hand side, and a sequence of zero or more nonterminal and terminal symbols as its right-hand side. For each grammar, the terminal symbols are drawn from a specified alphabet.
A chain production is a production that has exactly one nonterminal symbol on its right-hand side along with zero or more terminal symbols.
Starting from a sentence consisting of a single distinguished nonterminal, called the goal symbol, a given context-free grammar specifies a language, namely, the (perhaps infinite) set of possible sequences of terminal symbols that can result from repeatedly replacing any nonterminal in the sequence with a right-hand side of a production for which the nonterminal is the left-hand side.
Input elements other than white space and comments form the terminal symbols for the syntactic grammar for ECMAScript and are called ECMAScript tokens. These tokens are the reserved words, identifiers, literals, and punctuators of the ECMAScript language. Moreover, line terminators, although not considered to be tokens, also become part of the stream of input elements and guide the process of automatic semicolon insertion (12.10). Simple white space and single-line comments are discarded and do not appear in the stream of input elements for the syntactic grammar. A MultiLineComment (that is, a comment of the form /*…*/ regardless of whether it spans more than one line) is likewise simply discarded if it contains no line terminator; but if a MultiLineComment contains one or more line terminators, then it is replaced by a single line terminator, which becomes part of the stream of input elements for the syntactic grammar.
A RegExp grammar for ECMAScript is given in 22.2.1. This grammar also has as its terminal symbols the code points as defined by SourceCharacter. It defines a set of productions, starting from the goal symbolPattern, that describe how sequences of code points are translated into regular expression patterns.
Productions of the lexical and RegExp grammars are distinguished by having two colons “::” as separating punctuation. The lexical and RegExp grammars share some productions.
Productions of the numeric string grammar are distinguished by having three colons “:::” as punctuation, and are never used for parsing source text.
5.1.4 The Syntactic Grammar
The syntactic grammar for ECMAScript is given in clauses 13 through 16. This grammar has ECMAScript tokens defined by the lexical grammar as its terminal symbols (5.1.2). It defines a set of productions, starting from two alternative goal symbolsScript and Module, that describe how sequences of tokens form syntactically correct independent components of ECMAScript programs.
When a stream of code points is to be parsed as an ECMAScript Script or Module, it is first converted to a stream of input elements by repeated application of the lexical grammar; this stream of input elements is then parsed by a single application of the syntactic grammar. The input stream is syntactically in error if the tokens in the stream of input elements cannot be parsed as a single instance of the goal nonterminal (Script or Module), with no tokens left over.
When a parse is successful, it constructs a parse tree, a rooted tree structure in which each node is a Parse Node. Each Parse Node is an instance of a symbol in the grammar; it represents a span of the source text that can be derived from that symbol. The root node of the parse tree, representing the whole of the source text, is an instance of the parse's goal symbol. When a Parse Node is an instance of a nonterminal, it is also an instance of some production that has that nonterminal as its left-hand side. Moreover, it has zero or more children, one for each symbol on the production's right-hand side: each child is a Parse Node that is an instance of the corresponding symbol.
New Parse Nodes are instantiated for each invocation of the parser and never reused between parses even of identical source text. Parse Nodes are considered the same Parse Node if and only if they represent the same span of source text, are instances of the same grammar symbol, and resulted from the same parser invocation.
Note 1
Parsing the same String multiple times will lead to different Parse Nodes. For example, consider:
let str = "1 + 1;";
eval(str);
eval(str);
Each call to eval converts the value of str into ECMAScript source text and performs an independent parse that creates its own separate tree of Parse Nodes. The trees are distinct even though each parse operates upon a source text that was derived from the same String value.
Note 2
Parse Nodes are specification artefacts, and implementations are not required to use an analogous data structure.
Productions of the syntactic grammar are distinguished by having just one colon “:” as punctuation.
The syntactic grammar as presented in clauses 13 through 16 is not a complete account of which token sequences are accepted as a correct ECMAScript Script or Module. Certain additional token sequences are also accepted, namely, those that would be described by the grammar if only semicolons were added to the sequence in certain places (such as before line terminator characters). Furthermore, certain token sequences that are described by the grammar are not considered acceptable if a line terminator character appears in certain “awkward” places.
In certain cases, in order to avoid ambiguities, the syntactic grammar uses generalized productions that permit token sequences that do not form a valid ECMAScript Script or Module. For example, this technique is used for object literals and object destructuring patterns. In such cases a more restrictive supplemental grammar is provided that further restricts the acceptable token sequences. Typically, an early error rule will then state that, in certain contexts, "P must cover an N", where P is a Parse Node (an instance of the generalized production) and N is a nonterminal from the supplemental grammar. This means:
The sequence of tokens originally matched by P is parsed again using N as the goal symbol. If N takes grammatical parameters, then they are set to the same values used when P was originally parsed.
If the sequence of tokens can be parsed as a single instance of N, with no tokens left over, then:
We refer to that instance of N (a Parse Node, unique for a given P) as "the N that is covered by P".
All Early Error rules for N and its derived productions also apply to the N that is covered by P.
Otherwise (if the parse fails), it is an early Syntax Error.
5.1.5 Grammar Notation
5.1.5.1 Terminal Symbols
In the ECMAScript grammars, some terminal symbols are shown in fixed-width font. These are to appear in a source text exactly as written. All terminal symbol code points specified in this way are to be understood as the appropriate Unicode code points from the Basic Latin block, as opposed to any similar-looking code points from other Unicode ranges. A code point in a terminal symbol cannot be expressed by a \UnicodeEscapeSequence.
In grammars whose terminal symbols are individual Unicode code points (i.e., the lexical, RegExp, and numeric string grammars), a contiguous run of multiple fixed-width code points appearing in a production is a simple shorthand for the same sequence of code points, written as standalone terminal symbols.
In contrast, in the syntactic grammar, a contiguous run of fixed-width code points is a single terminal symbol.
Terminal symbols come in two other forms:
In the lexical and RegExp grammars, Unicode code points without a conventional printed representation are instead shown in the form "<ABBREV>" where "ABBREV" is a mnemonic for the code point or set of code points. These forms are defined in Unicode Format-Control Characters, White Space, and Line Terminators.
In the syntactic grammar, certain terminal symbols (e.g. IdentifierName and RegularExpressionLiteral) are shown in italics, as they refer to the nonterminals of the same name in the lexical grammar.
5.1.5.2 Nonterminal Symbols and Productions
Nonterminal symbols are shown in italic type. The definition of a nonterminal (also called a “production”) is introduced by the name of the nonterminal being defined followed by one or more colons. (The number of colons indicates to which grammar the production belongs.) One or more alternative right-hand sides for the nonterminal then follow on succeeding lines. For example, the syntactic definition:
states that the nonterminal WhileStatement represents the token while, followed by a left parenthesis token, followed by an Expression, followed by a right parenthesis token, followed by a Statement. The occurrences of Expression and Statement are themselves nonterminals. As another example, the syntactic definition:
states that an ArgumentList may represent either a single AssignmentExpression or an ArgumentList, followed by a comma, followed by an AssignmentExpression. This definition of ArgumentList is recursive, that is, it is defined in terms of itself. The result is that an ArgumentList may contain any positive number of arguments, separated by commas, where each argument expression is an AssignmentExpression. Such recursive definitions of nonterminals are common.
5.1.5.3 Optional Symbols
The subscripted suffix “opt”, which may appear after a terminal or nonterminal, indicates an optional symbol. The alternative containing the optional symbol actually specifies two right-hand sides, one that omits the optional element and one that includes it. This means that:
so, in this example, the nonterminal ForStatement actually has four alternative right-hand sides.
5.1.5.4 Grammatical Parameters
A production may be parameterized by a subscripted annotation of the form “[parameters]”, which may appear as a suffix to the nonterminal symbol defined by the production. “parameters” may be either a single name or a comma separated list of names. A parameterized production is shorthand for a set of productions defining all combinations of the parameter names, preceded by an underscore, appended to the parameterized nonterminal symbol. This means that:
Prefixing a parameter name with “?” on a right-hand side nonterminal reference makes that parameter value dependent upon the occurrence of the parameter name on the reference to the current production's left-hand side symbol. For example:
If a right-hand side alternative is prefixed with “[+parameter]” that alternative is only available if the named parameter was used in referencing the production's nonterminal symbol. If a right-hand side alternative is prefixed with “[~parameter]” that alternative is only available if the named parameter was not used in referencing the production's nonterminal symbol. This means that:
When the words “one of” follow the colon(s) in a grammar definition, they signify that each of the terminal symbols on the following line or lines is an alternative definition. For example, the lexical grammar for ECMAScript contains the production:
If the phrase “[empty]” appears as the right-hand side of a production, it indicates that the production's right-hand side contains no terminals or nonterminals.
5.1.5.7 Lookahead Restrictions
If the phrase “[lookahead = seq]” appears in the right-hand side of a production, it indicates that the production may only be used if the token sequence seq is a prefix of the immediately following input token sequence. Similarly, “[lookahead ∈ set]”, where set is a finite non-empty set of token sequences, indicates that the production may only be used if some element of set is a prefix of the immediately following token sequence. For convenience, the set can also be written as a nonterminal, in which case it represents the set of all token sequences to which that nonterminal could expand. It is considered an editorial error if the nonterminal could expand to infinitely many distinct token sequences.
These conditions may be negated. “[lookahead ≠ seq]” indicates that the containing production may only be used if seq is not a prefix of the immediately following input token sequence, and “[lookahead ∉ set]” indicates that the production may only be used if no element of set is a prefix of the immediately following token sequence.
matches either the letter n followed by one or more decimal digits the first of which is even, or a decimal digit not followed by another decimal digit.
Note that when these phrases are used in the syntactic grammar, it may not be possible to unambiguously identify the immediately following token sequence because determining later tokens requires knowing which lexical goal symbol to use at later positions. As such, when these are used in the syntactic grammar, it is considered an editorial error for a token sequence seq to appear in a lookahead restriction (including as part of a set of sequences) if the choices of lexical goal symbols to use could change whether or not seq would be a prefix of the resulting token sequence.
If the phrase “[no LineTerminator here]” appears in the right-hand side of a production of the syntactic grammar, it indicates that the production is a restricted production: it may not be used if a LineTerminator occurs in the input stream at the indicated position. For example, the production:
indicates that the production may not be used if a LineTerminator occurs in the script between the throw token and the Expression.
Unless the presence of a LineTerminator is forbidden by a restricted production, any number of occurrences of LineTerminator may appear between any two consecutive tokens in the stream of input elements without affecting the syntactic acceptability of the script.
5.1.5.9 but not
The right-hand side of a production may specify that certain expansions are not permitted by using the phrase “but not” and then indicating the expansions to be excluded. For example, the production:
means that the nonterminal Identifier may be replaced by any sequence of code points that could replace IdentifierName provided that the same sequence of code points could not replace ReservedWord.
5.1.5.10 Descriptive Phrases
Finally, a few nonterminal symbols are described by a descriptive phrase in sans-serif type in cases where it would be impractical to list all the alternatives:
The specification often uses a numbered list to specify steps in an algorithm. These algorithms are used to precisely specify the required semantics of ECMAScript language constructs. The algorithms are not intended to imply the use of any specific implementation technique. In practice, there may be more efficient algorithms available to implement a given feature.
Algorithms may be explicitly parameterized with an ordered, comma-separated sequence of alias names which may be used within the algorithm steps to reference the argument passed in that position. Optional parameters are denoted with surrounding brackets ([ , name ]) and are no different from required parameters within algorithm steps. A rest parameter may appear at the end of a parameter list, denoted with leading ellipsis (, ...name). The rest parameter captures all of the arguments provided following the required and optional parameters into a List. If there are no such additional arguments, that List is empty.
Algorithm steps may be subdivided into sequential substeps. Substeps are indented and may themselves be further divided into indented substeps. Outline numbering conventions are used to identify substeps with the first level of substeps labelled with lowercase alphabetic characters and the second level of substeps labelled with lowercase roman numerals. If more than three levels are required these rules repeat with the fourth level using numeric labels. For example:
Top-level step
Substep.
Substep.
Subsubstep.
Subsubsubstep
Subsubsubsubstep
Subsubsubsubsubstep
A step or substep may be written as an “if” predicate that conditions its substeps. In this case, the substeps are only applied if the predicate is true. If a step or substep begins with the word “else”, it is a predicate that is the negation of the preceding “if” predicate step at the same level.
A step may specify the iterative application of its substeps.
A step that begins with “Assert:” asserts an invariant condition of its algorithm. Such assertions are used to make explicit algorithmic invariants that would otherwise be implicit. Such assertions add no additional semantic requirements and hence need not be checked by an implementation. They are used simply to clarify algorithms.
Algorithm steps may declare named aliases for any value using the form “Let x be someValue”. These aliases are reference-like in that both x and someValue refer to the same underlying data and modifications to either are visible to both. Algorithm steps that want to avoid this reference-like behaviour should explicitly make a copy of the right-hand side: “Let x be a copy of someValue” creates a shallow copy of someValue.
Once declared, an alias may be referenced in any subsequent steps and must not be referenced from steps prior to the alias's declaration. Aliases may be modified using the form “Set x to someOtherValue”.
5.2.1 Abstract Operations
In order to facilitate their use in multiple parts of this specification, some algorithms, called abstract operations, are named and written in parameterized functional form so that they may be referenced by name from within other algorithms. Abstract operations are typically referenced using a functional application style such as OperationName(arg1, arg2). Some abstract operations are treated as polymorphically dispatched methods of class-like specification abstractions. Such method-like abstract operations are typically referenced using a method application style such as someValue.OperationName(arg1, arg2).
5.2.2 Syntax-Directed Operations
A syntax-directed operation is a named operation whose definition consists of algorithms, each of which is associated with one or more productions from one of the ECMAScript grammars. A production that has multiple alternative definitions will typically have a distinct algorithm for each alternative. When an algorithm is associated with a grammar production, it may reference the terminal and nonterminal symbols of the production alternative as if they were parameters of the algorithm. When used in this manner, nonterminal symbols refer to the actual alternative definition that is matched when parsing the source text. The source text matched by a grammar production or Parse Node derived from it is the portion of the source text that starts at the beginning of the first terminal that participated in the match and ends at the end of the last terminal that participated in the match.
When an algorithm is associated with a production alternative, the alternative is typically shown without any “[ ]” grammar annotations. Such annotations should only affect the syntactic recognition of the alternative and have no effect on the associated semantics for the alternative.
Syntax-directed operations are invoked with a parse node and, optionally, other parameters by using the conventions on steps 1, 3, and 4 in the following algorithm:
Let status be SyntaxDirectedOperation of SomeNonTerminal.
Let someParseNode be the parse of some source text.
Perform SyntaxDirectedOperation of someParseNode.
Perform SyntaxDirectedOperation of someParseNode with argument "value".
Unless explicitly specified otherwise, all chain productions have an implicit definition for every operation that might be applied to that production's left-hand side nonterminal. The implicit definition simply reapplies the same operation with the same parameters, if any, to the chain production's sole right-hand side nonterminal and then returns the result. For example, assume that some algorithm has a step of the form: “Return Evaluation of Block” and that there is a production:
but the Evaluation operation does not associate an algorithm with that production. In that case, the Evaluation operation implicitly includes an association of the form:
The abstract operation Completion takes argument completionRecord (a Completion Record) and returns a Completion Record. It is used to emphasize that a Completion Record is being returned. It performs the following steps when called:
Similarly, prefix ! is used to indicate that the following invocation of an abstract or syntax-directed operation will never return an abrupt completion and that the resulting Completion Record's [[Value]] field should be used in place of the return value of the operation. For example, the step:
In algorithms within abstract operations which are declared to return a Completion Record, and within all built-in functions, the returned value is first passed to NormalCompletion, and the result is used instead. This rule does not apply within the Completion algorithm or when the value being returned is clearly marked as a Completion Record in that step; these cases are:
when the result of constructing a Completion Record is directly returned
It is an editorial error if a Completion Record is returned from such an abstract operation through any other means. For example, within these abstract operations,
Note that, through the ReturnIfAbrupt expansion, the following example is allowed, as within the expanded steps, the result of applying Completion is returned directly in the abrupt case and the implicit NormalCompletion application occurs after unwrapping in the normal case.
Return ? completion.
The following example would be an editorial error because a Completion Record is being returned without being annotated in that step.
Context-free grammars are not sufficiently powerful to express all the rules that define whether a stream of input elements form a valid ECMAScript Script or Module that may be evaluated. In some situations additional rules are needed that may be expressed using either ECMAScript algorithm conventions or prose requirements. Such rules are always associated with a production of a grammar and are called the static semantics of the production.
Static Semantic Rules have names and typically are defined using an algorithm. Named Static Semantic Rules are associated with grammar productions and a production that has multiple alternative definitions will typically have for each alternative a distinct algorithm for each applicable named static semantic rule.
A special kind of static semantic rule is an Early Error Rule. Early error rules define early error conditions (see clause 17) that are associated with specific grammar productions. Evaluation of most early error rules are not explicitly invoked within the algorithms of this specification. A conforming implementation must, prior to the first evaluation of a Script or Module, validate all of the early error rules of the productions used to parse that Script or Module. If any of the early error rules are violated the Script or Module is invalid and cannot be evaluated.
5.2.5 Mathematical Operations
This specification makes reference to these kinds of numeric values:
Mathematical values: Arbitrary real numbers, used as the default numeric type.
In the language of this specification, numerical values are distinguished among different numeric kinds using subscript suffixes. The subscript 𝔽 refers to Numbers, and the subscript ℤ refers to BigInts. Numeric values without a subscript suffix refer to mathematical values. This specification denotes most numeric values in base 10; it also uses numeric values of the form 0x followed by digits 0-9 or A-F as base-16 values.
In general, when this specification refers to a numerical value, such as in the phrase, "the length of y" or "the integer represented by the four hexadecimal digits ...", without explicitly specifying a numeric kind, the phrase refers to a mathematical value. Phrases which refer to a Number or a BigInt value are explicitly annotated as such; for example, "the Number value for the number of code points in …" or "the BigInt value for …".
When the term integer is used in this specification, it refers to a mathematical value which is in the set of integers, unless otherwise stated. When the term integral Number is used in this specification, it refers to a finite Number value whose mathematical value is in the set of integers.
Numeric operators such as +, ×, =, and ≥ refer to those operations as determined by the type of the operands. When applied to mathematical values, the operators refer to the usual mathematical operations. When applied to extended mathematical values, the operators refer to the usual mathematical operations over the extended real numbers; indeterminate forms are not defined and their use in this specification should be considered an editorial error. When applied to Numbers, the operators refer to the relevant operations within IEEE 754-2019. When applied to BigInts, the operators refer to the usual mathematical operations applied to the mathematical value of the BigInt. Numeric operators applied to mixed-type operands (such as a Number and a mathematical value) are not defined and should be considered an editorial error in this specification.
The mathematical function abs(x) produces the absolute value of x, which is -x if x < 0 and otherwise is x itself.
The mathematical function min(x1, x2, … , xN) produces the mathematically smallest of x1 through xN. The mathematical function max(x1, x2, ..., xN) produces the mathematically largest of x1 through xN. The domain and range of these mathematical functions are the extended mathematical values.
The notation “x modulo y” (y must be finite and non-zero) computes a value k of the same sign as y (or zero) such that abs(k) < abs(y) and x - k = q × y for some integerq.
The phrase "the result of clamping x between lower and upper" (where x is an extended mathematical value and lower and upper are mathematical values such that lower ≤ upper) produces lower if x < lower, produces upper if x > upper, and otherwise produces x.
The mathematical function floor(x) produces the largest integer (closest to +∞) that is not larger than x.
The mathematical function truncate(x) removes the fractional part of x by rounding towards zero, producing -floor(-x) if x < 0 and otherwise producing floor(x).
Mathematical functions min, max, abs, floor, and truncate are not defined for Numbers and BigInts, and any usage of those methods that have non-mathematical value arguments would be an editorial error in this specification.
An interval from lower bound a to upper bound b is a possibly-infinite, possibly-empty set of numeric values of the same numeric type. Each bound will be described as either inclusive or exclusive, but not both. There are four kinds of intervals, as follows:
An interval from a (inclusive) to b (inclusive), also called an inclusive interval from a to b, includes all values x of the same numeric type such that a ≤ x ≤ b, and no others.
An interval from a (inclusive) to b (exclusive) includes all values x of the same numeric type such that a ≤ x < b, and no others.
An interval from a (exclusive) to b (inclusive) includes all values x of the same numeric type such that a < x ≤ b, and no others.
An interval from a (exclusive) to b (exclusive) includes all values x of the same numeric type such that a < x < b, and no others.
For example, the interval from 1 (inclusive) to 2 (exclusive) consists of all mathematical values between 1 and 2, including 1 and not including 2. For the purpose of defining intervals, -0𝔽 < +0𝔽, so, for example, an inclusive interval with a lower bound of +0𝔽 includes +0𝔽 but not -0𝔽. NaN is never included in an interval.
5.2.6 Value Notation
In this specification, ECMAScript language values are displayed in bold. Examples include null, true, or "hello". These are distinguished from ECMAScript source text such as Function.prototype.apply or let n = 42;.
5.2.7 Identity
In this specification, both specification values and ECMAScript language values are compared for equality. When comparing for equality, values fall into one of two categories. Values without identity are equal to other values without identity if all of their innate characteristics are the same — characteristics such as the magnitude of an integer or the length of a sequence. Values without identity may be manifest without prior reference by fully describing their characteristics. In contrast, each value with identity is unique and therefore only equal to itself. Values with identity are like values without identity but with an additional unguessable, unchangeable, universally-unique characteristic called identity. References to existing values with identity cannot be manifest simply by describing them, as the identity itself is indescribable; instead, references to these values must be explicitly passed from one place to another. Some values with identity are mutable and therefore can have their characteristics (except their identity) changed in-place, causing all holders of the value to observe the new characteristics. A value without identity is never equal to a value with identity.
From the perspective of this specification, the word “is” is used to compare two values for equality, as in “If bool is true, then ...”, and the word “contains” is used to search for a value inside lists using equality comparisons, as in "If list contains a Recordr such that r.[[Foo]] is true, then ...". The specification identity of values determines the result of these comparisons and is axiomatic in this specification.
From the perspective of the ECMAScript language, language values are compared for equality using the SameValue abstract operation and the abstract operations it transitively calls. The algorithms of these comparison abstract operations determine language identity of ECMAScript language values.
Algorithms within this specification manipulate values each of which has an associated type. The possible value types are exactly those defined in this clause. Types are further classified into ECMAScript language types and specification types.
Within this specification, the notation “Type(x)” is used as shorthand for “the type of x” where “type” refers to the ECMAScript language and specification types defined in this clause.
6.1 ECMAScript Language Types
An ECMAScript language type corresponds to values that are directly manipulated by an ECMAScript programmer using the ECMAScript language. The ECMAScript language types are Undefined, Null, Boolean, String, Symbol, Number, BigInt, and Object. An ECMAScript language value is a value that is characterized by an ECMAScript language type.
6.1.1 The Undefined Type
The Undefined type has exactly one value, called undefined. Any variable that has not been assigned a value has the value undefined.
6.1.2 The Null Type
The Null type has exactly one value, called null.
6.1.3 The Boolean Type
The Boolean type represents a logical entity having two values, called true and false.
6.1.4 The String Type
The String type is the set of all ordered sequences of zero or more 16-bit unsigned integer values (“elements”) up to a maximum length of 253 - 1 elements. The String type is generally used to represent textual data in a running ECMAScript program, in which case each element in the String is treated as a UTF-16 code unit value. Each element is regarded as occupying a position within the sequence. These positions are indexed with non-negative integers. The first element (if any) is at index 0, the next element (if any) at index 1, and so on. The length of a String is the number of elements (i.e., 16-bit values) within it. The empty String has length zero and therefore contains no elements.
ECMAScript operations that do not interpret String contents apply no further semantics. Operations that do interpret String values treat each element as a single UTF-16 code unit. However, ECMAScript does not restrict the value of or relationships between these code units, so operations that further interpret String contents as sequences of Unicode code points encoded in UTF-16 must account for ill-formed subsequences. Such operations apply special treatment to every code unit with a numeric value in the inclusive interval from 0xD800 to 0xDBFF (defined by the Unicode Standard as a leading surrogate, or more formally as a high-surrogate code unit) and every code unit with a numeric value in the inclusive interval from 0xDC00 to 0xDFFF (defined as a trailing surrogate, or more formally as a low-surrogate code unit) using the following rules:
A sequence of two code units, where the first code unit c1 is a leading surrogate and the second code unit c2 a trailing surrogate, is a surrogate pair and is interpreted as a code point with the value (c1 - 0xD800) × 0x400 + (c2 - 0xDC00) + 0x10000. (See 11.1.3)
The function String.prototype.normalize (see 22.1.3.15) can be used to explicitly normalize a String value. String.prototype.localeCompare (see 22.1.3.12) internally normalizes String values, but no other operations implicitly normalize the strings upon which they operate. Operation results are not language- and/or locale-sensitive unless stated otherwise.
Note
The rationale behind this design was to keep the implementation of Strings as simple and high-performing as possible. If ECMAScript source text is in Normalized Form C, string literals are guaranteed to also be normalized, as long as they do not contain any Unicode escape sequences.
In this specification, the phrase "the string-concatenation of A, B, ..." (where each argument is a String value, a code unit, or a sequence of code units) denotes the String value whose sequence of code units is the concatenation of the code units (in order) of each of the arguments (in order).
The phrase "the substring of S from inclusiveStart to exclusiveEnd" (where S is a String value or a sequence of code units and inclusiveStart and exclusiveEnd are integers) denotes the String value consisting of the consecutive code units of S beginning at index inclusiveStart and ending immediately before index exclusiveEnd (which is the empty String when inclusiveStart = exclusiveEnd). If the "to" suffix is omitted, the length of S is used as the value of exclusiveEnd.
The phrase "the ASCII word characters" denotes the following String value, which consists solely of every letter and number in the Unicode Basic Latin block along with U+005F (LOW LINE): "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_".
For historical reasons, it has significance to various algorithms.
The abstract operation StringIndexOf takes arguments string (a String), searchValue (a String), and fromIndex (a non-negative integer) and returns a non-negative integer or not-found. It performs the following steps when called:
Let len be the length of string.
If searchValue is the empty String and fromIndex ≤ len, return fromIndex.
Let searchLen be the length of searchValue.
For each integeri such that fromIndex ≤ i ≤ len - searchLen, in ascending order, do
Let candidate be the substring of string from i to i + searchLen.
If candidate is searchValue, return i.
Return not-found.
Note 1
If searchValue is the empty String and fromIndex ≤ the length of string, this algorithm returns fromIndex. The empty String is effectively found at every position within a string, including after the last code unit.
Note 2
This algorithm always returns not-found if fromIndex + the length of searchValue > the length of string.
The abstract operation StringLastIndexOf takes arguments string (a String), searchValue (a String), and fromIndex (a non-negative integer) and returns a non-negative integer or not-found. It performs the following steps when called:
For each integeri such that 0 ≤ i ≤ fromIndex, in descending order, do
Let candidate be the substring of string from i to i + searchLen.
If candidate is searchValue, return i.
Return not-found.
Note
If searchValue is the empty String, this algorithm returns fromIndex. The empty String is effectively found at every position within a string, including after the last code unit.
6.1.5 The Symbol Type
The Symbol type is the set of all non-String values that may be used as the key of an Object property (6.1.7).
Each possible Symbol value is unique and immutable.
Each Symbol value immutably holds an associated value called [[Description]] that is either undefined or a String value.
6.1.5.1 Well-Known Symbols
Well-known symbols are built-in Symbol values that are explicitly referenced by algorithms of this specification. They are typically used as the keys of properties whose values serve as extension points of a specification algorithm. Unless otherwise specified, well-known symbols values are shared by all realms (9.3).
Within this specification a well-known symbol is referred to using the standard intrinsic notation where the intrinsic is one of the values listed in Table 1.
Note
Previous editions of this specification used a notation of the form @@name, where the current edition would use %Symbol.name%. In particular, the following names were used: @@asyncIterator, @@hasInstance, @@isConcatSpreadable, @@iterator, @@match, @@matchAll, @@replace, @@search, @@species, @@split, @@toPrimitive, @@toStringTag, and @@unscopables.
Table 1: Well-known Symbols
Specification Name
[[Description]]
Value and Purpose
%Symbol.asyncIterator%
"Symbol.asyncIterator"
A method that returns the default AsyncIterator for an object. Called by the semantics of the for-await-of statement.
%Symbol.hasInstance%
"Symbol.hasInstance"
A method that determines if a constructor object recognizes an object as one of the constructor's instances. Called by the semantics of the instanceof operator.
%Symbol.isConcatSpreadable%
"Symbol.isConcatSpreadable"
A Boolean valued property that if true indicates that an object should be flattened to its array elements by Array.prototype.concat.
%Symbol.iterator%
"Symbol.iterator"
A method that returns the default Iterator for an object. Called by the semantics of the for-of statement.
%Symbol.match%
"Symbol.match"
A regular expression method that matches the regular expression against a string. Called by the String.prototype.match method.
%Symbol.matchAll%
"Symbol.matchAll"
A regular expression method that returns an iterator, that yields matches of the regular expression against a string. Called by the String.prototype.matchAll method.
%Symbol.replace%
"Symbol.replace"
A regular expression method that replaces matched substrings of a string. Called by the String.prototype.replace method.
%Symbol.search%
"Symbol.search"
A regular expression method that returns the index within a string that matches the regular expression. Called by the String.prototype.search method.
%Symbol.species%
"Symbol.species"
A function valued property that is the constructor function that is used to create derived objects.
%Symbol.split%
"Symbol.split"
A regular expression method that splits a string at the indices that match the regular expression. Called by the String.prototype.split method.
%Symbol.toPrimitive%
"Symbol.toPrimitive"
A method that converts an object to a corresponding primitive value. Called by the ToPrimitive abstract operation.
%Symbol.toStringTag%
"Symbol.toStringTag"
A String valued property that is used in the creation of the default string description of an object. Accessed by the built-in method Object.prototype.toString.
%Symbol.unscopables%
"Symbol.unscopables"
An object valued property whose own and inherited property names are property names that are excluded from the with environment bindings of the associated object.
6.1.6 Numeric Types
ECMAScript has two built-in numeric types: Number and BigInt. The following abstract operations are defined over these numeric types. The "Result" column shows the return type, along with an indication if it is possible for some invocations of the operation to return an abrupt completion.
Because the numeric types are in general not convertible without loss of precision or truncation, the ECMAScript language provides no implicit conversion among these types. Programmers must explicitly call Number and BigInt functions to convert among types when calling a function which requires another type.
Note
The first and subsequent editions of ECMAScript have provided, for certain operators, implicit numeric conversions that could lose precision or truncate. These legacy implicit conversions are maintained for backward compatibility, but not provided for BigInt in order to minimize opportunity for programmer error, and to leave open the option of generalized value types in a future edition.
6.1.6.1 The Number Type
The Number type has exactly 18,437,736,874,454,810,627 (that is, 264 - 253 + 3) values, representing the double-precision floating point IEEE 754-2019 binary64 values as specified in the IEEE Standard for Binary Floating-Point Arithmetic, except that the 9,007,199,254,740,990 (that is, 253 - 2) distinct “Not-a-Number” values of the IEEE Standard are represented in ECMAScript as a single special NaN value. (Note that the NaN value is produced by the program expression NaN.) In some implementations, external code might be able to detect a difference between various Not-a-Number values, but such behaviour is implementation-defined; to ECMAScript code, all NaN values are indistinguishable from each other.
Note
The bit pattern that might be observed in an ArrayBuffer (see 25.1) or a SharedArrayBuffer (see 25.2) after a Number value has been stored into it is not necessarily the same as the internal representation of that Number value used by the ECMAScript implementation.
There are two other special values, called positive Infinity and negative Infinity. For brevity, these values are also referred to for expository purposes by the symbols +∞𝔽 and -∞𝔽, respectively. (Note that these two infinite Number values are produced by the program expressions +Infinity (or simply Infinity) and -Infinity.)
The other 18,437,736,874,454,810,624 (that is, 264 - 253) values are called the finite numbers. Half of these are positive numbers and half are negative numbers; for every finite positive Number value there is a corresponding negative value having the same magnitude.
Note that there is both a positive zero and a negative zero. For brevity, these values are also referred to for expository purposes by the symbols +0𝔽 and -0𝔽, respectively. (Note that these two different zero Number values are produced by the program expressions +0 (or simply 0) and -0.)
The 18,437,736,874,454,810,622 (that is, 264 - 253 - 2) finite non-zero values are of two kinds:
18,428,729,675,200,069,632 (that is, 264 - 254) of them are normalized, having the form
The remaining 9,007,199,254,740,990 (that is, 253 - 2) values are denormalized, having the form
s × m × 2e
where s is 1 or -1, m is an integer in the interval from 0 (exclusive) to 252 (exclusive), and e is -1074.
Note that all the positive and negative integers whose magnitude is no greater than 253 are representable in the Number type. The integer 0 has two representations in the Number type: +0𝔽 and -0𝔽.
A finite number has an odd significand if it is non-zero and the integerm used to express it (in one of the two forms shown above) is odd. Otherwise, it has an even significand.
In this specification, the phrase “the Number value for x” where x represents an exact real mathematical quantity (which might even be an irrational number such as π) means a Number value chosen in the following manner. Consider the set of all finite values of the Number type, with -0𝔽 removed and with two additional values added to it that are not representable in the Number type, namely 21024 (which is +1 × 253 × 2971) and -21024 (which is -1 × 253 × 2971). Choose the member of this set that is closest in value to x. If two values of the set are equally close, then the one with an even significand is chosen; for this purpose, the two extra values 21024 and -21024 are considered to have even significands. Finally, if 21024 was chosen, replace it with +∞𝔽; if -21024 was chosen, replace it with -∞𝔽; if +0𝔽 was chosen, replace it with -0𝔽 if and only if x < 0; any other chosen value is used unchanged. The result is the Number value forx. (This procedure corresponds exactly to the behaviour of the IEEE 754-2019 roundTiesToEven mode.)
Some ECMAScript operators deal only with integers in specific ranges such as the inclusive interval from -231 to 231 - 1 or the inclusive interval from 0 to 216 - 1. These operators accept any value of the Number type but first convert each such value to an integer value in the expected range. See the descriptions of the numeric conversion operations in 7.1.
6.1.6.1.1 Number::unaryMinus ( x )
The abstract operation Number::unaryMinus takes argument x (a Number) and returns a Number. It performs the following steps when called:
If x is NaN, return NaN.
Return the negation of x; that is, compute a Number with the same magnitude but opposite sign.
6.1.6.1.2 Number::bitwiseNOT ( x )
The abstract operation Number::bitwiseNOT takes argument x (a Number) and returns an integral Number. It performs the following steps when called:
Return the bitwise complement of oldValue. The mathematical value of the result is exactly representable as a 32-bit two's complement bit string.
6.1.6.1.3 Number::exponentiate ( base, exponent )
The abstract operation Number::exponentiate takes arguments base (a Number) and exponent (a Number) and returns a Number. It returns an implementation-approximated value representing the result of raising base to the exponent power. It performs the following steps when called:
If exponent is NaN, return NaN.
If exponent is either +0𝔽 or -0𝔽, return 1𝔽.
If base is NaN, return NaN.
If base is +∞𝔽, then
If exponent > +0𝔽, return +∞𝔽. Otherwise, return +0𝔽.
If base is -∞𝔽, then
If exponent > +0𝔽, then
If exponent is an odd integral Number, return -∞𝔽. Otherwise, return +∞𝔽.
Else,
If exponent is an odd integral Number, return -0𝔽. Otherwise, return +0𝔽.
If base is +0𝔽, then
If exponent > +0𝔽, return +0𝔽. Otherwise, return +∞𝔽.
If base is -0𝔽, then
If exponent > +0𝔽, then
If exponent is an odd integral Number, return -0𝔽. Otherwise, return +0𝔽.
Else,
If exponent is an odd integral Number, return -∞𝔽. Otherwise, return +∞𝔽.
Assert: base is finite and is neither +0𝔽 nor -0𝔽.
Assert: exponent is finite and is neither +0𝔽 nor -0𝔽.
If base < -0𝔽 and exponent is not an integral Number, return NaN.
Return an implementation-approximated Number value representing the result of raising ℝ(base) to the ℝ(exponent) power.
Note
The result of base**exponent when base is 1𝔽 or -1𝔽 and exponent is +∞𝔽 or -∞𝔽, or when base is 1𝔽 and exponent is NaN, differs from IEEE 754-2019. The first edition of ECMAScript specified a result of NaN for this operation, whereas later revisions of IEEE 754 specified 1𝔽. The historical ECMAScript behaviour is preserved for compatibility reasons.
6.1.6.1.4 Number::multiply ( x, y )
The abstract operation Number::multiply takes arguments x (a Number) and y (a Number) and returns a Number. It performs multiplication according to the rules of IEEE 754-2019 binary double-precision arithmetic, producing the product of x and y. It performs the following steps when called:
Finite-precision multiplication is commutative, but not always associative.
6.1.6.1.5 Number::divide ( x, y )
The abstract operation Number::divide takes arguments x (a Number) and y (a Number) and returns a Number. It performs division according to the rules of IEEE 754-2019 binary double-precision arithmetic, producing the quotient of x and y where x is the dividend and y is the divisor. It performs the following steps when called:
If x is NaN or y is NaN, return NaN.
If x is either +∞𝔽 or -∞𝔽, then
If y is either +∞𝔽 or -∞𝔽, return NaN.
If y is +0𝔽 or y > +0𝔽, return x.
Return -x.
If y is +∞𝔽, then
If x is +0𝔽 or x > +0𝔽, return +0𝔽. Otherwise, return -0𝔽.
If y is -∞𝔽, then
If x is +0𝔽 or x > +0𝔽, return -0𝔽. Otherwise, return +0𝔽.
The abstract operation Number::remainder takes arguments n (a Number) and d (a Number) and returns a Number. It yields the remainder from an implied division of its operands where n is the dividend and d is the divisor. It performs the following steps when called:
In C and C++, the remainder operator accepts only integral operands; in ECMAScript, it also accepts floating-point operands.
Note 2
The result of a floating-point remainder operation as computed by the % operator is not the same as the “remainder” operation defined by IEEE 754-2019. The IEEE 754-2019 “remainder” operation computes the remainder from a rounding division, not a truncating division, and so its behaviour is not analogous to that of the usual integer remainder operator. Instead the ECMAScript language defines % on floating-point operations to behave in a manner analogous to that of the Java integer remainder operator; this may be compared with the C library function fmod.
6.1.6.1.7 Number::add ( x, y )
The abstract operation Number::add takes arguments x (a Number) and y (a Number) and returns a Number. It performs addition according to the rules of IEEE 754-2019 binary double-precision arithmetic, producing the sum of its arguments. It performs the following steps when called:
Finite-precision addition is commutative, but not always associative.
6.1.6.1.8 Number::subtract ( x, y )
The abstract operation Number::subtract takes arguments x (a Number) and y (a Number) and returns a Number. It performs subtraction, producing the difference of its operands; x is the minuend and y is the subtrahend. It performs the following steps when called:
It is always the case that x - y produces the same result as x + (-y).
6.1.6.1.9 Number::leftShift ( x, y )
The abstract operation Number::leftShift takes arguments x (a Number) and y (a Number) and returns an integral Number. It performs the following steps when called:
Return the result of left shifting lNum by shiftCount bits. The mathematical value of the result is exactly representable as a 32-bit two's complement bit string.
6.1.6.1.10 Number::signedRightShift ( x, y )
The abstract operation Number::signedRightShift takes arguments x (a Number) and y (a Number) and returns an integral Number. It performs the following steps when called:
Return the result of performing a sign-extending right shift of lNum by shiftCount bits. The most significant bit is propagated. The mathematical value of the result is exactly representable as a 32-bit two's complement bit string.
6.1.6.1.11 Number::unsignedRightShift ( x, y )
The abstract operation Number::unsignedRightShift takes arguments x (a Number) and y (a Number) and returns an integral Number. It performs the following steps when called:
Return the result of performing a zero-filling right shift of lNum by shiftCount bits. Vacated bits are filled with zero. The mathematical value of the result is exactly representable as a 32-bit unsigned bit string.
6.1.6.1.12 Number::lessThan ( x, y )
The abstract operation Number::lessThan takes arguments x (a Number) and y (a Number) and returns a Boolean or undefined. It performs the following steps when called:
If ℝ(x) < ℝ(y), return true. Otherwise, return false.
6.1.6.1.13 Number::equal ( x, y )
The abstract operation Number::equal takes arguments x (a Number) and y (a Number) and returns a Boolean. It performs the following steps when called:
If x is NaN, return false.
If y is NaN, return false.
If x is y, return true.
If x is +0𝔽 and y is -0𝔽, return true.
If x is -0𝔽 and y is +0𝔽, return true.
Return false.
6.1.6.1.14 Number::sameValue ( x, y )
The abstract operation Number::sameValue takes arguments x (a Number) and y (a Number) and returns a Boolean. It performs the following steps when called:
If x is NaN and y is NaN, return true.
If x is +0𝔽 and y is -0𝔽, return false.
If x is -0𝔽 and y is +0𝔽, return false.
If x is y, return true.
Return false.
6.1.6.1.15 Number::sameValueZero ( x, y )
The abstract operation Number::sameValueZero takes arguments x (a Number) and y (a Number) and returns a Boolean. It performs the following steps when called:
If x is NaN and y is NaN, return true.
If x is +0𝔽 and y is -0𝔽, return true.
If x is -0𝔽 and y is +0𝔽, return true.
If x is y, return true.
Return false.
6.1.6.1.16 NumberBitwiseOp ( op, x, y )
The abstract operation NumberBitwiseOp takes arguments op (&, ^, or |), x (a Number), and y (a Number) and returns an integral Number. It performs the following steps when called:
Let result be the result of applying the bitwise inclusive OR operation to lBits and rBits.
Return the Number value for the integer represented by the 32-bit two's complement bit string result.
6.1.6.1.17 Number::bitwiseAND ( x, y )
The abstract operation Number::bitwiseAND takes arguments x (a Number) and y (a Number) and returns an integral Number. It performs the following steps when called:
The abstract operation Number::bitwiseXOR takes arguments x (a Number) and y (a Number) and returns an integral Number. It performs the following steps when called:
The abstract operation Number::bitwiseOR takes arguments x (a Number) and y (a Number) and returns an integral Number. It performs the following steps when called:
The abstract operation Number::toString takes arguments x (a Number) and radix (an integer in the inclusive interval from 2 to 36) and returns a String. It represents x as a String using a positional numeral system with radix radix. The digits used in the representation of a number using radix r are taken from the first r code units of "0123456789abcdefghijklmnopqrstuvwxyz" in order. The representation of numbers with magnitude greater than or equal to 1𝔽 never includes leading zeroes. It performs the following steps when called:
Let n, k, and s be integers such that k ≥ 1, radixk - 1 ≤ s < radixk, 𝔽(s × radixn - k) is x, and k is as small as possible. Note that k is the number of digits in the representation of s using radix radix, that s is not divisible by radix, and that the least significant digit of s is not necessarily uniquely determined by these criteria.
the code unit of the most significant digit of the decimal representation of s
the code unit 0x002E (FULL STOP)
the code units of the remaining k - 1 digits of the decimal representation of s
the code unit 0x0065 (LATIN SMALL LETTER E)
exponentSign
the code units of the decimal representation of abs(n - 1)
Note 1
The following observations may be useful as guidelines for implementations, but are not part of the normative requirements of this Standard:
If x is any Number value other than -0𝔽, then ToNumber(ToString(x)) is x.
The least significant digit of s is not always uniquely determined by the requirements listed in step 5.
Note 2
For implementations that provide more accurate conversions than required by the rules above, it is recommended that the following alternative version of step 5 be used as a guideline:
Let n, k, and s be integers such that k ≥ 1, radixk - 1 ≤ s < radixk, 𝔽(s × radixn - k) is x, and k is as small as possible. If there are multiple possibilities for s, choose the value of s for which s × radixn - k is closest in value to ℝ(x). If there are two such possible values of s, choose the one that is even. Note that k is the number of digits in the representation of s using radix radix and that s is not divisible by radix.
Note 3
Implementers of ECMAScript may find useful the paper and code written by David M. Gay for binary-to-decimal conversion of floating-point numbers:
The BigInt type represents an integer value. The value may be any size and is not limited to a particular bit-width. Generally, where not otherwise noted, operations are designed to return exact mathematically-based answers. For binary operations, BigInts act as two's complement binary strings, with negative numbers treated as having bits set infinitely to the left.
6.1.6.2.1 BigInt::unaryMinus ( x )
The abstract operation BigInt::unaryMinus takes argument x (a BigInt) and returns a BigInt. It performs the following steps when called:
If x = 0ℤ, return 0ℤ.
Return -x.
6.1.6.2.2 BigInt::bitwiseNOT ( x )
The abstract operation BigInt::bitwiseNOT takes argument x (a BigInt) and returns a BigInt. It returns the one's complement of x. It performs the following steps when called:
Return -x - 1ℤ.
6.1.6.2.3 BigInt::exponentiate ( base, exponent )
The abstract operation BigInt::exponentiate takes arguments base (a BigInt) and exponent (a BigInt) and returns either a normal completion containing a BigInt or a throw completion. It performs the following steps when called:
If exponent < 0ℤ, throw a RangeError exception.
If base = 0ℤ and exponent = 0ℤ, return 1ℤ.
Return base raised to the power exponent.
6.1.6.2.4 BigInt::multiply ( x, y )
The abstract operation BigInt::multiply takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:
Return x × y.
Note
Even if the result has a much larger bit width than the input, the exact mathematical answer is given.
6.1.6.2.5 BigInt::divide ( x, y )
The abstract operation BigInt::divide takes arguments x (a BigInt) and y (a BigInt) and returns either a normal completion containing a BigInt or a throw completion. It performs the following steps when called:
The abstract operation BigInt::remainder takes arguments n (a BigInt) and d (a BigInt) and returns either a normal completion containing a BigInt or a throw completion. It performs the following steps when called:
The sign of the result is the sign of the dividend.
6.1.6.2.7 BigInt::add ( x, y )
The abstract operation BigInt::add takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:
Return x + y.
6.1.6.2.8 BigInt::subtract ( x, y )
The abstract operation BigInt::subtract takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:
Return x - y.
6.1.6.2.9 BigInt::leftShift ( x, y )
The abstract operation BigInt::leftShift takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:
Semantics here should be equivalent to a bitwise shift, treating the BigInt as an infinite length string of binary two's complement digits.
6.1.6.2.10 BigInt::signedRightShift ( x, y )
The abstract operation BigInt::signedRightShift takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:
The abstract operation BigInt::unsignedRightShift takes arguments x (a BigInt) and y (a BigInt) and returns a throw completion. It performs the following steps when called:
Throw a TypeError exception.
6.1.6.2.12 BigInt::lessThan ( x, y )
The abstract operation BigInt::lessThan takes arguments x (a BigInt) and y (a BigInt) and returns a Boolean. It performs the following steps when called:
If ℝ(x) < ℝ(y), return true; otherwise return false.
6.1.6.2.13 BigInt::equal ( x, y )
The abstract operation BigInt::equal takes arguments x (a BigInt) and y (a BigInt) and returns a Boolean. It performs the following steps when called:
If ℝ(x) = ℝ(y), return true; otherwise return false.
6.1.6.2.14 BinaryAnd ( x, y )
The abstract operation BinaryAnd takes arguments x (0 or 1) and y (0 or 1) and returns 0 or 1. It performs the following steps when called:
If x = 1 and y = 1, return 1.
Else, return 0.
6.1.6.2.15 BinaryOr ( x, y )
The abstract operation BinaryOr takes arguments x (0 or 1) and y (0 or 1) and returns 0 or 1. It performs the following steps when called:
If x = 1 or y = 1, return 1.
Else, return 0.
6.1.6.2.16 BinaryXor ( x, y )
The abstract operation BinaryXor takes arguments x (0 or 1) and y (0 or 1) and returns 0 or 1. It performs the following steps when called:
If x = 1 and y = 0, return 1.
Else if x = 0 and y = 1, return 1.
Else, return 0.
6.1.6.2.17 BigIntBitwiseOp ( op, x, y )
The abstract operation BigIntBitwiseOp takes arguments op (&, ^, or |), x (a BigInt), and y (a BigInt) and returns a BigInt. It performs the following steps when called:
The abstract operation BigInt::bitwiseAND takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:
The abstract operation BigInt::bitwiseXOR takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:
The abstract operation BigInt::bitwiseOR takes arguments x (a BigInt) and y (a BigInt) and returns a BigInt. It performs the following steps when called:
The abstract operation BigInt::toString takes arguments x (a BigInt) and radix (an integer in the inclusive interval from 2 to 36) and returns a String. It represents x as a String using a positional numeral system with radix radix. The digits used in the representation of a BigInt using radix r are taken from the first r code units of "0123456789abcdefghijklmnopqrstuvwxyz" in order. The representation of BigInts other than 0ℤ never includes leading zeroes. It performs the following steps when called:
Return the String value consisting of the representation of x using radix radix.
6.1.7 The Object Type
Each instance of the Object type, also referred to simply as “an Object”, represents a collection of properties. Each property is either a data property, or an accessor property:
A data property associates a key value with an ECMAScript language value and a set of Boolean attributes.
An accessor property associates a key value with one or two accessor functions, and a set of Boolean attributes. The accessor functions are used to store or retrieve an ECMAScript language value that is associated with the property.
The properties of an object are uniquely identified using property keys. A property key is either a String or a Symbol. All Strings and Symbols, including the empty String, are valid as property keys. A property name is a property key that is a String.
Property keys are used to access properties and their values. There are two kinds of access for properties: get and set, corresponding to value retrieval and assignment, respectively. The properties accessible via get and set access includes both own properties that are a direct part of an object and inherited properties which are provided by another associated object via a property inheritance relationship. Inherited properties may be either own or inherited properties of the associated object. Each own property of an object must each have a key value that is distinct from the key values of the other own properties of that object.
All objects are logically collections of properties, but there are multiple forms of objects that differ in their semantics for accessing and manipulating their properties. Please see 6.1.7.2 for definitions of the multiple forms of objects.
In addition, some objects are callable; these are referred to as functions or function objects and are described further below. All functions in ECMAScript are members of the Object type.
6.1.7.1 Property Attributes
Attributes are used in this specification to define and explain the state of Object properties as described in Table 3. Unless specified explicitly, the initial value of each attribute is its Default Value.
If the value is an Object it must be a function object. The function's [[Call]] internal method (Table 5) is called with an empty arguments list to retrieve the property value each time a get access of the property is performed.
If the value is an Object it must be a function object. The function's [[Call]] internal method (Table 5) is called with an arguments list containing the assigned value as its sole argument each time a set access of the property is performed. The effect of a property's [[Set]] internal method may, but is not required to, have an effect on the value returned by subsequent calls to the property's [[Get]] internal method.
If false, attempts to delete the property, change it from a data property to an accessor property or from an accessor property to a data property, or make any changes to its attributes (other than replacing an existing [[Value]] or setting [[Writable]] to false) will fail.
6.1.7.2 Object Internal Methods and Internal Slots
The actual semantics of objects, in ECMAScript, are specified via algorithms called internal methods. Each object in an ECMAScript engine is associated with a set of internal methods that defines its runtime behaviour. These internal methods are not part of the ECMAScript language. They are defined by this specification purely for expository purposes. However, each object within an implementation of ECMAScript must behave as specified by the internal methods associated with it. The exact manner in which this is accomplished is determined by the implementation.
Internal method names are polymorphic. This means that different object values may perform different algorithms when a common internal method name is invoked upon them. That actual object upon which an internal method is invoked is the “target” of the invocation. If, at runtime, the implementation of an algorithm attempts to use an internal method of an object that the object does not support, a TypeError exception is thrown.
Internal slots correspond to internal state that is associated with objects and used by various ECMAScript specification algorithms. Internal slots are not object properties and they are not inherited. Depending upon the specific internal slot specification, such state may consist of values of any ECMAScript language type or of specific ECMAScript specification type values. Unless explicitly specified otherwise, internal slots are allocated as part of the process of creating an object and may not be dynamically added to an object. Unless specified otherwise, the initial value of an internal slot is the value undefined. Various algorithms within this specification create objects that have internal slots. However, the ECMAScript language provides no direct way to associate internal slots with an object.
All objects have an internal slot named [[PrivateElements]], which is a List of PrivateElements. This List represents the values of the private fields, methods, and accessors for the object. Initially, it is an empty List.
Internal methods and internal slots are identified within this specification using names enclosed in double square brackets [[ ]].
Table 4 summarizes the essential internal methods used by this specification that are applicable to all objects created or manipulated by ECMAScript code. Every object must have algorithms for all of the essential internal methods. However, all objects do not necessarily use the same algorithms for those methods.
An ordinary object is an object that satisfies all of the following criteria:
For the internal methods listed in Table 4, the object uses those defined in 10.1.
If the object has a [[Call]] internal method, it uses either the one defined in 10.2.1 or the one defined in 10.3.1.
If the object has a [[Construct]] internal method, it uses either the one defined in 10.2.2 or the one defined in 10.3.2.
An exotic object is an object that is not an ordinary object.
This specification recognizes different kinds of exotic objects by those objects' internal methods. An object that is behaviourally equivalent to a particular kind of exotic object (such as an Array exotic object or a bound function exotic object), but does not have the same collection of internal methods specified for that kind, is not recognized as that kind of exotic object.
The “Signature” column of Table 4 and other similar tables describes the invocation pattern for each internal method. The invocation pattern always includes a parenthesized list of descriptive parameter names. If a parameter name is the same as an ECMAScript type name then the name describes the required type of the parameter value. If an internal method explicitly returns a value, its parameter list is followed by the symbol “→” and the type name of the returned value. The type names used in signatures refer to the types defined in clause 6 augmented by the following additional names. “any” means the value may be any ECMAScript language type.
In addition to its parameters, an internal method always has access to the object that is the target of the method invocation.
Determine the object that provides inherited properties for this object. A null value indicates that there are no inherited properties.
[[SetPrototypeOf]]
(Object | Null) → Boolean
Associate this object with another object that provides inherited properties. Passing null indicates that there are no inherited properties. Returns true indicating that the operation was completed successfully or false indicating that the operation was not successful.
[[IsExtensible]]
( ) → Boolean
Determine whether it is permitted to add additional properties to this object.
[[PreventExtensions]]
( ) → Boolean
Control whether new properties may be added to this object. Returns true if the operation was successful or false if the operation was unsuccessful.
Return a Property Descriptor for the own property of this object whose key is propertyKey, or undefined if no such property exists.
[[DefineOwnProperty]]
(propertyKey, PropertyDescriptor) → Boolean
Create or alter the own property, whose key is propertyKey, to have the state described by PropertyDescriptor. Return true if that property was successfully created/updated or false if the property could not be created or updated.
[[HasProperty]]
(propertyKey) → Boolean
Return a Boolean value indicating whether this object already has either an own or inherited property whose key is propertyKey.
[[Get]]
(propertyKey, Receiver) →any
Return the value of the property whose key is propertyKey from this object. If any ECMAScript code must be executed to retrieve the property value, Receiver is used as the this value when evaluating the code.
[[Set]]
(propertyKey, value, Receiver) → Boolean
Set the value of the property whose key is propertyKey to value. If any ECMAScript code must be executed to set the property value, Receiver is used as the this value when evaluating the code. Returns true if the property value was set or false if it could not be set.
[[Delete]]
(propertyKey) → Boolean
Remove the own property whose key is propertyKey from this object. Return false if the property was not deleted and is still present. Return true if the property was deleted or is not present.
Return a List whose elements are all of the own property keys for the object.
Table 5 summarizes additional essential internal methods that are supported by objects that may be called as functions. A function object is an object that supports the [[Call]] internal method. A constructor is an object that supports the [[Construct]] internal method. Every object that supports [[Construct]] must support [[Call]]; that is, every constructor must be a function object. Therefore, a constructor may also be referred to as a constructor function or constructorfunction object.
Table 5: Additional Essential Internal Methods of Function Objects
Executes code associated with this object. Invoked via a function call expression. The arguments to the internal method are a this value and a List whose elements are the arguments passed to the function by a call expression. Objects that implement this internal method are callable.
Creates an object. Invoked via the new operator or a super call. The first argument to the internal method is a List whose elements are the arguments of the constructor invocation or the super call. The second argument is the object to which the new operator was initially applied. Objects that implement this internal method are called constructors. A function object is not necessarily a constructor and such non-constructorfunction objects do not have a [[Construct]] internal method.
The semantics of the essential internal methods for ordinary objects and standard exotic objects are specified in clause 10. If any specified use of an internal method of an exotic object is not supported by an implementation, that usage must throw a TypeError exception when attempted.
6.1.7.3 Invariants of the Essential Internal Methods
The Internal Methods of Objects of an ECMAScript engine must conform to the list of invariants specified below. Ordinary ECMAScript Objects as well as all standard exotic objects in this specification maintain these invariants. ECMAScript Proxy objects maintain these invariants by means of runtime checks on the result of traps invoked on the [[ProxyHandler]] object.
Any implementation provided exotic objects must also maintain these invariants for those objects. Violation of these invariants may cause ECMAScript code to have unpredictable behaviour and create security issues. However, violation of these invariants must never compromise the memory safety of an implementation.
An implementation must not allow these invariants to be circumvented in any manner such as by providing alternative interfaces that implement the functionality of the essential internal methods without enforcing their invariants.
Definitions:
The target of an internal method is the object upon which the internal method is called.
A target is non-extensible if it has been observed to return false from its [[IsExtensible]] internal method, or true from its [[PreventExtensions]] internal method.
A non-existent property is a property that does not exist as an own property on a non-extensible target.
All references to SameValue are according to the definition of the SameValue algorithm.
Return value:
The value returned by any internal method must be a Completion Record with either:
[[Type]] = normal, [[Target]] = empty, and [[Value]] = a value of the "normal return type" shown below for that internal method, or
If target is non-extensible, and [[GetPrototypeOf]] returns a value V, then any future calls to [[GetPrototypeOf]] should return the SameValue as V.
Note 2
An object's prototype chain should have finite length (that is, starting from any object, recursively applying the [[GetPrototypeOf]] internal method to its result should eventually lead to the value null). However, this requirement is not enforceable as an object level invariant if the prototype chain includes any exotic objects that do not use the ordinary object definition of [[GetPrototypeOf]]. Such a circular prototype chain may result in infinite loops when accessing object properties.
[[SetPrototypeOf]] ( V )
The normal return type is Boolean.
If target is non-extensible, [[SetPrototypeOf]] must return false, unless V is the SameValue as the target's observed [[GetPrototypeOf]] value.
[[IsExtensible]] ( )
The normal return type is Boolean.
If [[IsExtensible]] returns false, all future calls to [[IsExtensible]] on the target must return false.
[[PreventExtensions]] ( )
The normal return type is Boolean.
If [[PreventExtensions]] returns true, all future calls to [[IsExtensible]] on the target must return false and the target is now considered non-extensible.
If P is described as a non-configurable, non-writable own data property, all future calls to [[GetOwnProperty]] ( P ) must return Property Descriptor whose [[Value]] is SameValue as P's [[Value]] attribute.
If P's attributes other than [[Writable]] and [[Value]] may change over time, or if the property might be deleted, then P's [[Configurable]] attribute must be true.
If the [[Writable]] attribute may change from false to true, then the [[Configurable]] attribute must be true.
If the target is non-extensible and P is non-existent, then all future calls to [[GetOwnProperty]] (P) on the target must describe P as non-existent (i.e. [[GetOwnProperty]] (P) must return undefined).
Note 3
As a consequence of the third invariant, if a property is described as a data property and it may return different values over time, then either or both of the [[Writable]] and [[Configurable]] attributes must be true even if no mechanism to change the value is exposed via the other essential internal methods.
[[DefineOwnProperty]] ( P, Desc )
The normal return type is Boolean.
[[DefineOwnProperty]] must return false if P has previously been observed as a non-configurable own property of the target, unless either:
All attributes of Desc are the SameValue as P's attributes.
[[DefineOwnProperty]] (P, Desc) must return false if target is non-extensible and P is a non-existent own property. That is, a non-extensible target object cannot be extended with new properties.
[[HasProperty]] ( P )
The normal return type is Boolean.
If P was previously observed as a non-configurable own data or accessor property of the target, [[HasProperty]] must return true.
If P was previously observed as a non-configurable, non-writable own data property of the target with value V, then [[Get]] must return the SameValue as V.
If P was previously observed as a non-configurable own accessor property of the target whose [[Get]] attribute is undefined, the [[Get]] operation must return undefined.
[[Set]] ( P, V, Receiver )
The normal return type is Boolean.
If P was previously observed as a non-configurable, non-writable own data property of the target, then [[Set]] must return false unless V is the SameValue as P's [[Value]] attribute.
If P was previously observed as a non-configurable own accessor property of the target whose [[Set]] attribute is undefined, the [[Set]] operation must return false.
[[Delete]] ( P )
The normal return type is Boolean.
If P was previously observed as a non-configurable own data or accessor property of the target, [[Delete]] must return false.
The returned List must not contain any duplicate entries.
The Type of each element of the returned List is either String or Symbol.
The returned List must contain at least the keys of all non-configurable own properties that have previously been observed.
If the target is non-extensible, the returned List must contain only the keys of all own properties of the target that are observable using [[GetOwnProperty]].
The target must also have a [[Call]] internal method.
6.1.7.4 Well-Known Intrinsic Objects
Well-known intrinsics are built-in objects that are explicitly referenced by the algorithms of this specification and which usually have realm-specific identities. Unless otherwise specified each intrinsic object actually corresponds to a set of similar objects, one per realm.
Within this specification a reference such as %name% means the intrinsic object, associated with the current realm, corresponding to the name. A reference such as %name.a.b% means, as if the "b" property of the value of the "a" property of the intrinsic object %name% was accessed prior to any ECMAScript code being evaluated. Determination of the current realm and its intrinsics is described in 9.4. The well-known intrinsics are listed in Table 6.
A specification type corresponds to meta-values that are used within algorithms to describe the semantics of ECMAScript language constructs and ECMAScript language types. The specification types include Reference, List, Completion Record, Property Descriptor, Environment Record, Abstract Closure, and Data Block. Specification type values are specification artefacts that do not necessarily correspond to any specific entity within an ECMAScript implementation. Specification type values may be used to describe intermediate results of ECMAScript expression evaluation but such values cannot be stored as properties of objects or values of ECMAScript language variables.
6.2.1 The Enum Specification Type
Enums are values which are internal to the specification and not directly observable from ECMAScript code. Enums are denoted using a sans-serif typeface. For instance, a Completion Record's [[Type]] field takes on values like normal, return, or throw. Enums have no characteristics other than their name. The name of an enum serves no purpose other than to distinguish it from other enums, and implies nothing about its usage or meaning in context.
6.2.2 The List and Record Specification Types
The List type is used to explain the evaluation of argument lists (see 13.3.8) in new expressions, in function calls, and in other algorithms where a simple ordered list of values is needed. Values of the List type are simply ordered sequences of list elements containing the individual values. These sequences may be of any length. The elements of a list may be randomly accessed using 0-origin indices. For notational convenience an array-like syntax can be used to access List elements. For example, arguments[2] is shorthand for saying the 3rd element of the List arguments.
When an algorithm iterates over the elements of a List without specifying an order, the order used is the order of the elements in the List.
For notational convenience within this specification, a literal syntax can be used to express a new List value. For example, « 1, 2 » defines a List value that has two elements each of which is initialized to a specific value. A new empty List can be expressed as « ».
In this specification, the phrase "the list-concatenation of A, B, ..." (where each argument is a possibly empty List) denotes a new List value whose elements are the concatenation of the elements (in order) of each of the arguments (in order).
As applied to a List of Strings, the phrase "sorted according to lexicographic code unit order" means sorting by the numeric value of each code unit up to the length of the shorter string, and sorting the shorter string before the longer string if all are equal, as described in the abstract operation IsLessThan.
The Record type is used to describe data aggregations within the algorithms of this specification. A Record type value consists of one or more named fields. The value of each field is an ECMAScript language value or specification value. Field names are always enclosed in double brackets, for example [[Value]].
For notational convenience within this specification, an object literal-like syntax can be used to express a Record value. For example, { [[Field1]]: 42, [[Field2]]: false, [[Field3]]: empty } defines a Record value that has three fields, each of which is initialized to a specific value. Field name order is not significant. Any fields that are not explicitly listed are considered to be absent.
In specification text and algorithms, dot notation may be used to refer to a specific field of a Record value. For example, if R is the record shown in the previous paragraph then R.[[Field2]] is shorthand for “the field of R named [[Field2]]”.
Schema for commonly used Record field combinations may be named, and that name may be used as a prefix to a literal Record value to identify the specific kind of aggregations that is being described. For example: PropertyDescriptor { [[Value]]: 42, [[Writable]]: false, [[Configurable]]: true }.
6.2.3 The Set and Relation Specification Types
The Set type is used to explain a collection of unordered elements for use in the memory model. It is distinct from the ECMAScript collection type of the same name. To disambiguate, instances of the ECMAScript collection are consistently referred to as "Set objects" within this specification. Values of the Set type are simple collections of elements, where no element appears more than once. Elements may be added to and removed from Sets. Sets may be unioned, intersected, or subtracted from each other.
The Relation type is used to explain constraints on Sets. Values of the Relation type are Sets of ordered pairs of values from its value domain. For example, a Relation on events is a set of ordered pairs of events. For a Relation R and two values a and b in the value domain of R, aRb is shorthand for saying the ordered pair (a, b) is a member of R. A Relation is the least Relation with respect to some conditions when it is the smallest Relation that satisfies those conditions.
A strict partial order is a Relation value R that satisfies the following.
For all a, b, and c in R's domain:
It is not the case that aRa, and
If aRb and bRc, then aRc.
Note 1
The two properties above are called irreflexivity and transitivity, respectively.
A strict total order is a Relation value R that satisfies the following.
For all a, b, and c in R's domain:
a is b or aRb or bRa, and
It is not the case that aRa, and
If aRb and bRc, then aRc.
Note 2
The three properties above are called totality, irreflexivity, and transitivity, respectively.
6.2.4 The Completion Record Specification Type
The Completion Record specification type is used to explain the runtime propagation of values and control flow such as the behaviour of statements (break, continue, return and throw) that perform nonlocal transfers of control.
Completion Records have the fields defined in Table 7.
The following shorthand terms are sometimes used to refer to Completion Records.
normal completion refers to any Completion Record with a [[Type]] value of normal.
break completion refers to any Completion Record with a [[Type]] value of break.
continue completion refers to any Completion Record with a [[Type]] value of continue.
return completion refers to any Completion Record with a [[Type]] value of return.
throw completion refers to any Completion Record with a [[Type]] value of throw.
abrupt completion refers to any Completion Record with a [[Type]] value other than normal.
a normal completion containing some type of value refers to a normal completion that has a value of that type in its [[Value]] field.
Callable objects that are defined in this specification only return a normal completion or a throw completion. Returning any other kind of Completion Record is considered an editorial error.
Implementation-defined callable objects must return either a normal completion or a throw completion.
6.2.4.1 NormalCompletion ( value )
The abstract operation NormalCompletion takes argument value (any value except a Completion Record) and returns a normal completion. It performs the following steps when called:
The abstract operation ThrowCompletion takes argument value (an ECMAScript language value) and returns a throw completion. It performs the following steps when called:
The abstract operation UpdateEmpty takes arguments completionRecord (a Completion Record) and value (any value except a Completion Record) and returns a Completion Record. It performs the following steps when called:
The Reference Record type is used to explain the behaviour of such operators as delete, typeof, the assignment operators, the superkeyword and other language features. For example, the left-hand operand of an assignment is expected to produce a Reference Record.
A Reference Record is a resolved name or (possibly not-yet-resolved) property binding; its fields are defined by Table 8.
If not empty, the Reference Record represents a property binding that was expressed using the superkeyword; it is called a Super Reference Record and its [[Base]] value will never be an Environment Record. In that case, the [[ThisValue]] field holds the this value at the time the Reference Record was created.
The following abstract operations are used in this specification to operate upon Reference Records:
6.2.5.1 IsPropertyReference ( V )
The abstract operation IsPropertyReference takes argument V (a Reference Record) and returns a Boolean. It performs the following steps when called:
If V.[[Base]] is unresolvable, return false.
If V.[[Base]] is an Environment Record, return false; otherwise return true.
6.2.5.2 IsUnresolvableReference ( V )
The abstract operation IsUnresolvableReference takes argument V (a Reference Record) and returns a Boolean. It performs the following steps when called:
If V.[[Base]] is unresolvable, return true; otherwise return false.
6.2.5.3 IsSuperReference ( V )
The abstract operation IsSuperReference takes argument V (a Reference Record) and returns a Boolean. It performs the following steps when called:
If V.[[ThisValue]] is not empty, return true; otherwise return false.
6.2.5.4 IsPrivateReference ( V )
The abstract operation IsPrivateReference takes argument V (a Reference Record) and returns a Boolean. It performs the following steps when called:
If V.[[ReferencedName]] is a Private Name, return true; otherwise return false.
Return ? base.GetBindingValue(V.[[ReferencedName]], V.[[Strict]]) (see 9.1).
Note
The object that may be created in step 3.a is not accessible outside of the above abstract operation and the ordinary object[[Get]] internal method. An implementation might choose to avoid the actual creation of the object.
Return ? base.SetMutableBinding(V.[[ReferencedName]], W, V.[[Strict]]) (see 9.1).
Note
The object that may be created in step 3.a is not accessible outside of the above abstract operation and the ordinary object[[Set]] internal method. An implementation might choose to avoid the actual creation of that object.
The abstract operation MakePrivateReference takes arguments baseValue (an ECMAScript language value) and privateIdentifier (a String) and returns a Reference Record. It performs the following steps when called:
Return the Reference Record { [[Base]]: baseValue, [[ReferencedName]]: privateName, [[Strict]]: true, [[ThisValue]]: empty }.
6.2.6 The Property Descriptor Specification Type
The Property Descriptor type is used to explain the manipulation and reification of Object property attributes. A Property Descriptor is a Record with zero or more fields, where each field's name is an attribute name and its value is a corresponding attribute value as specified in 6.1.7.1. The schema name used within this specification to tag literal descriptions of Property Descriptor records is “PropertyDescriptor”.
Property Descriptor values may be further classified as data Property Descriptors and accessor Property Descriptors based upon the existence or use of certain fields. A data Property Descriptor is one that includes any fields named either [[Value]] or [[Writable]]. An accessor Property Descriptor is one that includes any fields named either [[Get]] or [[Set]]. Any Property Descriptor may have fields named [[Enumerable]] and [[Configurable]]. A Property Descriptor value may not be both a data Property Descriptor and an accessor Property Descriptor; however, it may be neither (in which case it is a generic Property Descriptor). A fully populated Property Descriptor is one that is either an accessor Property Descriptor or a data Property Descriptor and that has all of the corresponding fields defined in Table 3.
The following abstract operations are used in this specification to operate upon Property Descriptor values:
6.2.6.1 IsAccessorDescriptor ( Desc )
The abstract operation IsAccessorDescriptor takes argument Desc (a Property Descriptor or undefined) and returns a Boolean. It performs the following steps when called:
If Desc is undefined, return false.
If Desc has a [[Get]] field, return true.
If Desc has a [[Set]] field, return true.
Return false.
6.2.6.2 IsDataDescriptor ( Desc )
The abstract operation IsDataDescriptor takes argument Desc (a Property Descriptor or undefined) and returns a Boolean. It performs the following steps when called:
If Desc is undefined, return false.
If Desc has a [[Value]] field, return true.
If Desc has a [[Writable]] field, return true.
Return false.
6.2.6.3 IsGenericDescriptor ( Desc )
The abstract operation IsGenericDescriptor takes argument Desc (a Property Descriptor or undefined) and returns a Boolean. It performs the following steps when called:
The abstract operation FromPropertyDescriptor takes argument Desc (a Property Descriptor or undefined) and returns an Object or undefined. It performs the following steps when called:
If IsCallable(setter) is false and setter is not undefined, throw a TypeError exception.
Set desc.[[Set]] to setter.
If desc has a [[Get]] field or desc has a [[Set]] field, then
If desc has a [[Value]] field or desc has a [[Writable]] field, throw a TypeError exception.
Return desc.
6.2.6.6 CompletePropertyDescriptor ( Desc )
The abstract operation CompletePropertyDescriptor takes argument Desc (a Property Descriptor) and returns unused. It performs the following steps when called:
Let like be the Record { [[Value]]: undefined, [[Writable]]: false, [[Get]]: undefined, [[Set]]: undefined, [[Enumerable]]: false, [[Configurable]]: false }.
If Desc does not have a [[Value]] field, set Desc.[[Value]] to like.[[Value]].
If Desc does not have a [[Writable]] field, set Desc.[[Writable]] to like.[[Writable]].
Else,
If Desc does not have a [[Get]] field, set Desc.[[Get]] to like.[[Get]].
If Desc does not have a [[Set]] field, set Desc.[[Set]] to like.[[Set]].
If Desc does not have an [[Enumerable]] field, set Desc.[[Enumerable]] to like.[[Enumerable]].
If Desc does not have a [[Configurable]] field, set Desc.[[Configurable]] to like.[[Configurable]].
Return unused.
6.2.7 The Environment Record Specification Type
The Environment Record type is used to explain the behaviour of name resolution in nested functions and blocks. This type and the operations upon it are defined in 9.1.
6.2.8 The Abstract Closure Specification Type
The Abstract Closure specification type is used to refer to algorithm steps together with a collection of values. Abstract Closures are meta-values and are invoked using function application style such as closure(arg1, arg2). Like abstract operations, invocations perform the algorithm steps described by the Abstract Closure.
In algorithm steps that create an Abstract Closure, values are captured with the verb "capture" followed by a list of aliases. When an Abstract Closure is created, it captures the value that is associated with each alias at that time. In steps that specify the algorithm to be performed when an Abstract Closure is called, each captured value is referred to by the alias that was used to capture the value.
The Data Block specification type is used to describe a distinct and mutable sequence of byte-sized (8 bit) numeric values. A byte value is an integer in the inclusive interval from 0 to 255. A Data Block value is created with a fixed number of bytes that each have the initial value 0.
For notational convenience within this specification, an array-like syntax can be used to access the individual bytes of a Data Block value. This notation presents a Data Block value as a 0-based integer-indexed sequence of bytes. For example, if db is a 5 byte Data Block value then db[2] can be used to access its 3rd byte.
A data block that resides in memory that can be referenced from multiple agents concurrently is designated a Shared Data Block. A Shared Data Block has an identity (for the purposes of equality testing Shared Data Block values) that is address-free: it is tied not to the virtual addresses the block is mapped to in any process, but to the set of locations in memory that the block represents. Two data blocks are equal only if the sets of the locations they contain are equal; otherwise, they are not equal and the intersection of the sets of locations they contain is empty. Finally, Shared Data Blocks can be distinguished from Data Blocks.
Let db be a new Shared Data Block value consisting of size bytes. If it is impossible to create such a Shared Data Block, throw a RangeError exception.
The abstract operation CopyDataBlockBytes takes arguments toBlock (a Data Block or a Shared Data Block), toIndex (a non-negative integer), fromBlock (a Data Block or a Shared Data Block), fromIndex (a non-negative integer), and count (a non-negative integer) and returns unused. It performs the following steps when called:
Assert: fromBlock and toBlock are distinct values.
Let bytes be a List whose sole element is a nondeterministically chosen byte value.
NOTE: In implementations, bytes is the result of a non-atomic read instruction on the underlying hardware. The nondeterminism is a semantic prescription of the memory model to describe observable behaviour of hardware with weak consistency.
Let readEvent be ReadSharedMemory { [[Order]]: unordered, [[NoTear]]: true, [[Block]]: fromBlock, [[ByteIndex]]: fromIndex, [[ElementSize]]: 1 }.
Append readEvent to eventsRecord.[[EventList]].
Append Chosen Value Record { [[Event]]: readEvent, [[ChosenValue]]: bytes } to execution.[[ChosenValues]].
The PrivateElement type is a Record used in the specification of private class fields, methods, and accessors. Although Property Descriptors are not used for private elements, private fields behave similarly to non-configurable, non-enumerable, writable data properties, private methods behave similarly to non-configurable, non-enumerable, non-writable data properties, and private accessors behave similarly to non-configurable, non-enumerable accessor properties.
Values of the PrivateElement type are Record values whose fields are defined by Table 9. Such values are referred to as PrivateElements.
6.2.11 The ClassFieldDefinition Record Specification Type
The ClassFieldDefinition type is a Record used in the specification of class fields.
Values of the ClassFieldDefinition type are Record values whose fields are defined by Table 10. Such values are referred to as ClassFieldDefinition Records.
The Private Name specification type is used to describe a globally unique value (one which differs from any other Private Name, even if they are otherwise indistinguishable) which represents the key of a private class element (field, method, or accessor). Each Private Name has an associated immutable [[Description]] which is a String value. A Private Name may be installed on any ECMAScript object with PrivateFieldAdd or PrivateMethodOrAccessorAdd, and then read or written using PrivateGet and PrivateSet.
6.2.13 The ClassStaticBlockDefinition Record Specification Type
A ClassStaticBlockDefinition Record is a Record value used to encapsulate the executable code for a class static initialization block.
ClassStaticBlockDefinition Records have the fields listed in Table 11.
The function object to be called during static initialization of a class.
7 Abstract Operations
These operations are not a part of the ECMAScript language; they are defined here solely to aid the specification of the semantics of the ECMAScript language. Other, more specialized abstract operations are defined throughout this specification.
7.1 Type Conversion
The ECMAScript language implicitly performs automatic type conversion as needed. To clarify the semantics of certain constructs it is useful to define a set of conversion abstract operations. The conversion abstract operations are polymorphic; they can accept a value of any ECMAScript language type. But no other specification types are used with these operations.
The BigInt type has no implicit conversions in the ECMAScript language; programmers must call BigInt explicitly to convert values from other types.
7.1.1 ToPrimitive ( input [ , preferredType ] )
The abstract operation ToPrimitive takes argument input (an ECMAScript language value) and optional argument preferredType (string or number) and returns either a normal completion containing an ECMAScript language value or a throw completion. It converts its input argument to a non-Object type. If an object is capable of converting to more than one primitive type, it may use the optional hint preferredType to favour that type. It performs the following steps when called:
When ToPrimitive is called without a hint, then it generally behaves as if the hint were number. However, objects may over-ride this behaviour by defining a %Symbol.toPrimitive% method. Of the objects defined in this specification only Dates (see 21.4.4.45) and Symbol objects (see 20.4.3.5) over-ride the default ToPrimitive behaviour. Dates treat the absence of a hint as if the hint were string.
The abstract operation ToBoolean takes argument argument (an ECMAScript language value) and returns a Boolean. It converts argument to a value of type Boolean. It performs the following steps when called:
The abstract operation ToNumeric takes argument value (an ECMAScript language value) and returns either a normal completion containing either a Number or a BigInt, or a throw completion. It returns value converted to a Number or a BigInt. It performs the following steps when called:
The abstract operation RoundMVResult takes argument n (a mathematical value) and returns a Number. It converts n to a Number in an implementation-defined manner. For the purposes of this abstract operation, a digit is significant if it is not zero or there is a non-zero digit to its left and there is a non-zero digit to its right. For the purposes of this abstract operation, "the mathematical value denoted by" a representation of a mathematical value is the inverse of "the decimal representation of" a mathematical value. It performs the following steps when called:
If the decimal representation of n has 20 or fewer significant digits, return 𝔽(n).
Let option1 be the mathematical value denoted by the result of replacing each significant digit in the decimal representation of n after the 20th with a 0 digit.
Let option2 be the mathematical value denoted by the result of replacing each significant digit in the decimal representation of n after the 20th with a 0 digit and then incrementing it at the 20th position (with carrying as necessary).
The abstract operation ToIntegerOrInfinity takes argument argument (an ECMAScript language value) and returns either a normal completion containing either an integer, +∞, or -∞, or a throw completion. It converts argument to an integer representing its Number value with fractional part truncated, or to +∞ or -∞ when that Number value is infinite. It performs the following steps when called:
𝔽(ToIntegerOrInfinity(x)) never returns -0𝔽 for any value of x. The truncation of the fractional part is performed after converting x to a mathematical value.
Step 5 is the only difference between ToUint32 and ToInt32.
The ToUint32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged.
ToUint32(ToInt32(x)) is the same value as ToUint32(x) for all values of x. (It is to preserve this latter property that +∞𝔽 and -∞𝔽 are mapped to +0𝔽.)
If f is even, return 𝔽(f). Otherwise, return 𝔽(f + 1).
Note
Unlike most other ECMAScript integer conversion operations, ToUint8Clamp rounds rather than truncates non-integral values. It also uses “round half to even” tie-breaking, which differs from the “round half up” tie-breaking of Math.round.
7.1.13 ToBigInt ( argument )
The abstract operation ToBigInt takes argument argument (an ECMAScript language value) and returns either a normal completion containing a BigInt or a throw completion. It converts argument to a BigInt value, or throws if an implicit conversion from Number would be required. It performs the following steps when called:
The abstract operation CanonicalNumericIndexString takes argument argument (a String) and returns a Number or undefined. If argument is either "-0" or exactly matches ToString(n) for some Number value n, it returns the respective Number value. Otherwise, it returns undefined. It performs the following steps when called:
The abstract operation IsCallable takes argument argument (an ECMAScript language value) and returns a Boolean. It determines if argument is a callable function with a [[Call]] internal method. It performs the following steps when called:
If argument has a [[Call]] internal method, return true.
Return false.
7.2.4 IsConstructor ( argument )
The abstract operation IsConstructor takes argument argument (an ECMAScript language value) and returns a Boolean. It determines if argument is a function object with a [[Construct]] internal method. It performs the following steps when called:
If argument has a [[Construct]] internal method, return true.
Return false.
7.2.5 IsExtensible ( O )
The abstract operation IsExtensible takes argument O (an Object) and returns either a normal completion containing a Boolean or a throw completion. It is used to determine whether additional properties can be added to O. It performs the following steps when called:
The abstract operation IsStringWellFormedUnicode takes argument string (a String) and returns a Boolean. It interprets string as a sequence of UTF-16 encoded code points, as described in 6.1.4, and determines whether it is a well formed UTF-16 sequence. It performs the following steps when called:
If cp.[[IsUnpairedSurrogate]] is true, return false.
Set k to k + cp.[[CodeUnitCount]].
Return true.
7.2.8 SameValue ( x, y )
The abstract operation SameValue takes arguments x (an ECMAScript language value) and y (an ECMAScript language value) and returns a Boolean. It determines whether or not the two arguments are the same value. It performs the following steps when called:
This algorithm differs from the IsStrictlyEqual Algorithm by treating all NaN values as equivalent and by differentiating +0𝔽 from -0𝔽.
7.2.9 SameValueZero ( x, y )
The abstract operation SameValueZero takes arguments x (an ECMAScript language value) and y (an ECMAScript language value) and returns a Boolean. It determines whether or not the two arguments are the same value (ignoring the difference between +0𝔽 and -0𝔽). It performs the following steps when called:
SameValueZero differs from SameValue only in that it treats +0𝔽 and -0𝔽 as equivalent.
7.2.10 SameValueNonNumber ( x, y )
The abstract operation SameValueNonNumber takes arguments x (an ECMAScript language value, but not a Number) and y (an ECMAScript language value, but not a Number) and returns a Boolean. It performs the following steps when called:
For expository purposes, some cases are handled separately within this algorithm even if it is unnecessary to do so.
Note 2
The specifics of what "x is y" means are detailed in 5.2.7.
7.2.11 IsLessThan ( x, y, LeftFirst )
The abstract operation IsLessThan takes arguments x (an ECMAScript language value), y (an ECMAScript language value), and LeftFirst (a Boolean) and returns either a normal completion containing either a Boolean or undefined, or a throw completion. It provides the semantics for the comparison x < y, returning true, false, or undefined (which indicates that at least one operand is NaN). The LeftFirst flag is used to control the order in which operations with potentially visible side-effects are performed upon x and y. It is necessary because ECMAScript specifies left to right evaluation of expressions. If LeftFirst is true, the x parameter corresponds to an expression that occurs to the left of the y parameter's corresponding expression. If LeftFirst is false, the reverse is the case and operations must be performed upon y before x. It performs the following steps when called:
If ℝ(nx) < ℝ(ny), return true; otherwise return false.
Note 1
Step 3 differs from step 1.c in the algorithm that handles the addition operator + (13.15.3) by using the logical-and operation instead of the logical-or operation.
Note 2
The comparison of Strings uses a simple lexicographic ordering on sequences of UTF-16 code unit values. There is no attempt to use the more complex, semantically oriented definitions of character or string equality and collating order defined in the Unicode specification. Therefore String values that are canonically equal according to the Unicode Standard but not in the same normalization form could test as unequal. Also note that lexicographic ordering by code unit differs from ordering by code point for Strings containing surrogate pairs.
If x is not finite or y is not finite, return false.
If ℝ(x) = ℝ(y), return true; otherwise return false.
Return false.
7.2.13 IsStrictlyEqual ( x, y )
The abstract operation IsStrictlyEqual takes arguments x (an ECMAScript language value) and y (an ECMAScript language value) and returns a Boolean. It provides the semantics for the === operator. It performs the following steps when called:
This algorithm differs from the SameValue Algorithm in its treatment of signed zeroes and NaNs.
7.3 Operations on Objects
7.3.1 MakeBasicObject ( internalSlotsList )
The abstract operation MakeBasicObject takes argument internalSlotsList (a List of internal slot names) and returns an Object. It is the source of all ECMAScript objects that are created algorithmically, including both ordinary objects and exotic objects. It factors out common steps used in creating all objects, and centralizes object creation. It performs the following steps when called:
Set internalSlotsList to the list-concatenation of internalSlotsList and « [[PrivateElements]] ».
Let obj be a newly created object with an internal slot for each name in internalSlotsList.
Set obj's essential internal methods to the default ordinary object definitions specified in 10.1.
Assert: If the caller will not be overriding both obj's [[GetPrototypeOf]] and [[SetPrototypeOf]] essential internal methods, then internalSlotsList contains [[Prototype]].
Assert: If the caller will not be overriding all of obj's [[SetPrototypeOf]], [[IsExtensible]], and [[PreventExtensions]] essential internal methods, then internalSlotsList contains [[Extensible]].
If internalSlotsList contains [[Extensible]], set obj.[[Extensible]] to true.
Return obj.
Note
Within this specification, exotic objects are created in abstract operations such as ArrayCreate and BoundFunctionCreate by first calling MakeBasicObject to obtain a basic, foundational object, and then overriding some or all of that object's internal methods. In order to encapsulate exotic object creation, the object's essential internal methods are never modified outside those operations.
The abstract operation Set takes arguments O (an Object), P (a property key), V (an ECMAScript language value), and Throw (a Boolean) and returns either a normal completion containingunused or a throw completion. It is used to set the value of a specific property of an object. V is the new value for the property. It performs the following steps when called:
Let success be ? O.[[Set]](P, V, O).
If success is false and Throw is true, throw a TypeError exception.
Let newDesc be the PropertyDescriptor { [[Value]]: V, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true }.
Return ? O.[[DefineOwnProperty]](P, newDesc).
Note
This abstract operation creates a property whose attributes are set to the same defaults used for properties created by the ECMAScript language assignment operator. Normally, the property will not already exist. If it does exist and is not configurable or if O is not extensible, [[DefineOwnProperty]] will return false.
7.3.6 CreateDataPropertyOrThrow ( O, P, V )
The abstract operation CreateDataPropertyOrThrow takes arguments O (an Object), P (a property key), and V (an ECMAScript language value) and returns either a normal completion containingunused or a throw completion. It is used to create a new own property of an object. It throws a TypeError exception if the requested property update cannot be performed. It performs the following steps when called:
This abstract operation creates a property whose attributes are set to the same defaults used for properties created by the ECMAScript language assignment operator. Normally, the property will not already exist. If it does exist and is not configurable or if O is not extensible, [[DefineOwnProperty]] will return false causing this operation to throw a TypeError exception.
7.3.7 CreateNonEnumerableDataPropertyOrThrow ( O, P, V )
The abstract operation CreateNonEnumerableDataPropertyOrThrow takes arguments O (an Object), P (a property key), and V (an ECMAScript language value) and returns unused. It is used to create a new non-enumerable own property of an ordinary object. It performs the following steps when called:
Assert: O is an ordinary, extensible object with no non-configurable properties.
Let newDesc be the PropertyDescriptor { [[Value]]: V, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }.
This abstract operation creates a property whose attributes are set to the same defaults used for properties created by the ECMAScript language assignment operator except it is not enumerable. Normally, the property will not already exist. If it does exist, DefinePropertyOrThrow is guaranteed to complete normally.
7.3.8 DefinePropertyOrThrow ( O, P, desc )
The abstract operation DefinePropertyOrThrow takes arguments O (an Object), P (a property key), and desc (a Property Descriptor) and returns either a normal completion containingunused or a throw completion. It is used to call the [[DefineOwnProperty]] internal method of an object in a manner that will throw a TypeError exception if the requested property update cannot be performed. It performs the following steps when called:
Let success be ? O.[[DefineOwnProperty]](P, desc).
If success is false, throw a TypeError exception.
Return unused.
7.3.9 DeletePropertyOrThrow ( O, P )
The abstract operation DeletePropertyOrThrow takes arguments O (an Object) and P (a property key) and returns either a normal completion containingunused or a throw completion. It is used to remove a specific own property of an object. It throws an exception if the property is not configurable. It performs the following steps when called:
If func is either undefined or null, return undefined.
If IsCallable(func) is false, throw a TypeError exception.
Return func.
7.3.11 HasProperty ( O, P )
The abstract operation HasProperty takes arguments O (an Object) and P (a property key) and returns either a normal completion containing a Boolean or a throw completion. It is used to determine whether an object has a property with the specified property key. The property may be either own or inherited. It performs the following steps when called:
Return ? O.[[HasProperty]](P).
7.3.12 HasOwnProperty ( O, P )
The abstract operation HasOwnProperty takes arguments O (an Object) and P (a property key) and returns either a normal completion containing a Boolean or a throw completion. It is used to determine whether an object has an own property with the specified property key. It performs the following steps when called:
The abstract operation Construct takes argument F (a constructor) and optional arguments argumentsList (a List of ECMAScript language values) and newTarget (a constructor) and returns either a normal completion containing an Object or a throw completion. It is used to call the [[Construct]] internal method of a function object. argumentsList and newTarget are the values to be passed as the corresponding arguments of the internal method. If argumentsList is not present, a new empty List is used as its value. If newTarget is not present, F is used as its value. It performs the following steps when called:
If newTarget is not present, set newTarget to F.
If argumentsList is not present, set argumentsList to a new empty List.
If newTarget is not present, this operation is equivalent to: new F(...argumentsList)
7.3.15 SetIntegrityLevel ( O, level )
The abstract operation SetIntegrityLevel takes arguments O (an Object) and level (sealed or frozen) and returns either a normal completion containing a Boolean or a throw completion. It is used to fix the set of own properties of an object. It performs the following steps when called:
The abstract operation TestIntegrityLevel takes arguments O (an Object) and level (sealed or frozen) and returns either a normal completion containing a Boolean or a throw completion. It is used to determine if the set of own properties of an object are fixed. It performs the following steps when called:
NOTE: If the object is extensible, none of its properties are examined.
Let keys be ? O.[[OwnPropertyKeys]]().
For each element k of keys, do
Let currentDesc be ? O.[[GetOwnProperty]](k).
If currentDesc is not undefined, then
If currentDesc.[[Configurable]] is true, return false.
If level is frozen and IsDataDescriptor(currentDesc) is true, then
If currentDesc.[[Writable]] is true, return false.
Return true.
7.3.17 CreateArrayFromList ( elements )
The abstract operation CreateArrayFromList takes argument elements (a List of ECMAScript language values) and returns an Array. It is used to create an Array whose elements are provided by elements. It performs the following steps when called:
The abstract operation LengthOfArrayLike takes argument obj (an Object) and returns either a normal completion containing a non-negative integer or a throw completion. It returns the value of the "length" property of an array-like object. It performs the following steps when called:
The abstract operation CreateListFromArrayLike takes argument obj (an ECMAScript language value) and optional argument elementTypes (a List of names of ECMAScript Language Types) and returns either a normal completion containing a List of ECMAScript language values or a throw completion. It is used to create a List value whose elements are provided by the indexed properties of obj. elementTypes contains the names of ECMAScript Language Types that are allowed for element values of the List that is created. It performs the following steps when called:
If elementTypes is not present, set elementTypes to « Undefined, Null, Boolean, String, Symbol, Number, BigInt, Object ».
The abstract operation OrdinaryHasInstance takes arguments C (an ECMAScript language value) and O (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It implements the default algorithm for determining if O inherits from the instance object inheritance path provided by C. It performs the following steps when called:
7.3.22 SpeciesConstructor ( O, defaultConstructor )
The abstract operation SpeciesConstructor takes arguments O (an Object) and defaultConstructor (a constructor) and returns either a normal completion containing a constructor or a throw completion. It is used to retrieve the constructor that should be used to create new objects that are derived from O. defaultConstructor is the constructor to use if a constructor%Symbol.species% property cannot be found starting from O. It performs the following steps when called:
The target passed in here is always a newly created object which is not directly accessible in case of an error being thrown.
7.3.26 PrivateElementFind ( O, P )
The abstract operation PrivateElementFind takes arguments O (an Object) and P (a Private Name) and returns a PrivateElement or empty. It performs the following steps when called:
If O.[[PrivateElements]] contains a PrivateElementpe such that pe.[[Key]] is P, then
If entry is not empty, throw a TypeError exception.
Append PrivateElement { [[Key]]: P, [[Kind]]: field, [[Value]]: value } to O.[[PrivateElements]].
Return unused.
7.3.28 PrivateMethodOrAccessorAdd ( O, method )
The abstract operation PrivateMethodOrAccessorAdd takes arguments O (an Object) and method (a PrivateElement) and returns either a normal completion containingunused or a throw completion. It performs the following steps when called:
Assert: method.[[Kind]] is either method or accessor.
7.3.33 InitializeInstanceElements ( O, constructor )
The abstract operation InitializeInstanceElements takes arguments O (an Object) and constructor (an ECMAScript function object) and returns either a normal completion containingunused or a throw completion. It performs the following steps when called:
Let methods be the value of constructor.[[PrivateMethods]].
The abstract operation IteratorComplete takes argument iteratorResult (an Object) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:
The abstract operation IteratorStep takes argument iteratorRecord (an Iterator Record) and returns either a normal completion containing either an Object or done, or a throw completion. It requests the next value from iteratorRecord.[[Iterator]] by calling iteratorRecord.[[NextMethod]] and returns either done indicating that the iterator has reached its end or the IteratorResult object if a next value is available. It performs the following steps when called:
The abstract operation IteratorStepValue takes argument iteratorRecord (an Iterator Record) and returns either a normal completion containing either an ECMAScript language value or done, or a throw completion. It requests the next value from iteratorRecord.[[Iterator]] by calling iteratorRecord.[[NextMethod]] and returns either done indicating that the iterator has reached its end or the value from the IteratorResult object if a next value is available. It performs the following steps when called:
The abstract operation IteratorClose takes arguments iteratorRecord (an Iterator Record) and completion (a Completion Record) and returns a Completion Record. It is used to notify an iterator that it should perform any actions it would normally perform when it has reached its completed state. It performs the following steps when called:
The abstract operation AsyncIteratorClose takes arguments iteratorRecord (an Iterator Record) and completion (a Completion Record) and returns a Completion Record. It is used to notify an async iterator that it should perform any actions it would normally perform when it has reached its completed state. It performs the following steps when called:
If innerResult.[[Value]]is not an Object, throw a TypeError exception.
Return ? completion.
7.4.12 CreateIteratorResultObject ( value, done )
The abstract operation CreateIteratorResultObject takes arguments value (an ECMAScript language value) and done (a Boolean) and returns an Object that conforms to the IteratorResult interface. It creates an object that conforms to the IteratorResult interface. It performs the following steps when called:
The abstract operation CreateListIteratorRecord takes argument list (a List of ECMAScript language values) and returns an Iterator Record. It creates an Iterator (27.1.1.2) object record whose next method returns the successive elements of list. It performs the following steps when called:
Let closure be a new Abstract Closure with no parameters that captures list and performs the following steps when called:
The definitions for this operation are distributed over the "ECMAScript Language" sections of this specification. Each definition appears after the defining occurrence of the relevant productions.
"*default*" is used within this specification as a synthetic name for a module's default export when it does not have another name. An entry in the module's [[Environment]] is created with that name and holds the corresponding value, and resolving the export named "default" by calling ResolveExport ( exportName [ , resolveSet ] ) for the module will return a ResolvedBinding Record whose [[BindingName]] is "*default*", which will then resolve in the module's [[Environment]] to the above-mentioned value. This is done only for ease of specification, so that anonymous default exports can be resolved like any other export. This "*default*" string is never accessible to ECMAScript code or to the module linking algorithm.
It is defined piecewise over the following productions:
It is not necessary to treat export defaultAssignmentExpression as a constant declaration because there is no syntax that permits assignment to the internal bound name used to reference a module's default object.
8.2.4 Static Semantics: LexicallyDeclaredNames
The syntax-directed operation LexicallyDeclaredNames takes no arguments and returns a List of Strings. It is defined piecewise over the following productions:
The syntax-directed operation LexicallyScopedDeclarations takes no arguments and returns a List of Parse Nodes. It is defined piecewise over the following productions:
The syntax-directed operation VarDeclaredNames takes no arguments and returns a List of Strings. It is defined piecewise over the following productions:
The syntax-directed operation VarScopedDeclarations takes no arguments and returns a List of Parse Nodes. It is defined piecewise over the following productions:
The syntax-directed operation TopLevelLexicallyDeclaredNames takes no arguments and returns a List of Strings. It is defined piecewise over the following productions:
The syntax-directed operation TopLevelLexicallyScopedDeclarations takes no arguments and returns a List of Parse Nodes. It is defined piecewise over the following productions:
The syntax-directed operation TopLevelVarDeclaredNames takes no arguments and returns a List of Strings. It is defined piecewise over the following productions:
The syntax-directed operation TopLevelVarScopedDeclarations takes no arguments and returns a List of Parse Nodes. It is defined piecewise over the following productions:
The syntax-directed operation ContainsDuplicateLabels takes argument labelSet (a List of Strings) and returns a Boolean. It is defined piecewise over the following productions:
The syntax-directed operation ContainsUndefinedBreakTarget takes argument labelSet (a List of Strings) and returns a Boolean. It is defined piecewise over the following productions:
The syntax-directed operation ContainsUndefinedContinueTarget takes arguments iterationSet (a List of Strings) and labelSet (a List of Strings) and returns a Boolean. It is defined piecewise over the following productions:
The abstract operation IsAnonymousFunctionDefinition takes argument expr (an AssignmentExpressionParse Node, an InitializerParse Node, or an ExpressionParse Node) and returns a Boolean. It determines if its argument is a function definition that does not bind a name. It performs the following steps when called:
The syntax-directed operation ComputedPropertyContains takes argument symbol (a grammar symbol) and returns a Boolean. It is defined piecewise over the following productions:
undefined is passed for environment to indicate that a PutValue operation should be used to assign the initialization value. This is the case for var statements and formal parameter lists of some non-strict functions (See 10.2.11). In those cases a lexical binding is hoisted and preinitialized prior to evaluation of its initializer.
It is defined piecewise over the following productions:
When undefined is passed for environment it indicates that a PutValue operation should be used to assign the initialization value. This is the case for formal parameter lists of non-strict functions. In that case the formal parameter bindings are preinitialized in order to deal with the possibility of multiple parameters with the same name.
It is defined piecewise over the following productions:
The syntax-directed operation AssignmentTargetType takes no arguments and returns simple or invalid. It is defined piecewise over the following productions:
Environment Record is a specification type used to define the association of Identifiers to specific variables and functions, based upon the lexical nesting structure of ECMAScript code. Usually an Environment Record is associated with some specific syntactic structure of ECMAScript code such as a FunctionDeclaration, a BlockStatement, or a Catch clause of a TryStatement. Each time such code is evaluated, a new Environment Record is created to record the identifier bindings that are created by that code.
Every Environment Record has an [[OuterEnv]] field, which is either null or a reference to an outer Environment Record. This is used to model the logical nesting of Environment Record values. The outer reference of an (inner) Environment Record is a reference to the Environment Record that logically surrounds the inner Environment Record. An outer Environment Record may, of course, have its own outer Environment Record. An Environment Record may serve as the outer environment for multiple inner Environment Records. For example, if a FunctionDeclaration contains two nested FunctionDeclarations then the Environment Records of each of the nested functions will have as their outer Environment Record the Environment Record of the current evaluation of the surrounding function.
Environment Records are purely specification mechanisms and need not correspond to any specific artefact of an ECMAScript implementation. It is impossible for an ECMAScript program to directly access or manipulate such values.
A Function Environment Record corresponds to the invocation of an ECMAScript function object, and contains bindings for the top-level declarations within that function. It may establish a new this binding. It also captures the state necessary to support super method invocations.
An Object Environment Record is used to define the effect of ECMAScript elements such as WithStatement that associate identifier bindings with the properties of some object.
A Global Environment Record is used for Script global declarations. It does not have an outer environment; its [[OuterEnv]] is null. It may be prepopulated with identifier bindings and it includes an associated global object whose properties provide some of the global environment's identifier bindings. As ECMAScript code is executed, additional properties may be added to the global object and the initial properties may be modified.
The Environment Record abstract class includes the abstract specification methods defined in Table 16. These abstract methods have distinct concrete algorithms for each of the concrete subclasses.
Determine if an Environment Record has a binding for the String value N. Return true if it does and false if it does not.
CreateMutableBinding(N, D)
Create a new but uninitialized mutable binding in an Environment Record. The String value N is the text of the bound name. If the Boolean argument D is true the binding may be subsequently deleted.
CreateImmutableBinding(N, S)
Create a new but uninitialized immutable binding in an Environment Record. The String value N is the text of the bound name. If S is true then attempts to set it after it has been initialized will always throw an exception, regardless of the strict mode setting of operations that reference that binding.
InitializeBinding(N, V)
Set the value of an already existing but uninitialized binding in an Environment Record. The String value N is the text of the bound name. V is the value for the binding and is a value of any ECMAScript language type.
SetMutableBinding(N, V, S)
Set the value of an already existing mutable binding in an Environment Record. The String value N is the text of the bound name. V is the value for the binding and may be a value of any ECMAScript language type. Sis a Boolean flag. If S is true and the binding cannot be set throw a TypeError exception.
GetBindingValue(N, S)
Returns the value of an already existing binding from an Environment Record. The String value N is the text of the bound name. S is used to identify references originating in strict mode code or that otherwise require strict mode reference semantics. If S is true and the binding does not exist throw a ReferenceError exception. If the binding exists but is uninitialized a ReferenceError is thrown, regardless of the value of S.
DeleteBinding(N)
Delete a binding from an Environment Record. The String value N is the text of the bound name. If a binding for N exists, remove the binding and return true. If the binding exists but cannot be removed return false. If the binding does not exist return true.
HasThisBinding()
Determine if an Environment Record establishes a this binding. Return true if it does and false if it does not.
HasSuperBinding()
Determine if an Environment Record establishes a super method binding. Return true if it does and false if it does not.
WithBaseObject()
If this Environment Record is associated with a with statement, return the with object. Otherwise, return undefined.
9.1.1.1 Declarative Environment Records
Each Declarative Environment Record is associated with an ECMAScript program scope containing variable, constant, let, class, module, import, and/or function declarations. A Declarative Environment Record binds the set of identifiers defined by the declarations contained within its scope.
The behaviour of the concrete specification methods for Declarative Environment Records is defined by the following algorithms.
9.1.1.1.1 HasBinding ( N )
The HasBinding concrete method of a Declarative Environment RecordenvRec takes argument N (a String) and returns a normal completion containing a Boolean. It determines if the argument identifier is one of the identifiers bound by the record. It performs the following steps when called:
If envRec has a binding for N, return true.
Return false.
9.1.1.1.2 CreateMutableBinding ( N, D )
The CreateMutableBinding concrete method of a Declarative Environment RecordenvRec takes arguments N (a String) and D (a Boolean) and returns a normal completion containingunused. It creates a new mutable binding for the name N that is uninitialized. A binding must not already exist in this Environment Record for N. If D is true, the new binding is marked as being subject to deletion. It performs the following steps when called:
Assert: envRec does not already have a binding for N.
Create a mutable binding in envRec for N and record that it is uninitialized. If D is true, record that the newly created binding may be deleted by a subsequent DeleteBinding call.
Return unused.
9.1.1.1.3 CreateImmutableBinding ( N, S )
The CreateImmutableBinding concrete method of a Declarative Environment RecordenvRec takes arguments N (a String) and S (a Boolean) and returns a normal completion containingunused. It creates a new immutable binding for the name N that is uninitialized. A binding must not already exist in this Environment Record for N. If S is true, the new binding is marked as a strict binding. It performs the following steps when called:
Assert: envRec does not already have a binding for N.
Create an immutable binding in envRec for N and record that it is uninitialized. If S is true, record that the newly created binding is a strict binding.
Return unused.
9.1.1.1.4 InitializeBinding ( N, V )
The InitializeBinding concrete method of a Declarative Environment RecordenvRec takes arguments N (a String) and V (an ECMAScript language value) and returns a normal completion containingunused. It is used to set the bound value of the current binding of the identifier whose name is N to the value V. An uninitialized binding for N must already exist. It performs the following steps when called:
Assert: envRec must have an uninitialized binding for N.
Set the bound value for N in envRec to V.
Record that the binding for N in envRec has been initialized.
Return unused.
9.1.1.1.5 SetMutableBinding ( N, V, S )
The SetMutableBinding concrete method of a Declarative Environment RecordenvRec takes arguments N (a String), V (an ECMAScript language value), and S (a Boolean) and returns either a normal completion containingunused or a throw completion. It attempts to change the bound value of the current binding of the identifier whose name is N to the value V. A binding for N normally already exists, but in rare cases it may not. If the binding is an immutable binding, a TypeError is thrown if S is true. It performs the following steps when called:
If envRec does not have a binding for N, then
If S is true, throw a ReferenceError exception.
Perform ! envRec.CreateMutableBinding(N, true).
Perform ! envRec.InitializeBinding(N, V).
Return unused.
If the binding for N in envRec is a strict binding, set S to true.
If the binding for N in envRec has not yet been initialized, then
Throw a ReferenceError exception.
Else if the binding for N in envRec is a mutable binding, then
Change its bound value to V.
Else,
Assert: This is an attempt to change the value of an immutable binding.
If S is true, throw a TypeError exception.
Return unused.
Note
An example of ECMAScript code that results in a missing binding at step 1 is:
functionf() { eval("var x; x = (delete x, 0);"); }
9.1.1.1.6 GetBindingValue ( N, S )
The GetBindingValue concrete method of a Declarative Environment RecordenvRec takes arguments N (a String) and S (a Boolean) and returns either a normal completion containing an ECMAScript language value or a throw completion. It returns the value of its bound identifier whose name is N. If the binding exists but is uninitialized a ReferenceError is thrown, regardless of the value of S. It performs the following steps when called:
If the binding for N in envRec is an uninitialized binding, throw a ReferenceError exception.
Return the value currently bound to N in envRec.
9.1.1.1.7 DeleteBinding ( N )
The DeleteBinding concrete method of a Declarative Environment RecordenvRec takes argument N (a String) and returns a normal completion containing a Boolean. It can only delete bindings that have been explicitly designated as being subject to deletion. It performs the following steps when called:
If the binding for N in envRec cannot be deleted, return false.
Remove the binding for N from envRec.
Return true.
9.1.1.1.8 HasThisBinding ( )
The HasThisBinding concrete method of a Declarative Environment RecordenvRec takes no arguments and returns false. It performs the following steps when called:
The HasSuperBinding concrete method of a Declarative Environment RecordenvRec takes no arguments and returns false. It performs the following steps when called:
The WithBaseObject concrete method of a Declarative Environment RecordenvRec takes no arguments and returns undefined. It performs the following steps when called:
Return undefined.
9.1.1.2 Object Environment Records
Each Object Environment Record is associated with an object called its binding object. An Object Environment Record binds the set of string identifier names that directly correspond to the property names of its binding object. Property keys that are not strings in the form of an IdentifierName are not included in the set of bound identifiers. Both own and inherited properties are included in the set regardless of the setting of their [[Enumerable]] attribute. Because properties can be dynamically added and deleted from objects, the set of identifiers bound by an Object Environment Record may potentially change as a side-effect of any operation that adds or deletes properties. Any bindings that are created as a result of such a side-effect are considered to be a mutable binding even if the Writable attribute of the corresponding property is false. Immutable bindings do not exist for Object Environment Records.
Object Environment Records created for with statements (14.11) can provide their binding object as an implicit this value for use in function calls. The capability is controlled by a Boolean [[IsWithEnvironment]] field.
Object Environment Records have the additional state fields listed in Table 17.
Indicates whether this Environment Record is created for a with statement.
The behaviour of the concrete specification methods for Object Environment Records is defined by the following algorithms.
9.1.1.2.1 HasBinding ( N )
The HasBinding concrete method of an Object Environment RecordenvRec takes argument N (a String) and returns either a normal completion containing a Boolean or a throw completion. It determines if its associated binding object has a property whose name is N. It performs the following steps when called:
Let bindingObject be envRec.[[BindingObject]].
Let foundBinding be ? HasProperty(bindingObject, N).
If foundBinding is false, return false.
If envRec.[[IsWithEnvironment]] is false, return true.
The CreateMutableBinding concrete method of an Object Environment RecordenvRec takes arguments N (a String) and D (a Boolean) and returns either a normal completion containingunused or a throw completion. It creates in an Environment Record's associated binding object a property whose name is N and initializes it to the value undefined. If D is true, the new property's [[Configurable]] attribute is set to true; otherwise it is set to false. It performs the following steps when called:
Let bindingObject be envRec.[[BindingObject]].
Perform ? DefinePropertyOrThrow(bindingObject, N, PropertyDescriptor { [[Value]]: undefined, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: D }).
Return unused.
Note
Normally envRec will not have a binding for N but if it does, the semantics of DefinePropertyOrThrow may result in an existing binding being replaced or shadowed or cause an abrupt completion to be returned.
9.1.1.2.3 CreateImmutableBinding ( N, S )
The CreateImmutableBinding concrete method of an Object Environment Record is never used within this specification.
In this specification, all uses of CreateMutableBinding for Object Environment Records are immediately followed by a call to InitializeBinding for the same name. Hence, this specification does not explicitly track the initialization state of bindings in Object Environment Records.
9.1.1.2.5 SetMutableBinding ( N, V, S )
The SetMutableBinding concrete method of an Object Environment RecordenvRec takes arguments N (a String), V (an ECMAScript language value), and S (a Boolean) and returns either a normal completion containingunused or a throw completion. It attempts to set the value of the Environment Record's associated binding object's property whose name is N to the value V. A property named N normally already exists but if it does not or is not currently writable, error handling is determined by S. It performs the following steps when called:
Let bindingObject be envRec.[[BindingObject]].
Let stillExists be ? HasProperty(bindingObject, N).
If stillExists is false and S is true, throw a ReferenceError exception.
The GetBindingValue concrete method of an Object Environment RecordenvRec takes arguments N (a String) and S (a Boolean) and returns either a normal completion containing an ECMAScript language value or a throw completion. It returns the value of its associated binding object's property whose name is N. The property should already exist but if it does not the result depends upon S. It performs the following steps when called:
The DeleteBinding concrete method of an Object Environment RecordenvRec takes argument N (a String) and returns either a normal completion containing a Boolean or a throw completion. It can only delete bindings that correspond to properties of the environment object whose [[Configurable]] attribute have the value true. It performs the following steps when called:
Let bindingObject be envRec.[[BindingObject]].
Return ? bindingObject.[[Delete]](N).
9.1.1.2.8 HasThisBinding ( )
The HasThisBinding concrete method of an Object Environment RecordenvRec takes no arguments and returns false. It performs the following steps when called:
The HasSuperBinding concrete method of an Object Environment RecordenvRec takes no arguments and returns false. It performs the following steps when called:
The WithBaseObject concrete method of an Object Environment RecordenvRec takes no arguments and returns an Object or undefined. It performs the following steps when called:
If envRec.[[IsWithEnvironment]] is true, return envRec.[[BindingObject]].
Otherwise, return undefined.
9.1.1.3 Function Environment Records
A Function Environment Record is a Declarative Environment Record that is used to represent the top-level scope of a function and, if the function is not an ArrowFunction, provides a this binding. If a function is not an ArrowFunction function and references super, its Function Environment Record also contains the state that is used to perform super method invocations from within the function.
Function Environment Records have the additional state fields listed in Table 18.
If this Environment Record was created by the [[Construct]] internal method, [[NewTarget]] is the value of the [[Construct]]newTarget parameter. Otherwise, its value is undefined.
Function Environment Records support all of the Declarative Environment Record methods listed in Table 16 and share the same specifications for all of those methods except for HasThisBinding and HasSuperBinding. In addition, Function Environment Records support the methods listed in Table 19:
Set the [[ThisValue]] and record that it has been initialized.
GetThisBinding()
Return the value of this Environment Record's this binding. Throws a ReferenceError if the this binding has not been initialized.
GetSuperBase()
Return the object that is the base for super property accesses bound in this Environment Record. The value undefined indicates that such accesses will produce runtime errors.
The behaviour of the additional concrete specification methods for Function Environment Records is defined by the following algorithms:
Assert: envRec.[[ThisBindingStatus]] is not lexical.
If envRec.[[ThisBindingStatus]] is initialized, throw a ReferenceError exception.
Set envRec.[[ThisValue]] to V.
Set envRec.[[ThisBindingStatus]] to initialized.
Return V.
9.1.1.3.2 HasThisBinding ( )
The HasThisBinding concrete method of a Function Environment RecordenvRec takes no arguments and returns a Boolean. It performs the following steps when called:
If envRec.[[ThisBindingStatus]] is lexical, return false; otherwise, return true.
9.1.1.3.3 HasSuperBinding ( )
The HasSuperBinding concrete method of a Function Environment RecordenvRec takes no arguments and returns a Boolean. It performs the following steps when called:
If envRec.[[ThisBindingStatus]] is lexical, return false.
If envRec.[[FunctionObject]].[[HomeObject]] is undefined, return false; otherwise, return true.
A Global Environment Record is used to represent the outer most scope that is shared by all of the ECMAScript Script elements that are processed in a common realm. A Global Environment Record provides the bindings for built-in globals (clause 19), properties of the global object, and for all top-level declarations (8.2.9, 8.2.11) that occur within a Script.
Determines if the argument is the name of a global object property that may not be shadowed by a global lexical binding.
CanDeclareGlobalVar (N)
Determines if a corresponding CreateGlobalVarBinding call would succeed if called for the same argument N.
CanDeclareGlobalFunction (N)
Determines if a corresponding CreateGlobalFunctionBinding call would succeed if called for the same argument N.
CreateGlobalVarBinding(N, D)
Used to create and initialize to undefined a global var binding in the [[ObjectRecord]] component of a Global Environment Record. The binding will be a mutable binding. The corresponding global object property will have attribute values appropriate for a var. The String value N is the bound name. If D is true, the binding may be deleted. Logically equivalent to CreateMutableBinding followed by a SetMutableBinding but it allows var declarations to receive special treatment.
CreateGlobalFunctionBinding(N, V, D)
Create and initialize a global function binding in the [[ObjectRecord]] component of a Global Environment Record. The binding will be a mutable binding. The corresponding global object property will have attribute values appropriate for a function. The String value N is the bound name. V is the initialization value. If the Boolean argument D is true, the binding may be deleted. Logically equivalent to CreateMutableBinding followed by a SetMutableBinding but it allows function declarations to receive special treatment.
The behaviour of the concrete specification methods for Global Environment Records is defined by the following algorithms.
9.1.1.4.1 HasBinding ( N )
The HasBinding concrete method of a Global Environment RecordenvRec takes argument N (a String) and returns either a normal completion containing a Boolean or a throw completion. It determines if the argument identifier is one of the identifiers bound by the record. It performs the following steps when called:
Let DclRec be envRec.[[DeclarativeRecord]].
If ! DclRec.HasBinding(N) is true, return true.
Let ObjRec be envRec.[[ObjectRecord]].
Return ? ObjRec.HasBinding(N).
9.1.1.4.2 CreateMutableBinding ( N, D )
The CreateMutableBinding concrete method of a Global Environment RecordenvRec takes arguments N (a String) and D (a Boolean) and returns either a normal completion containingunused or a throw completion. It creates a new mutable binding for the name N that is uninitialized. The binding is created in the associated DeclarativeRecord. A binding for N must not already exist in the DeclarativeRecord. If D is true, the new binding is marked as being subject to deletion. It performs the following steps when called:
Let DclRec be envRec.[[DeclarativeRecord]].
If ! DclRec.HasBinding(N) is true, throw a TypeError exception.
Return ! DclRec.CreateMutableBinding(N, D).
9.1.1.4.3 CreateImmutableBinding ( N, S )
The CreateImmutableBinding concrete method of a Global Environment RecordenvRec takes arguments N (a String) and S (a Boolean) and returns either a normal completion containingunused or a throw completion. It creates a new immutable binding for the name N that is uninitialized. A binding must not already exist in this Environment Record for N. If S is true, the new binding is marked as a strict binding. It performs the following steps when called:
Let DclRec be envRec.[[DeclarativeRecord]].
If ! DclRec.HasBinding(N) is true, throw a TypeError exception.
Return ! DclRec.CreateImmutableBinding(N, S).
9.1.1.4.4 InitializeBinding ( N, V )
The InitializeBinding concrete method of a Global Environment RecordenvRec takes arguments N (a String) and V (an ECMAScript language value) and returns either a normal completion containingunused or a throw completion. It is used to set the bound value of the current binding of the identifier whose name is N to the value V. An uninitialized binding for N must already exist. It performs the following steps when called:
The SetMutableBinding concrete method of a Global Environment RecordenvRec takes arguments N (a String), V (an ECMAScript language value), and S (a Boolean) and returns either a normal completion containingunused or a throw completion. It attempts to change the bound value of the current binding of the identifier whose name is N to the value V. If the binding is an immutable binding and S is true, a TypeError is thrown. A property named N normally already exists but if it does not or is not currently writable, error handling is determined by S. It performs the following steps when called:
Let DclRec be envRec.[[DeclarativeRecord]].
If ! DclRec.HasBinding(N) is true, then
Return ? DclRec.SetMutableBinding(N, V, S).
Let ObjRec be envRec.[[ObjectRecord]].
Return ? ObjRec.SetMutableBinding(N, V, S).
9.1.1.4.6 GetBindingValue ( N, S )
The GetBindingValue concrete method of a Global Environment RecordenvRec takes arguments N (a String) and S (a Boolean) and returns either a normal completion containing an ECMAScript language value or a throw completion. It returns the value of its bound identifier whose name is N. If the binding is an uninitialized binding throw a ReferenceError exception. A property named N normally already exists but if it does not or is not currently writable, error handling is determined by S. It performs the following steps when called:
Let DclRec be envRec.[[DeclarativeRecord]].
If ! DclRec.HasBinding(N) is true, then
Return ? DclRec.GetBindingValue(N, S).
Let ObjRec be envRec.[[ObjectRecord]].
Return ? ObjRec.GetBindingValue(N, S).
9.1.1.4.7 DeleteBinding ( N )
The DeleteBinding concrete method of a Global Environment RecordenvRec takes argument N (a String) and returns either a normal completion containing a Boolean or a throw completion. It can only delete bindings that have been explicitly designated as being subject to deletion. It performs the following steps when called:
If status is true and envRec.[[VarNames]] contains N, then
Remove N from envRec.[[VarNames]].
Return status.
Return true.
9.1.1.4.8 HasThisBinding ( )
The HasThisBinding concrete method of a Global Environment RecordenvRec takes no arguments and returns true. It performs the following steps when called:
The HasSuperBinding concrete method of a Global Environment RecordenvRec takes no arguments and returns false. It performs the following steps when called:
The WithBaseObject concrete method of a Global Environment RecordenvRec takes no arguments and returns undefined. It performs the following steps when called:
The HasVarDeclaration concrete method of a Global Environment RecordenvRec takes argument N (a String) and returns a Boolean. It determines if the argument identifier has a binding in this record that was created using a VariableStatement or a FunctionDeclaration. It performs the following steps when called:
Let varDeclaredNames be envRec.[[VarNames]].
If varDeclaredNames contains N, return true.
Return false.
9.1.1.4.13 HasLexicalDeclaration ( N )
The HasLexicalDeclaration concrete method of a Global Environment RecordenvRec takes argument N (a String) and returns a Boolean. It determines if the argument identifier has a binding in this record that was created using a lexical declaration such as a LexicalDeclaration or a ClassDeclaration. It performs the following steps when called:
Let DclRec be envRec.[[DeclarativeRecord]].
Return ! DclRec.HasBinding(N).
9.1.1.4.14 HasRestrictedGlobalProperty ( N )
The HasRestrictedGlobalProperty concrete method of a Global Environment RecordenvRec takes argument N (a String) and returns either a normal completion containing a Boolean or a throw completion. It determines if the argument identifier is the name of a property of the global object that must not be shadowed by a global lexical binding. It performs the following steps when called:
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be ObjRec.[[BindingObject]].
Let existingProp be ? globalObject.[[GetOwnProperty]](N).
If existingProp is undefined, return false.
If existingProp.[[Configurable]] is true, return false.
Return true.
Note
Properties may exist upon a global object that were directly created rather than being declared using a var or function declaration. A global lexical binding may not be created that has the same name as a non-configurable property of the global object. The global property "undefined" is an example of such a property.
9.1.1.4.15 CanDeclareGlobalVar ( N )
The CanDeclareGlobalVar concrete method of a Global Environment RecordenvRec takes argument N (a String) and returns either a normal completion containing a Boolean or a throw completion. It determines if a corresponding CreateGlobalVarBinding call would succeed if called for the same argument N. Redundant var declarations and var declarations for pre-existing global object properties are allowed. It performs the following steps when called:
The CanDeclareGlobalFunction concrete method of a Global Environment RecordenvRec takes argument N (a String) and returns either a normal completion containing a Boolean or a throw completion. It determines if a corresponding CreateGlobalFunctionBinding call would succeed if called for the same argument N. It performs the following steps when called:
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be ObjRec.[[BindingObject]].
Let existingProp be ? globalObject.[[GetOwnProperty]](N).
If existingProp is undefined, return ? IsExtensible(globalObject).
If existingProp.[[Configurable]] is true, return true.
If IsDataDescriptor(existingProp) is true and existingProp has attribute values { [[Writable]]: true, [[Enumerable]]: true }, return true.
Return false.
9.1.1.4.17 CreateGlobalVarBinding ( N, D )
The CreateGlobalVarBinding concrete method of a Global Environment RecordenvRec takes arguments N (a String) and D (a Boolean) and returns either a normal completion containingunused or a throw completion. It creates and initializes a mutable binding in the associated Object Environment Record and records the bound name in the associated [[VarNames]]List. If a binding already exists, it is reused and assumed to be initialized. It performs the following steps when called:
Global function declarations are always represented as own properties of the global object. If possible, an existing own property is reconfigured to have a standard set of attribute values. Step 7 is equivalent to what calling the InitializeBinding concrete method would do and if globalObject is a Proxy will produce the same sequence of Proxy trap calls.
9.1.1.5 Module Environment Records
A Module Environment Record is a Declarative Environment Record that is used to represent the outer scope of an ECMAScript Module. In additional to normal mutable and immutable bindings, Module Environment Records also provide immutable import bindings which are bindings that provide indirect access to a target binding that exists in another Environment Record.
Module Environment Records support all of the Declarative Environment Record methods listed in Table 16 and share the same specifications for all of those methods except for GetBindingValue, DeleteBinding, HasThisBinding and GetThisBinding. In addition, Module Environment Records support the methods listed in Table 22:
The behaviour of the additional concrete specification methods for Module Environment Records are defined by the following algorithms:
9.1.1.5.1 GetBindingValue ( N, S )
The GetBindingValue concrete method of a Module Environment RecordenvRec takes arguments N (a String) and S (a Boolean) and returns either a normal completion containing an ECMAScript language value or a throw completion. It returns the value of its bound identifier whose name is N. However, if the binding is an indirect binding the value of the target binding is returned. If the binding exists but is uninitialized a ReferenceError is thrown. It performs the following steps when called:
The HasThisBinding concrete method of a Module Environment RecordenvRec takes no arguments and returns true. It performs the following steps when called:
The CreateImportBinding concrete method of a Module Environment RecordenvRec takes arguments N (a String), M (a Module Record), and N2 (a String) and returns unused. It creates a new initialized immutable indirect binding for the name N. A binding must not already exist in this Environment Record for N. N2 is the name of a binding that exists in M's Module Environment Record. Accesses to the value of the new binding will indirectly access the bound value of the target binding. It performs the following steps when called:
Assert: envRec does not already have a binding for N.
Assert: When M.[[Environment]] is instantiated, it will have a direct binding for N2.
Create an immutable indirect binding in envRec for N that references M and N2 as its target binding and record that the binding is initialized.
The abstract operation NewObjectEnvironment takes arguments O (an Object), W (a Boolean), and E (an Environment Record or null) and returns an Object Environment Record. It performs the following steps when called:
The abstract operation NewFunctionEnvironment takes arguments F (an ECMAScript function object) and newTarget (an Object or undefined) and returns a Function Environment Record. It performs the following steps when called:
If F.[[ThisMode]] is lexical, set env.[[ThisBindingStatus]] to lexical.
Else, set env.[[ThisBindingStatus]] to uninitialized.
Set env.[[NewTarget]] to newTarget.
Set env.[[OuterEnv]] to F.[[Environment]].
Return env.
9.1.2.5 NewGlobalEnvironment ( G, thisValue )
The abstract operation NewGlobalEnvironment takes arguments G (an Object) and thisValue (an Object) and returns a Global Environment Record. It performs the following steps when called:
The abstract operation ResolvePrivateIdentifier takes arguments privateEnv (a PrivateEnvironment Record) and identifier (a String) and returns a Private Name. It performs the following steps when called:
Before it is evaluated, all ECMAScript code must be associated with a realm. Conceptually, a realm consists of a set of intrinsic objects, an ECMAScript global environment, all of the ECMAScript code that is loaded within the scope of that global environment, and other associated state and resources.
A realm is represented in this specification as a Realm Record with the fields specified in Table 24:
Template objects are canonicalized separately for each realm using its Realm Record's [[TemplateMap]]. Each [[Site]] value is a Parse Node that is a TemplateLiteral. The associated [[Array]] value is the corresponding template object that is passed to a tag function.
Note 1
Once a Parse Node becomes unreachable, the corresponding [[Array]] is also unreachable, and it would be unobservable if an implementation removed the pair from the [[TemplateMap]] list.
A map from the specifier strings imported by this realm to the resolved Module Record. The list does not contain two different Records with the same [[Specifier]].
Field reserved for use by hosts that need to associate additional information with a Realm Record.
9.3.1 InitializeHostDefinedRealm ( )
The abstract operation InitializeHostDefinedRealm takes no arguments and returns either a normal completion containingunused or a throw completion. It performs the following steps when called:
Set fields of realmRec.[[Intrinsics]] with the values listed in Table 6. The field names are the names listed in column one of the table. The value of each field is a new object value fully and recursively populated with property values as defined by the specification of each object in clauses 19 through 28. All object property values are newly created object values. All values that are built-in function objects are created by performing CreateBuiltinFunction(steps, length, name, slots, realmRec, prototype) where steps is the definition of that function provided by this specification, name is the initial value of the function's "name" property, length is the initial value of the function's "length" property, slots is a list of the names, if any, of the function's specified internal slots, and prototype is the specified value of the function's [[Prototype]] internal slot. The creation of the intrinsics and their properties must be ordered to avoid any dependencies upon objects that have not yet been created.
Let desc be the fully populated data Property Descriptor for the property, containing the specified attributes for the property. For properties listed in 19.2, 19.3, or 19.4 the value of the [[Value]] attribute is the corresponding intrinsic object from realmRec.
An execution context is a specification device that is used to track the runtime evaluation of code by an ECMAScript implementation. At any point in time, there is at most one execution context per agent that is actually executing code. This is known as the agent's running execution context. All references to the running execution context in this specification denote the running execution context of the surrounding agent.
The execution context stack is used to track execution contexts. The running execution context is always the top element of this stack. A new execution context is created whenever control is transferred from the executable code associated with the currently running execution context to executable code that is not associated with that execution context. The newly created execution context is pushed onto the stack and becomes the running execution context.
An execution context contains whatever implementation specific state is necessary to track the execution progress of its associated code. Each execution context has at least the state components listed in Table 25.
Table 25: State Components for All Execution Contexts
Component
Purpose
code evaluation state
Any state needed to perform, suspend, and resume evaluation of the code associated with this execution context.
Evaluation of code by the running execution context may be suspended at various points defined within this specification. Once the running execution context has been suspended a different execution context may become the running execution context and commence evaluating its code. At some later time a suspended execution context may again become the running execution context and continue evaluating its code at the point where it had previously been suspended. Transition of the running execution context status among execution contexts usually occurs in stack-like last-in/first-out manner. However, some ECMAScript features require non-LIFO transitions of the running execution context.
In most situations only the running execution context (the top of the execution context stack) is directly manipulated by algorithms within this specification. Hence when the terms “LexicalEnvironment”, and “VariableEnvironment” are used without qualification they are in reference to those components of the running execution context.
An execution context is purely a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation. It is impossible for ECMAScript code to directly access or observe an execution context.
9.4.1 GetActiveScriptOrModule ( )
The abstract operation GetActiveScriptOrModule takes no arguments and returns a Script Record, a Module Record, or null. It is used to determine the running script or module, based on the running execution context. It performs the following steps when called:
If no such execution context exists, return null. Otherwise, return ec's ScriptOrModule.
9.4.2 ResolveBinding ( name [ , env ] )
The abstract operation ResolveBinding takes argument name (a String) and optional argument env (an Environment Record or undefined) and returns either a normal completion containing a Reference Record or a throw completion. It is used to determine the binding of name. env can be used to explicitly provide the Environment Record that is to be searched for the binding. It performs the following steps when called:
The result of ResolveBinding is always a Reference Record whose [[ReferencedName]] field is name.
9.4.3 GetThisEnvironment ( )
The abstract operation GetThisEnvironment takes no arguments and returns an Environment Record. It finds the Environment Record that currently supplies the binding of the keywordthis. It performs the following steps when called:
The abstract operation GetNewTarget takes no arguments and returns an Object or undefined. It determines the NewTarget value using the LexicalEnvironment of the running execution context. It performs the following steps when called:
The abstract operation GetGlobalObject takes no arguments and returns an Object. It returns the global object used by the currently running execution context. It performs the following steps when called:
A Job is an Abstract Closure with no parameters that initiates an ECMAScript computation when no other ECMAScript computation is currently in progress.
At some future point in time, when there is no running context in the agent for which the job is scheduled and that agent's execution context stack is empty, the implementation must:
Host environments are not required to treat Jobs uniformly with respect to scheduling. For example, web browsers and Node.js treat Promise-handling Jobs as a higher priority than other work; future features may add Jobs that are not treated at such a high priority.
At any particular time, scriptOrModule (a Script Record, a Module Record, or null) is the active script or module if all of the following conditions are true:
The specific choice of Realm is up to the host environment. This initial execution context and Realm is only in use before any callback function is invoked. When a callback function related to a Job, like a Promise handler, is invoked, the invocation pushes its own execution context and Realm.
Particular kinds of Jobs have additional conformance requirements.
An implementation of HostMakeJobCallback must conform to the following requirements:
It must return a JobCallback Record whose [[Callback]] field is callback.
The default implementation of HostMakeJobCallback performs the following steps when called:
Return the JobCallback Record { [[Callback]]: callback, [[HostDefined]]: empty }.
ECMAScript hosts that are not web browsers must use the default implementation of HostMakeJobCallback.
Note
This is called at the time that the callback is passed to the function that is responsible for its being eventually scheduled and run. For example, promise.then(thenAction) calls MakeJobCallback on thenAction at the time of invoking Promise.prototype.then, not at the time of scheduling the reaction Job.
ECMAScript hosts that are not web browsers must use the default implementation of HostCallJobCallback.
9.5.4 HostEnqueueGenericJob ( job, realm )
The host-defined abstract operation HostEnqueueGenericJob takes arguments job (a JobAbstract Closure) and realm (a Realm Record) and returns unused. It schedules job in the realmrealm in the agent signified by realm.[[AgentSignifier]] to be performed at some future time. The Abstract Closures used with this algorithm are intended to be scheduled without additional constraints, such as priority and ordering.
An implementation of HostEnqueueGenericJob must conform to the requirements in 9.5.
9.5.5 HostEnqueuePromiseJob ( job, realm )
The host-defined abstract operation HostEnqueuePromiseJob takes arguments job (a JobAbstract Closure) and realm (a Realm Record or null) and returns unused. It schedules job to be performed at some future time. The Abstract Closures used with this algorithm are intended to be related to the handling of Promises, or otherwise, to be scheduled with equal priority to Promise handling operations.
An implementation of HostEnqueuePromiseJob must conform to the requirements in 9.5 as well as the following:
Let scriptOrModule be GetActiveScriptOrModule() at the time HostEnqueuePromiseJob is invoked. If realm is not null, each time job is invoked the implementation must perform implementation-defined steps such that scriptOrModule is the active script or module at the time of job's invocation.
Jobs must run in the same order as the HostEnqueuePromiseJob invocations that scheduled them.
The host-defined abstract operation HostEnqueueTimeoutJob takes arguments timeoutJob (a JobAbstract Closure), realm (a Realm Record), and milliseconds (a non-negative finite Number) and returns unused. It schedules timeoutJob in the realmrealm in the agent signified by realm.[[AgentSignifier]] to be performed after at least milliseconds milliseconds.
An implementation of HostEnqueueTimeoutJob must conform to the requirements in 9.5.
The default value computed for the isLittleEndian parameter when it is needed by the algorithms GetValueFromBuffer and SetValueInBuffer. The choice is implementation-defined and should be the alternative that is most efficient for the implementation. Once the value has been observed it cannot change.
Initially a new empty List, representing the list of objects and/or symbols to be kept alive until the end of the current Job
Once the values of [[Signifier]], [[IsLockFree1]], and [[IsLockFree2]] have been observed by any agent in the agent cluster they cannot change.
Note 2
The values of [[IsLockFree1]] and [[IsLockFree2]] are not necessarily determined by the hardware, but may also reflect implementation choices that can vary over time and between ECMAScript implementations.
There is no [[IsLockFree4]] field: 4-byte atomic operations are always lock-free.
In practice, if an atomic operation is implemented with any type of lock the operation is not lock-free. Lock-free does not imply wait-free: there is no upper bound on how many machine steps may be required to complete a lock-free atomic operation.
That an atomic access of size n is lock-free does not imply anything about the (perceived) atomicity of non-atomic accesses of size n, specifically, non-atomic accesses may still be performed as a sequence of several separate memory accesses. See ReadSharedMemory and WriteSharedMemory for details.
Note 3
An agent is a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation.
9.6.1 AgentSignifier ( )
The abstract operation AgentSignifier takes no arguments and returns an agent signifier. It performs the following steps when called:
In some environments it may not be reasonable for a given agent to suspend. For example, in a web browser environment, it may be reasonable to disallow suspending a document's main event handling thread, while still allowing workers' event handling threads to suspend.
9.7 Agent Clusters
An agent cluster is a maximal set of agents that can communicate by operating on shared memory.
Note 1
Programs within different agents may share memory by unspecified means. At a minimum, the backing memory for SharedArrayBuffers can be shared among the agents in the cluster.
There may be agents that can communicate by message passing that cannot share memory; they are never in the same agent cluster.
The agents in a cluster need not all be alive at some particular point in time. If agentA creates another agentB, after which A terminates and B creates agentC, the three agents are in the same cluster if A could share some memory with B and B could share some memory with C.
All agents within a cluster must have the same value for the [[LittleEndian]] field in their respective Agent Records.
Note 3
If different agents within an agent cluster have different values of [[LittleEndian]] it becomes hard to use shared memory for multi-byte data.
All agents within a cluster must have the same values for the [[IsLockFree1]] field in their respective Agent Records; similarly for the [[IsLockFree2]] field.
All agents within a cluster must have different values for the [[Signifier]] field in their respective Agent Records.
An embedding may deactivate (stop forward progress) or activate (resume forward progress) an agent without the agent's knowledge or cooperation. If the embedding does so, it must not leave some agents in the cluster active while other agents in the cluster are deactivated indefinitely.
Note 4
The purpose of the preceding restriction is to avoid a situation where an agent deadlocks or starves because another agent has been deactivated. For example, if an HTML shared worker that has a lifetime independent of documents in any windows were allowed to share memory with the dedicated worker of such an independent document, and the document and its dedicated worker were to be deactivated while the dedicated worker holds a lock (say, the document is pushed into its window's history), and the shared worker then tries to acquire the lock, then the shared worker will be blocked until the dedicated worker is activated again, if ever. Meanwhile other workers trying to access the shared worker from other windows will starve.
The implication of the restriction is that it will not be possible to share memory between agents that don't belong to the same suspend/wake collective within the embedding.
An embedding may terminate an agent without any of the agent's cluster's other agents' prior knowledge or cooperation. If an agent is terminated not by programmatic action of its own or of another agent in the cluster but by forces external to the cluster, then the embedding must choose one of two strategies: Either terminate all the agents in the cluster, or provide reliable APIs that allow the agents in the cluster to coordinate so that at least one remaining member of the cluster will be able to detect the termination, with the termination data containing enough information to identify the agent that was terminated.
Note 5
Examples of that type of termination are: operating systems or users terminating agents that are running in separate processes; the embedding itself terminating an agent that is running in-process with the other agents when per-agent resource accounting indicates that the agent is runaway.
Each of the following specification values, and values transitively reachable from them, belong to exactly one agent cluster.
An agent cluster is a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation.
9.8 Forward Progress
For an agent to make forward progress is for it to perform an evaluation step according to this specification.
An agent becomes blocked when its running execution context waits synchronously and indefinitely for an external event. Only agents whose Agent Record's [[CanBlock]] field is true can become blocked in this sense. An unblockedagent is one that is not blocked.
Implementations must ensure that:
every unblocked agent with a dedicated executing thread eventually makes forward progress
an agent does not cause another agent to become blocked except via explicit APIs that provide blocking.
Note
This, along with the liveness guarantee in the memory model, ensures that all seq-cst writes eventually become observable to all agents.
9.9 Processing Model of WeakRef and FinalizationRegistry Targets
9.9.1 Objectives
This specification does not make any guarantees that any object or symbol will be garbage collected. Objects or symbols which are not live may be released after long periods of time, or never at all. For this reason, this specification uses the term "may" when describing behaviour triggered by garbage collection.
The semantics of WeakRefs and FinalizationRegistrys is based on two operations which happen at particular points in time:
When WeakRef.prototype.deref is called, the referent (if undefined is not returned) is kept alive so that subsequent, synchronous accesses also return the same value. This list is reset when synchronous work is done using the ClearKeptObjects abstract operation.
Some ECMAScript implementations include garbage collector implementations which run in the background, including when ECMAScript is idle. Letting the host environment schedule CleanupFinalizationRegistry allows it to resume ECMAScript execution in order to run finalizer work, which may free up held values, reducing overall memory usage.
9.9.2 Liveness
For some set of objects and/or symbols S a hypothetical WeakRef-oblivious execution with respect to S is an execution whereby the abstract operation WeakRefDeref of a WeakRef whose referent is an element of S always returns undefined.
Note 1
WeakRef-obliviousness, together with liveness, capture two notions. One, that a WeakRef itself does not keep its referent alive. Two, that cycles in liveness does not imply that a value is live. To be concrete, if determining v's liveness depends on determining the liveness of a WeakRef referent, r, r's liveness cannot assume v's liveness, which would be circular reasoning.
Note 2
WeakRef-obliviousness is defined on sets of objects or symbols instead of individual values to account for cycles. If it were defined on individual values, then a WeakRef referent in a cycle will be considered live even though its identity is only observed via other WeakRef referents in the cycle.
Note 3
Colloquially, we say that an individual object or symbol is live if every set containing it is live.
At any point during evaluation, a set of objects and/or symbols S is considered live if either of the following conditions is met:
Any element in S is included in any agent's [[KeptAlive]]List.
There exists a valid future hypothetical WeakRef-oblivious execution with respect to S that observes the identity of any value in S.
Note 4
The second condition above intends to capture the intuition that a value is live if its identity is observable via non-WeakRef means. A value's identity may be observed by observing a strict equality comparison or observing the value being used as key in a Map.
Note 5
Presence of an object or a symbol in a field, an internal slot, or a property does not imply that the value is live. For example if the value in question is never passed back to the program, then it cannot be observed.
This is the case for keys in a WeakMap, members of a WeakSet, as well as the [[WeakRefTarget]] and [[UnregisterToken]] fields of a FinalizationRegistry Cell record.
The above definition implies that, if a key in a WeakMap is not live, then its corresponding value is not necessarily live either.
Note 6
Liveness is the lower bound for guaranteeing which WeakRefs engines must not empty. Liveness as defined here is undecidable. In practice, engines use conservative approximations such as reachability. There is expected to be significant implementation leeway.
9.9.3 Execution
At any time, if a set of objects and/or symbols S is not live, an ECMAScript implementation may perform the following steps atomically:
For each element value of S, do
For each WeakRefref such that ref.[[WeakRefTarget]] is value, do
Set ref.[[WeakRefTarget]] to empty.
For each FinalizationRegistryfg such that fg.[[Cells]] contains a Recordcell such that cell.[[WeakRefTarget]] is value, do
For each WeakMap map such that map.[[WeakMapData]] contains a Recordr such that r.[[Key]] is value, do
Set r.[[Key]] to empty.
Set r.[[Value]] to empty.
For each WeakSet set such that set.[[WeakSetData]] contains value, do
Replace the element of set.[[WeakSetData]] whose value is value with an element whose value is empty.
Note 1
Together with the definition of liveness, this clause prescribes optimizations that an implementation may apply regarding WeakRefs.
It is possible to access an object without observing its identity. Optimizations such as dead variable elimination and scalar replacement on properties of non-escaping objects whose identity is not observed are allowed. These optimizations are thus allowed to observably empty WeakRefs that point to such objects.
On the other hand, if an object's identity is observable, and that object is in the [[WeakRefTarget]] internal slot of a WeakRef, optimizations such as rematerialization that observably empty the WeakRef are prohibited.
Implementations are not obligated to empty WeakRefs for maximal sets of non-live objects or symbols.
If an implementation chooses a non-live set S in which to empty WeakRefs, this definition requires that it empties WeakRefs for all values in S simultaneously. In other words, it is not conformant for an implementation to empty a WeakRef pointing to a value v without emptying out other WeakRefs that, if not emptied, could result in an execution that observes the value of v.
The host-defined abstract operation HostEnqueueFinalizationRegistryCleanupJob takes argument finalizationRegistry (a FinalizationRegistry) and returns unused.
Let cleanupJob be a new JobAbstract Closure with no parameters that captures finalizationRegistry and performs the following steps when called:
An implementation of HostEnqueueFinalizationRegistryCleanupJob schedules cleanupJob to be performed at some future time, if possible. It must also conform to the requirements in 9.5.
9.10 ClearKeptObjects ( )
The abstract operation ClearKeptObjects takes no arguments and returns unused. ECMAScript implementations are expected to call ClearKeptObjects when a synchronous sequence of ECMAScript executions completes. It performs the following steps when called:
When the abstract operation AddToKeptObjects is called with a target object or symbol, it adds the target to a list that will point strongly at the target until ClearKeptObjects is called.
Assert: finalizationRegistry has [[Cells]] and [[CleanupCallback]] internal slots.
Let callback be finalizationRegistry.[[CleanupCallback]].
While finalizationRegistry.[[Cells]] contains a Recordcell such that cell.[[WeakRefTarget]] is empty, an implementation may perform the following steps:
The abstract operation CanBeHeldWeakly takes argument v (an ECMAScript language value) and returns a Boolean. It returns true if and only if v is suitable for use as a weak reference. Only values that are suitable for use as a weak reference may be a key of a WeakMap, an element of a WeakSet, the target of a WeakRef, or one of the targets of a FinalizationRegistry. It performs the following steps when called:
A language value without language identity can be manifested without prior reference and is unsuitable for use as a weak reference. A Symbol value produced by Symbol.for, unlike other Symbol values, does not have language identity and is unsuitable for use as a weak reference. Well-known symbols are likely to never be collected, but are nonetheless treated as suitable for use as a weak reference because they are limited in number and therefore manageable by a variety of implementation approaches. However, any value associated to a well-known symbol in a live WeakMap is unlikely to be collected and could “leak” memory resources in implementations.
10 Ordinary and Exotic Objects Behaviours
10.1 Ordinary Object Internal Methods and Internal Slots
All ordinary objects have an internal slot called [[Prototype]]. The value of this internal slot is either null or an object and is used for implementing inheritance. Assume a property named P is missing from an ordinary objectO but exists on its [[Prototype]] object. If P refers to a data property on the [[Prototype]] object, O inherits it for get access, making it behave as if P was a property of O. If P refers to a writable data property on the [[Prototype]] object, set access of P on O creates a new data property named P on O. If P refers to a non-writable data property on the [[Prototype]] object, set access of P on O fails. If P refers to an accessor property on the [[Prototype]] object, the accessor is inherited by O for both get access and set access.
Every ordinary object has a Boolean-valued [[Extensible]] internal slot which is used to fulfill the extensibility-related internal method invariants specified in 6.1.7.3. Namely, once the value of an object's [[Extensible]] internal slot has been set to false, it is no longer possible to add properties to the object, to modify the value of the object's [[Prototype]] internal slot, or to subsequently change the value of [[Extensible]] to true.
Each ordinary object internal method delegates to a similarly-named abstract operation. If such an abstract operation depends on another internal method, then the internal method is invoked on O rather than calling the similarly-named abstract operation directly. These semantics ensure that exotic objects have their overridden internal methods invoked when ordinary object internal methods are applied to them.
10.1.1[[GetPrototypeOf]] ( )
The [[GetPrototypeOf]] internal method of an ordinary objectO takes no arguments and returns a normal completion containing either an Object or null. It performs the following steps when called:
The abstract operation OrdinaryGetPrototypeOf takes argument O (an Object) and returns an Object or null. It performs the following steps when called:
Return O.[[Prototype]].
10.1.2[[SetPrototypeOf]] ( V )
The [[SetPrototypeOf]] internal method of an ordinary objectO takes argument V (an Object or null) and returns a normal completion containing a Boolean. It performs the following steps when called:
The abstract operation OrdinarySetPrototypeOf takes arguments O (an Object) and V (an Object or null) and returns a Boolean. It performs the following steps when called:
If p.[[GetPrototypeOf]] is not the ordinary object internal method defined in 10.1.1, set done to true.
Else, set p to p.[[Prototype]].
Set O.[[Prototype]] to V.
Return true.
Note
The loop in step 7 guarantees that there will be no cycles in any prototype chain that only includes objects that use the ordinary object definitions for [[GetPrototypeOf]] and [[SetPrototypeOf]].
10.1.3[[IsExtensible]] ( )
The [[IsExtensible]] internal method of an ordinary objectO takes no arguments and returns a normal completion containing a Boolean. It performs the following steps when called:
The abstract operation OrdinaryIsExtensible takes argument O (an Object) and returns a Boolean. It performs the following steps when called:
Return O.[[Extensible]].
10.1.4[[PreventExtensions]] ( )
The [[PreventExtensions]] internal method of an ordinary objectO takes no arguments and returns a normal completion containingtrue. It performs the following steps when called:
The abstract operation OrdinaryGetOwnProperty takes arguments O (an Object) and P (a property key) and returns a Property Descriptor or undefined. It performs the following steps when called:
If O does not have an own property with key P, return undefined.
10.1.6.2 IsCompatiblePropertyDescriptor ( Extensible, Desc, Current )
The abstract operation IsCompatiblePropertyDescriptor takes arguments Extensible (a Boolean), Desc (a Property Descriptor), and Current (a Property Descriptor or undefined) and returns a Boolean. It performs the following steps when called:
10.1.6.3 ValidateAndApplyPropertyDescriptor ( O, P, extensible, Desc, current )
The abstract operation ValidateAndApplyPropertyDescriptor takes arguments O (an Object or undefined), P (a property key), extensible (a Boolean), Desc (a Property Descriptor), and current (a Property Descriptor or undefined) and returns a Boolean. It returns true if and only if Desc can be applied as the property of an object with specified extensibility and current property current while upholding invariants. When such application is possible and O is not undefined, it is performed for the property named P (which is created if necessary). It performs the following steps when called:
Create an own accessor property named P of object O whose [[Get]], [[Set]], [[Enumerable]], and [[Configurable]] attributes are set to the value of the corresponding field in Desc if Desc has that field, or to the attribute's default value otherwise.
Else,
Create an own data property named P of object O whose [[Value]], [[Writable]], [[Enumerable]], and [[Configurable]] attributes are set to the value of the corresponding field in Desc if Desc has that field, or to the attribute's default value otherwise.
If Desc has a [[Configurable]] field, let configurable be Desc.[[Configurable]]; else let configurable be current.[[Configurable]].
If Desc has a [[Enumerable]] field, let enumerable be Desc.[[Enumerable]]; else let enumerable be current.[[Enumerable]].
Replace the property named P of object O with an accessor property whose [[Configurable]] and [[Enumerable]] attributes are set to configurable and enumerable, respectively, and whose [[Get]] and [[Set]] attributes are set to the value of the corresponding field in Desc if Desc has that field, or to the attribute's default value otherwise.
If Desc has a [[Configurable]] field, let configurable be Desc.[[Configurable]]; else let configurable be current.[[Configurable]].
If Desc has a [[Enumerable]] field, let enumerable be Desc.[[Enumerable]]; else let enumerable be current.[[Enumerable]].
Replace the property named P of object O with a data property whose [[Configurable]] and [[Enumerable]] attributes are set to configurable and enumerable, respectively, and whose [[Value]] and [[Writable]] attributes are set to the value of the corresponding field in Desc if Desc has that field, or to the attribute's default value otherwise.
Else,
For each field of Desc, set the corresponding attribute of the property named P of object O to the value of the field.
The abstract operation OrdinaryOwnPropertyKeys takes argument O (an Object) and returns a List of property keys. It performs the following steps when called:
For each own property keyP of O such that P is an array index, in ascending numeric index order, do
Append P to keys.
For each own property keyP of O such that Pis a String and P is not an array index, in ascending chronological order of property creation, do
Append P to keys.
For each own property keyP of O such that Pis a Symbol, in ascending chronological order of property creation, do
Append P to keys.
Return keys.
10.1.12 OrdinaryObjectCreate ( proto [ , additionalInternalSlotsList ] )
The abstract operation OrdinaryObjectCreate takes argument proto (an Object or null) and optional argument additionalInternalSlotsList (a List of names of internal slots) and returns an Object. It is used to specify the runtime creation of new ordinary objects. additionalInternalSlotsList contains the names of additional internal slots that must be defined as part of the object, beyond [[Prototype]] and [[Extensible]]. If additionalInternalSlotsList is not provided, a new empty List is used. It performs the following steps when called:
Let internalSlotsList be « [[Prototype]], [[Extensible]] ».
If additionalInternalSlotsList is present, set internalSlotsList to the list-concatenation of internalSlotsList and additionalInternalSlotsList.
Although OrdinaryObjectCreate does little more than call MakeBasicObject, its use communicates the intention to create an ordinary object, and not an exotic one. Thus, within this specification, it is not called by any algorithm that subsequently modifies the internal methods of the object in ways that would make the result non-ordinary. Operations that create exotic objects invoke MakeBasicObject directly.
The abstract operation OrdinaryCreateFromConstructor takes arguments constructor (a constructor) and intrinsicDefaultProto (a String) and optional argument internalSlotsList (a List of names of internal slots) and returns either a normal completion containing an Object or a throw completion. It creates an ordinary object whose [[Prototype]] value is retrieved from a constructor's "prototype" property, if it exists. Otherwise the intrinsic named by intrinsicDefaultProto is used for [[Prototype]]. internalSlotsList contains the names of additional internal slots that must be defined as part of the object. If internalSlotsList is not provided, a new empty List is used. It performs the following steps when called:
Assert: intrinsicDefaultProto is this specification's name of an intrinsic object. The corresponding object must be an intrinsic that is intended to be used as the [[Prototype]] value of an object.
The abstract operation GetPrototypeFromConstructor takes arguments constructor (a function object) and intrinsicDefaultProto (a String) and returns either a normal completion containing an Object or a throw completion. It determines the [[Prototype]] value that should be used to create an object corresponding to a specific constructor. The value is retrieved from the constructor's "prototype" property, if it exists. Otherwise the intrinsic named by intrinsicDefaultProto is used for [[Prototype]]. It performs the following steps when called:
Assert: intrinsicDefaultProto is this specification's name of an intrinsic object. The corresponding object must be an intrinsic that is intended to be used as the [[Prototype]] value of an object.
Set proto to realm's intrinsic object named intrinsicDefaultProto.
Return proto.
Note
If constructor does not supply a [[Prototype]] value, the default value that is used is obtained from the realm of the constructor function rather than from the running execution context.
10.1.15 RequireInternalSlot ( O, internalSlot )
The abstract operation RequireInternalSlot takes arguments O (an ECMAScript language value) and internalSlot (an internal slot name) and returns either a normal completion containingunused or a throw completion. It throws an exception unless Ois an Object and has the given internal slot. It performs the following steps when called:
The PrivateEnvironment Record for Private Names that the function was closed over. null if this function is not syntactically contained within a class. Used as the outer PrivateEnvironment for inner classes when evaluating the code of the function.
The script or module in which the function was created.
[[ThisMode]]
lexical, strict, or global
Defines how this references are interpreted within the formal parameters and code body of the function. lexical means that this refers to the this value of a lexically enclosing function. strict means that the this value is used exactly as provided by an invocation of the function. global means that a this value of undefined or null is interpreted as a reference to the global object, and any other this value is first passed to ToObject.
If the function is created as the initializer of a class field, the name to use for NamedEvaluation of the field; empty otherwise.
[[IsClassConstructor]]
a Boolean
Indicates whether the function is a class constructor. (If true, invoking the function's [[Call]] will immediately throw a TypeError exception.)
All ECMAScript function objects have the [[Call]] internal method defined here. ECMAScript functions that are also constructors in addition have the [[Construct]] internal method.
When calleeContext is removed from the execution context stack in step 7 it must not be destroyed if it is suspended and retained for later resumption by an accessible Generator.
10.2.1.1 PrepareForOrdinaryCall ( F, newTarget )
The abstract operation PrepareForOrdinaryCall takes arguments F (an ECMAScript function object) and newTarget (an Object or undefined) and returns an execution context. It performs the following steps when called:
The abstract operation OrdinaryCallBindThis takes arguments F (an ECMAScript function object), calleeContext (an execution context), and thisArgument (an ECMAScript language value) and returns unused. It performs the following steps when called:
Let thisMode be F.[[ThisMode]].
If thisMode is lexical, return unused.
Let calleeRealm be F.[[Realm]].
Let localEnv be the LexicalEnvironment of calleeContext.
Even though field initializers constitute a function boundary, calling FunctionDeclarationInstantiation does not have any observable effect and so is omitted.
The abstract operation OrdinaryFunctionCreate takes arguments functionPrototype (an Object), sourceText (a sequence of Unicode code points), ParameterList (a Parse Node), Body (a Parse Node), thisMode (lexical-this or non-lexical-this), env (an Environment Record), and privateEnv (a PrivateEnvironment Record or null) and returns an ECMAScript function object. It is used to specify the runtime creation of a new function with a default [[Call]] internal method and no [[Construct]] internal method (although one may be subsequently added by an operation such as MakeConstructor). sourceText is the source text of the syntactic definition of the function to be created. It performs the following steps when called:
Let internalSlotsList be the internal slots listed in Table 30.
The abstract operation AddRestrictedFunctionProperties takes arguments F (a function object) and realm (a Realm Record) and returns unused. It performs the following steps when called:
The abstract operation MakeConstructor takes argument F (an ECMAScript function object or a built-in function object) and optional arguments writablePrototype (a Boolean) and prototype (an Object) and returns unused. It converts F into a constructor. It performs the following steps when called:
The abstract operation MakeClassConstructor takes argument F (an ECMAScript function object) and returns unused. It performs the following steps when called:
The abstract operation MakeMethod takes arguments F (an ECMAScript function object) and homeObject (an Object) and returns unused. It configures F as a method. It performs the following steps when called:
The abstract operation SetFunctionName takes arguments F (a function object) and name (a property key or Private Name) and optional argument prefix (a String) and returns unused. It adds a "name" property to F. It performs the following steps when called:
Assert: F is an extensible object that does not have a "name" own property.
The abstract operation SetFunctionLength takes arguments F (a function object) and length (a non-negative integer or +∞) and returns unused. It adds a "length" property to F. It performs the following steps when called:
Assert: F is an extensible object that does not have a "length" own property.
When an execution context is established for evaluating an ECMAScript function a new Function Environment Record is created and bindings for each formal parameter are instantiated in that Environment Record. Each declaration in the function body is also instantiated. If the function's formal parameters do not include any default value initializers then the body declarations are instantiated in the same Environment Record as the parameters. If default value parameter initializers exist, a second Environment Record is created for the body declarations. Formal parameters and functions are initialized as part of FunctionDeclarationInstantiation. All other bindings are initialized during evaluation of the function body.
Let fn be the sole element of the BoundNames of d.
If functionNames does not contain fn, then
Insert fn as the first element of functionNames.
NOTE: If there are multiple function declarations for the same name, the last declaration is used.
Insert d as the first element of functionsToInitialize.
Let argumentsObjectNeeded be true.
If func.[[ThisMode]] is lexical, then
NOTE: Arrow functions never have an arguments object.
Set argumentsObjectNeeded to false.
Else if parameterNames contains "arguments", then
Set argumentsObjectNeeded to false.
Else if hasParameterExpressions is false, then
If functionNames contains "arguments" or lexicalNames contains "arguments", then
Set argumentsObjectNeeded to false.
If strict is true or hasParameterExpressions is false, then
NOTE: Only a single Environment Record is needed for the parameters, since calls to eval in strict mode code cannot create new bindings which are visible outside of the eval.
Let env be the LexicalEnvironment of calleeContext.
Else,
NOTE: A separate Environment Record is needed to ensure that bindings created by direct eval calls in the formal parameter list are outside the environment where parameters are declared.
Let calleeEnv be the LexicalEnvironment of calleeContext.
Set the LexicalEnvironment of calleeContext to env.
For each String paramName of parameterNames, do
Let alreadyDeclared be ! env.HasBinding(paramName).
NOTE: Early errors ensure that duplicate parameter names can only occur in non-strict functions that do not have parameter default values or rest parameters.
NOTE: A mapped argument object is only provided for non-strict functions that don't have a rest parameter, any parameter default value initializers, or any destructured parameters.
NOTE: Only a single Environment Record is needed for the parameters and top-level vars.
Let instantiatedVarNames be a copy of the ListparameterBindings.
For each element n of varNames, do
If instantiatedVarNames does not contain n, then
Append n to instantiatedVarNames.
Perform ! env.CreateMutableBinding(n, false).
Perform ! env.InitializeBinding(n, undefined).
Let varEnv be env.
Else,
NOTE: A separate Environment Record is needed to ensure that closures created by expressions in the formal parameter list do not have visibility of declarations in the function body.
NOTE: Non-strict functions use a separate Environment Record for top-level lexical declarations so that a direct eval can determine whether any var scoped declarations introduced by the eval code conflict with pre-existing top-level lexically scoped declarations. This is not needed for strict functions because a strict direct eval always places all declarations into a new Environment Record.
Else,
Let lexEnv be varEnv.
Set the LexicalEnvironment of calleeContext to lexEnv.
NOTE: A lexically declared name cannot be the same as a function/generator declaration, formal parameter, or a var name. Lexically declared names are only instantiated here but not initialized.
B.3.2 provides an extension to the above algorithm that is necessary for backwards compatibility with web browser implementations of ECMAScript that predate ECMAScript 2015.
In addition to the internal slots required of every ordinary object (see 10.1), a built-in function object must also have the following internal slots:
[[Realm]], a Realm Record that represents the realm in which the function was created.
[[InitialName]], a String that is the initial name of the function. It is used by 20.2.3.5.
A built-in function object must have a [[Call]] internal method that conforms to the definition in 10.3.1.
A built-in function object has a [[Construct]] internal method if and only if it is described as a “constructor”, or some algorithm in this specification explicitly sets its [[Construct]] internal method. Such a [[Construct]] internal method must conform to the definition in 10.3.2.
An implementation may provide additional built-in function objects that are not defined in this specification.
Let result be the Completion Record that is the result of evaluatingF in a manner that conforms to the specification of F. If thisArgument is uninitialized, the this value is uninitialized; otherwise, thisArgument provides the this value. argumentsList provides the named parameters. newTarget provides the NewTarget value.
NOTE: If F is defined in this document, “the specification of F” is the behaviour specified for it via algorithm steps or other means.
When calleeContext is removed from the execution context stack it must not be destroyed if it has been suspended and retained by an accessible Generator for later resumption.
The abstract operation CreateBuiltinFunction takes arguments behaviour (an Abstract Closure, a set of algorithm steps, or some other definition of a function's behaviour provided in this specification), length (a non-negative integer or +∞), name (a property key or a Private Name), and additionalInternalSlotsList (a List of names of internal slots) and optional arguments realm (a Realm Record), prototype (an Object or null), and prefix (a String) and returns a function object. additionalInternalSlotsList contains the names of additional internal slots that must be defined as part of the object. This operation creates a built-in function object. It performs the following steps when called:
If prototype is not present, set prototype to realm.[[Intrinsics]].[[%Function.prototype%]].
Let internalSlotsList be a List containing the names of all the internal slots that 10.3 requires for the built-in function object that is about to be created.
Append to internalSlotsList the elements of additionalInternalSlotsList.
Let func be a new built-in function object that, when called, performs the action described by behaviour using the provided arguments as the values of the corresponding parameters specified by behaviour. The new function object has internal slots whose names are the elements of internalSlotsList, and an [[InitialName]] internal slot.
Each built-in function defined in this specification is created by calling the CreateBuiltinFunction abstract operation.
10.4 Built-in Exotic Object Internal Methods and Slots
This specification defines several kinds of built-in exotic objects. These objects generally behave similar to ordinary objects except for a few specific situations. The following exotic objects use the ordinary object internal methods except where it is explicitly specified otherwise below:
An object is a bound function exotic object if its [[Call]] and (if applicable) [[Construct]] internal methods use the following implementations, and its other essential internal methods use the definitions found in 10.1. These methods are installed in BoundFunctionCreate.
Set obj.[[BoundTargetFunction]] to targetFunction.
Set obj.[[BoundThis]] to boundThis.
Set obj.[[BoundArguments]] to boundArgs.
Return obj.
10.4.2 Array Exotic Objects
An Array is an exotic object that gives special treatment to array indexproperty keys (see 6.1.7). A property whose property name is an array index is also called an element. Every Array has a non-configurable "length" property whose value is always a non-negative integral Number whose mathematical value is strictly less than 232. The value of the "length" property is numerically greater than the name of every own property whose name is an array index; whenever an own property of an Array is created or changed, other properties are adjusted as necessary to maintain this invariant. Specifically, whenever an own property is added whose name is an array index, the value of the "length" property is changed, if necessary, to be one more than the numeric value of that array index; and whenever the value of the "length" property is changed, every own property whose name is an array index whose value is not smaller than the new length is deleted. This constraint applies only to own properties of an Array and is unaffected by "length" or array index properties that may be inherited from its prototypes.
An object is an Array exotic object (or simply, an Array) if its [[DefineOwnProperty]] internal method uses the following implementation, and its other essential internal methods use the definitions found in 10.1. These methods are installed in ArrayCreate.
The abstract operation ArrayCreate takes argument length (a non-negative integer) and optional argument proto (an Object) and returns either a normal completion containing an Array exotic object or a throw completion. It is used to specify the creation of new Arrays. It performs the following steps when called:
If length > 232 - 1, throw a RangeError exception.
The abstract operation ArraySpeciesCreate takes arguments originalArray (an Object) and length (a non-negative integer) and returns either a normal completion containing an Object or a throw completion. It is used to specify the creation of a new Array or similar object using a constructor function that is derived from originalArray. It does not enforce that the constructor function returns an Array. It performs the following steps when called:
If originalArray was created using the standard built-in Array constructor for a realm that is not the realm of the running execution context, then a new Array is created using the realm of the running execution context. This maintains compatibility with Web browsers that have historically had that behaviour for the Array.prototype methods that now are defined using ArraySpeciesCreate.
In steps 3 and 4, if Desc.[[Value]] is an object then its valueOf method is called twice. This is legacy behaviour that was specified with this effect starting with the 2nd Edition of this specification.
10.4.3 String Exotic Objects
A String object is an exotic object that encapsulates a String value and exposes virtual integer-indexeddata properties corresponding to the individual code unit elements of the String value. String exotic objects always have a data property named "length" whose value is the length of the encapsulated String value. Both the code unit data properties and the "length" property are non-writable and non-configurable.
An object is a String exotic object (or simply, a String object) if its [[GetOwnProperty]], [[DefineOwnProperty]], and [[OwnPropertyKeys]] internal methods use the following implementations, and its other essential internal methods use the definitions found in 10.1. These methods are installed in StringCreate.
For each own property keyP of O such that Pis a String and P is not an array index, in ascending chronological order of property creation, do
Append P to keys.
For each own property keyP of O such that Pis a Symbol, in ascending chronological order of property creation, do
Append P to keys.
Return keys.
10.4.3.4 StringCreate ( value, prototype )
The abstract operation StringCreate takes arguments value (a String) and prototype (an Object) and returns a String exotic object. It is used to specify the creation of new String exotic objects. It performs the following steps when called:
Let S be MakeBasicObject(« [[Prototype]], [[Extensible]], [[StringData]] »).
Set S.[[Prototype]] to prototype.
Set S.[[StringData]] to value.
Set S.[[GetOwnProperty]] as specified in 10.4.3.1.
Set S.[[DefineOwnProperty]] as specified in 10.4.3.2.
Set S.[[OwnPropertyKeys]] as specified in 10.4.3.3.
The abstract operation StringGetOwnProperty takes arguments S (an Object that has a [[StringData]] internal slot) and P (a property key) and returns a Property Descriptor or undefined. It performs the following steps when called:
Most ECMAScript functions make an arguments object available to their code. Depending upon the characteristics of the function definition, its arguments object is either an ordinary object or an arguments exotic object. An arguments exotic object is an exotic object whose array index properties map to the formal parameters bindings of an invocation of its associated ECMAScript function.
An object is an arguments exotic object if its internal methods use the following implementations, with the ones not specified here using those found in 10.1. These methods are installed in CreateMappedArgumentsObject.
Arguments exotic objects have the same internal slots as ordinary objects. They also have a [[ParameterMap]] internal slot. Ordinary arguments objects also have a [[ParameterMap]] internal slot whose value is always undefined. For ordinary argument objects the [[ParameterMap]] internal slot is only used by Object.prototype.toString (20.1.3.6) to identify them as such.
Note 2
The integer-indexeddata properties of an arguments exotic object whose numeric name values are less than the number of formal parameters of the corresponding function object initially share their values with the corresponding argument bindings in the function's execution context. This means that changing the property changes the corresponding value of the argument binding and vice-versa. This correspondence is broken if such a property is deleted and then redefined or if the property is changed into an accessor property. If the arguments object is an ordinary object, the values of its properties are simply a copy of the arguments passed to the function and there is no dynamic linkage between the property values and the formal parameter values.
Note 3
The ParameterMap object and its property values are used as a device for specifying the arguments object correspondence to argument bindings. The ParameterMap object and the objects that are the values of its properties are not directly observable from ECMAScript code. An ECMAScript implementation does not need to actually create or use such objects to implement the specified semantics.
Note 4
Ordinary arguments objects define a non-configurable accessor property named "callee" which throws a TypeError exception on access. The "callee" property has a more specific meaning for arguments exotic objects, which are created only for some class of non-strict functions. The definition of this property in the ordinary variant exists to ensure that it is not defined in any other manner by conforming ECMAScript implementations.
Note 5
ECMAScript implementations of arguments exotic objects have historically contained an accessor property named "caller". Prior to ECMAScript 2017, this specification included the definition of a throwing "caller" property on ordinary arguments objects. Since implementations do not contain this extension any longer, ECMAScript 2017 dropped the requirement for a throwing "caller" accessor.
The abstract operation CreateUnmappedArgumentsObject takes argument argumentsList (a List of ECMAScript language values) and returns an ordinary object. It performs the following steps when called:
Let len be the number of elements in argumentsList.
The abstract operation MakeArgGetter takes arguments name (a String) and env (an Environment Record) and returns a function object. It creates a built-in function object that when executed returns the value bound for name in env. It performs the following steps when called:
Let getterClosure be a new Abstract Closure with no parameters that captures name and env and performs the following steps when called:
NOTE: getter is never directly accessible to ECMAScript code.
Return getter.
10.4.4.7.2 MakeArgSetter ( name, env )
The abstract operation MakeArgSetter takes arguments name (a String) and env (an Environment Record) and returns a function object. It creates a built-in function object that when executed sets the value bound for name in env. It performs the following steps when called:
Let setterClosure be a new Abstract Closure with parameters (value) that captures name and env and performs the following steps when called:
TypedArrays have the same internal slots as ordinary objects and additionally [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], [[ContentType]], and [[TypedArrayName]] internal slots.
An object is a TypedArray if its [[GetOwnProperty]], [[HasProperty]], [[DefineOwnProperty]], [[Get]], [[Set]], [[Delete]], and [[OwnPropertyKeys]] internal methods use the definitions in this section, and its other essential internal methods use the definitions found in 10.1. These methods are installed by TypedArrayCreate.
For each own property keyP of O such that Pis a Symbol, in ascending chronological order of property creation, do
Append P to keys.
Return keys.
10.4.5.8 TypedArray With Buffer Witness Records
An TypedArray With Buffer Witness Record is a Record value used to encapsulate a TypedArray along with a cached byte length of the viewed buffer. It is used to help ensure there is a single shared memory read event of the byte length data block when the viewed buffer is a growable SharedArrayBuffer.
TypedArray With Buffer Witness Records have the fields listed in Table 32.
The byte length of the object's [[ViewedArrayBuffer]] when the Record was created.
10.4.5.9 MakeTypedArrayWithBufferWitnessRecord ( obj, order )
The abstract operation MakeTypedArrayWithBufferWitnessRecord takes arguments obj (a TypedArray) and order (seq-cst or unordered) and returns a TypedArray With Buffer Witness Record. It performs the following steps when called:
The abstract operation TypedArrayCreate takes argument prototype (an Object) and returns a TypedArray. It is used to specify the creation of new TypedArrays. It performs the following steps when called:
Let internalSlotsList be « [[Prototype]], [[Extensible]], [[ViewedArrayBuffer]], [[TypedArrayName]], [[ContentType]], [[ByteLength]], [[ByteOffset]], [[ArrayLength]] ».
Set A.[[OwnPropertyKeys]] as specified in 10.4.5.7.
Set A.[[Prototype]] to prototype.
Return A.
10.4.5.11 TypedArrayByteLength ( taRecord )
The abstract operation TypedArrayByteLength takes argument taRecord (a TypedArray With Buffer Witness Record) and returns a non-negative integer. It performs the following steps when called:
The abstract operation TypedArrayLength takes argument taRecord (a TypedArray With Buffer Witness Record) and returns a non-negative integer. It performs the following steps when called:
The abstract operation IsTypedArrayOutOfBounds takes argument taRecord (a TypedArray With Buffer Witness Record) and returns a Boolean. It checks if any of the object's numeric properties reference a value at an index not contained within the underlying buffer's bounds. It performs the following steps when called:
Let O be taRecord.[[Object]].
Let bufferByteLength be taRecord.[[CachedBufferByteLength]].
Assert: IsDetachedBuffer(O.[[ViewedArrayBuffer]]) is true if and only if bufferByteLength is detached.
Let byteOffsetEnd be byteOffsetStart + O.[[ArrayLength]] × elementSize.
If byteOffsetStart > bufferByteLength or byteOffsetEnd > bufferByteLength, return true.
NOTE: 0-length TypedArrays are not considered out-of-bounds.
Return false.
10.4.5.14 IsValidIntegerIndex ( O, index )
The abstract operation IsValidIntegerIndex takes arguments O (a TypedArray) and index (a Number) and returns a Boolean. It performs the following steps when called:
If IsDetachedBuffer(O.[[ViewedArrayBuffer]]) is true, return false.
If ℝ(index) < 0 or ℝ(index) ≥ length, return false.
Return true.
10.4.5.15 TypedArrayGetElement ( O, index )
The abstract operation TypedArrayGetElement takes arguments O (a TypedArray) and index (a Number) and returns a Number, a BigInt, or undefined. It performs the following steps when called:
This operation always appears to succeed, but it has no effect when attempting to write past the end of a TypedArray or to a TypedArray which is backed by a detached ArrayBuffer.
10.4.5.17 IsArrayBufferViewOutOfBounds ( O )
The abstract operation IsArrayBufferViewOutOfBounds takes argument O (a TypedArray or a DataView) and returns a Boolean. It checks if either any of a TypedArray's numeric properties or a DataView object's methods can reference a value at an index not contained within the underlying data block's bounds. This abstract operation exists as a convenience for upstream specifications. It performs the following steps when called:
A module namespace exotic object is an exotic object that exposes the bindings exported from an ECMAScript Module (See 16.2.3). There is a one-to-one correspondence between the String-keyed own properties of a module namespace exotic object and the binding names exported by the Module. The exported bindings include any bindings that are indirectly exported using export * export items. Each String-valued own property key is the StringValue of the corresponding exported binding name. These are the only String-keyed properties of a module namespace exotic object. Each such property has the attributes { [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: false }. Module namespace exotic objects are not extensible.
An object is a module namespace exotic object if its [[GetPrototypeOf]], [[SetPrototypeOf]], [[IsExtensible]], [[PreventExtensions]], [[GetOwnProperty]], [[DefineOwnProperty]], [[HasProperty]], [[Get]], [[Set]], [[Delete]], and [[OwnPropertyKeys]] internal methods use the definitions in this section, and its other essential internal methods use the definitions found in 10.1. These methods are installed by ModuleNamespaceCreate.
A List whose elements are the String values of the exported names exposed as own properties of this object. The list is sorted according to lexicographic code unit order.
ResolveExport is side-effect free. Each time this operation is called with a specific exportName, resolveSet pair as arguments it must return the same result. An implementation might choose to pre-compute or cache the ResolveExport results for the [[Exports]] of each module namespace exotic object.
An object is an immutable prototype exotic object if its [[SetPrototypeOf]] internal method uses the following implementation. (Its other essential internal methods may use any implementation, depending on the specific immutable prototype exotic object in question.)
The abstract operation SetImmutablePrototype takes arguments O (an Object) and V (an Object or null) and returns either a normal completion containing a Boolean or a throw completion. It performs the following steps when called:
10.5 Proxy Object Internal Methods and Internal Slots
A Proxy object is an exotic object whose essential internal methods are partially implemented using ECMAScript code. Every Proxy object has an internal slot called [[ProxyHandler]]. The value of [[ProxyHandler]] is an object, called the proxy's handler object, or null. Methods (see Table 34) of a handler object may be used to augment the implementation for one or more of the Proxy object's internal methods. Every Proxy object also has an internal slot called [[ProxyTarget]] whose value is either an object or the null value. This object is called the proxy's target object.
An object is a Proxy exotic object if its essential internal methods (including [[Call]] and [[Construct]], if applicable) use the definitions in this section. These internal methods are installed in ProxyCreate.
Table 34: Proxy Handler Methods
Internal Method
Handler Method
[[GetPrototypeOf]]
getPrototypeOf
[[SetPrototypeOf]]
setPrototypeOf
[[IsExtensible]]
isExtensible
[[PreventExtensions]]
preventExtensions
[[GetOwnProperty]]
getOwnPropertyDescriptor
[[DefineOwnProperty]]
defineProperty
[[HasProperty]]
has
[[Get]]
get
[[Set]]
set
[[Delete]]
deleteProperty
[[OwnPropertyKeys]]
ownKeys
[[Call]]
apply
[[Construct]]
construct
When a handler method is called to provide the implementation of a Proxy object internal method, the handler method is passed the proxy's target object as a parameter. A proxy's handler object does not necessarily have a method corresponding to every essential internal method. Invoking an internal method on the proxy results in the invocation of the corresponding internal method on the proxy's target object if the handler object does not have a method corresponding to the internal trap.
The [[ProxyHandler]] and [[ProxyTarget]] internal slots of a Proxy object are always initialized when the object is created and typically may not be modified. Some Proxy objects are created in a manner that permits them to be subsequently revoked. When a proxy is revoked, its [[ProxyHandler]] and [[ProxyTarget]] internal slots are set to null causing subsequent invocations of internal methods on that Proxy object to throw a TypeError exception.
Because Proxy objects permit the implementation of internal methods to be provided by arbitrary ECMAScript code, it is possible to define a Proxy object whose handler methods violates the invariants defined in 6.1.7.3. Some of the internal method invariants defined in 6.1.7.3 are essential integrity invariants. These invariants are explicitly enforced by the Proxy object internal methods specified in this section. An ECMAScript implementation must be robust in the presence of all possible invariant violations.
If SameValue(handlerProto, targetProto) is false, throw a TypeError exception.
Return handlerProto.
Note
[[GetPrototypeOf]] for Proxy objects enforces the following invariants:
The result of [[GetPrototypeOf]] must be either an Object or null.
If the target object is not extensible, [[GetPrototypeOf]] applied to the Proxy object must return the same value as [[GetPrototypeOf]] applied to the Proxy object's target object.
[[IsExtensible]] applied to the Proxy object must return the same value as [[IsExtensible]] applied to the Proxy object's target object with the same argument.
If targetDesc.[[Writable]] is true, throw a TypeError exception.
Return resultDesc.
Note
[[GetOwnProperty]] for Proxy objects enforces the following invariants:
The result of [[GetOwnProperty]] must be either an Object or undefined.
A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.
A property cannot be reported as non-existent, if it exists as an own property of a non-extensible target object.
A property cannot be reported as existent, if it does not exist as an own property of the target object and the target object is not extensible.
A property cannot be reported as non-configurable, unless it exists as a non-configurable own property of the target object.
A property cannot be reported as both non-configurable and non-writable, unless it exists as a non-configurable, non-writable own property of the target object.
If settingConfigFalse is true and targetDesc.[[Configurable]] is true, throw a TypeError exception.
If IsDataDescriptor(targetDesc) is true, targetDesc.[[Configurable]] is false, and targetDesc.[[Writable]] is true, then
If Desc has a [[Writable]] field and Desc.[[Writable]] is false, throw a TypeError exception.
Return true.
Note
[[DefineOwnProperty]] for Proxy objects enforces the following invariants:
The result of [[DefineOwnProperty]]is a Boolean value.
A property cannot be added, if the target object is not extensible.
A property cannot be non-configurable, unless there exists a corresponding non-configurable own property of the target object.
A non-configurable property cannot be non-writable, unless there exists a corresponding non-configurable, non-writable own property of the target object.
If a property has a corresponding target object property then applying the Property Descriptor of the property to the target object using [[DefineOwnProperty]] will not throw an exception.
Let trapResult be ? Call(trap, handler, « target, P, Receiver »).
Let targetDesc be ? target.[[GetOwnProperty]](P).
If targetDesc is not undefined and targetDesc.[[Configurable]] is false, then
If IsDataDescriptor(targetDesc) is true and targetDesc.[[Writable]] is false, then
If SameValue(trapResult, targetDesc.[[Value]]) is false, throw a TypeError exception.
If IsAccessorDescriptor(targetDesc) is true and targetDesc.[[Get]] is undefined, then
If trapResult is not undefined, throw a TypeError exception.
Return trapResult.
Note
[[Get]] for Proxy objects enforces the following invariants:
The value reported for a property must be the same as the value of the corresponding target object property if the target object property is a non-writable, non-configurable own data property.
The value reported for a property must be undefined if the corresponding target object property is a non-configurable own accessor property that has undefined as its [[Get]] attribute.
Cannot change the value of a property to be different from the value of the corresponding target object property if the corresponding target object property is a non-writable, non-configurable own data property.
Cannot set the value of a property if the corresponding target object property is a non-configurable own accessor property that has undefined as its [[Set]] attribute.
A Proxy exotic object only has a [[Call]] internal method if the initial value of its [[ProxyTarget]] internal slot is an object that has a [[Call]] internal method.
A Proxy exotic object only has a [[Construct]] internal method if the initial value of its [[ProxyTarget]] internal slot is an object that has a [[Construct]] internal method.
Note 2
[[Construct]] for Proxy objects enforces the following invariants:
The result of [[Construct]] must be an Object.
10.5.14 ValidateNonRevokedProxy ( proxy )
The abstract operation ValidateNonRevokedProxy takes argument proxy (a Proxy exotic object) and returns either a normal completion containingunused or a throw completion. It throws a TypeError exception if proxy has been revoked. It performs the following steps when called:
If proxy.[[ProxyTarget]] is null, throw a TypeError exception.
ECMAScript source text is a sequence of Unicode code points. All Unicode code point values from U+0000 to U+10FFFF, including surrogate code points, may occur in ECMAScript source text where permitted by the ECMAScript grammars. The actual encodings used to store and interchange ECMAScript source text is not relevant to this specification. Regardless of the external source text encoding, a conforming ECMAScript implementation processes the source text as if it was an equivalent sequence of SourceCharacter values, each SourceCharacter being a Unicode code point. Conforming ECMAScript implementations are not required to perform any normalization of source text, or behave as though they were performing normalization of source text.
The components of a combining character sequence are treated as individual Unicode code points even though a user might think of the whole sequence as a single character.
Note
In string literals, regular expression literals, template literals and identifiers, any Unicode code point may also be expressed using Unicode escape sequences that explicitly express a code point's numeric value. Within a comment, such an escape sequence is effectively ignored as part of the comment.
ECMAScript differs from the Java programming language in the behaviour of Unicode escape sequences. In a Java program, if the Unicode escape sequence \u000A, for example, occurs within a single-line comment, it is interpreted as a line terminator (Unicode code point U+000A is LINE FEED (LF)) and therefore the next code point is not part of the comment. Similarly, if the Unicode escape sequence \u000A occurs within a string literal in a Java program, it is likewise interpreted as a line terminator, which is not allowed within a string literal—one must write \n instead of \u000A to cause a LINE FEED (LF) to be part of the String value of a string literal. In an ECMAScript program, a Unicode escape sequence occurring within a comment is never interpreted and therefore cannot contribute to termination of the comment. Similarly, a Unicode escape sequence occurring within a string literal in an ECMAScript program always contributes to the literal and is never interpreted as a line terminator or as a code point that might terminate the string literal.
The abstract operation UTF16EncodeCodePoint takes argument cp (a Unicode code point) and returns a String. It performs the following steps when called:
11.1.2 Static Semantics: CodePointsToString ( text )
The abstract operation CodePointsToString takes argument text (a sequence of Unicode code points) and returns a String. It converts text into a String value, as described in 6.1.4. It performs the following steps when called:
The abstract operation UTF16SurrogatePairToCodePoint takes arguments lead (a code unit) and trail (a code unit) and returns a code point. Two code units that form a UTF-16 surrogate pair are converted to a code point. It performs the following steps when called:
Let cp be (lead - 0xD800) × 0x400 + (trail - 0xDC00) + 0x10000.
Return the code point cp.
11.1.4 Static Semantics: CodePointAt ( string, position )
The abstract operation CodePointAt takes arguments string (a String) and position (a non-negative integer) and returns a Record with fields [[CodePoint]] (a code point), [[CodeUnitCount]] (a positive integer), and [[IsUnpairedSurrogate]] (a Boolean). It interprets string as a sequence of UTF-16 encoded code points, as described in 6.1.4, and reads from it a single code point starting with the code unit at index position. It performs the following steps when called:
The abstract operation StringToCodePoints takes argument string (a String) and returns a List of code points. It returns the sequence of Unicode code points that results from interpreting string as UTF-16 encoded Unicode text as described in 6.1.4. It performs the following steps when called:
The abstract operation ParseText takes arguments sourceText (a String or a sequence of Unicode code points) and goalSymbol (a nonterminal in one of the ECMAScript grammars) and returns a Parse Node or a non-empty List of SyntaxError objects. It performs the following steps when called:
If the parse succeeded and no early errors were found, return the Parse Node (an instance of goalSymbol) at the root of the parse tree resulting from the parse.
Otherwise, return a List of one or more SyntaxError objects representing the parsing errors and/or early errors. If more than one parsing error or early error is present, the number and ordering of error objects in the list is implementation-defined, but at least one must be present.
Note 1
Consider a text that has an early error at a particular point, and also a syntax error at a later point. An implementation that does a parse pass followed by an early errors pass might report the syntax error and not proceed to the early errors pass. An implementation that interleaves the two activities might report the early error and not proceed to find the syntax error. A third implementation might report both errors. All of these behaviours are conformant.
Eval code is the source text supplied to the built-in eval function. More precisely, if the parameter to the built-in eval function is a String, it is treated as an ECMAScript Script. The eval code for a particular invocation of eval is the global code portion of that Script.
then the source text matched by the BindingIdentifier (if any) of that declaration or expression is also included in the function code of the corresponding function.
Function code is generally provided as the bodies of Function Definitions (15.2), Arrow Function Definitions (15.3), Method Definitions (15.4), Generator Function Definitions (15.5), Async Function Definitions (15.8), Async Generator Function Definitions (15.6), and Async Arrow Functions (15.9). Function code is also derived from the arguments to the Function constructor (20.2.1.1), the GeneratorFunction constructor (27.3.1.1), and the AsyncFunction constructor (27.7.1.1).
Note 2
The practical effect of including the BindingIdentifier in function code is that the Early Errors for strict mode code are applied to a BindingIdentifier that is the name of a function whose body contains a "use strict" directive, even if the surrounding code is not strict mode code.
11.2.1 Directive Prologues and the Use Strict Directive
An ECMAScript syntactic unit may be processed using either unrestricted or strict mode syntax and semantics (4.3.2). Code is interpreted as strict mode code in the following situations:
An ECMAScript implementation may support the evaluation of function exotic objects whose evaluative behaviour is expressed in some host-defined form of executable code other than ECMAScript source text. Whether a function object is defined within ECMAScript code or is a built-in function is not observable from the perspective of ECMAScript code that calls or is called by such a function object.
12 ECMAScript Language: Lexical Grammar
The source text of an ECMAScript Script or Module is first converted into a sequence of input elements, which are tokens, line terminators, comments, or white space. The source text is scanned from left to right, repeatedly taking the longest possible sequence of code points as the next input element.
The use of multiple lexical goals ensures that there are no lexical ambiguities that would affect automatic semicolon insertion. For example, there are no syntactic grammar contexts where both a leading division or division-assignment, and a leading RegularExpressionLiteral are permitted. This is not affected by semicolon insertion (see 12.10); in examples such as the following:
a = b
/hi/g.exec(c).map(d);
where the first non-whitespace, non-comment code point after a LineTerminator is U+002F (SOLIDUS) and the syntactic context allows division or division-assignment, no semicolon is inserted at the LineTerminator. That is, the above example is interpreted in the same way as:
The Unicode format-control characters (i.e., the characters in category “Cf” in the Unicode Character Database such as LEFT-TO-RIGHT MARK or RIGHT-TO-LEFT MARK) are control codes used to control the formatting of a range of text in the absence of higher-level protocols for this (such as mark-up languages).
It is useful to allow format-control characters in source text to facilitate editing and display. All format control characters may be used within comments, and within string literals, template literals, and regular expression literals.
U+FEFF (ZERO WIDTH NO-BREAK SPACE) is a format-control character used primarily at the start of a text to mark it as Unicode and to allow detection of the text's encoding and byte order. <ZWNBSP> characters intended for this purpose can sometimes also appear after the start of a text, for example as a result of concatenating files. In ECMAScript source text <ZWNBSP> code points are treated as white space characters (see 12.2) outside of comments, string literals, template literals, and regular expression literals.
12.2 White Space
White space code points are used to improve source text readability and to separate tokens (indivisible lexical units) from each other, but are otherwise insignificant. White space code points may occur between any two tokens and at the start or end of input. White space code points may occur within a StringLiteral, a RegularExpressionLiteral, a Template, or a TemplateSubstitutionTail where they are considered significant code points forming part of a literal value. They may also occur within a Comment, but cannot appear within any other kind of token.
The ECMAScript white space code points are listed in Table 35.
Table 35: White Space Code Points
Code Points
Name
Abbreviation
U+0009
CHARACTER TABULATION
<TAB>
U+000B
LINE TABULATION
<VT>
U+000C
FORM FEED (FF)
<FF>
U+FEFF
ZERO WIDTH NO-BREAK SPACE
<ZWNBSP>
any code point in general category “Space_Separator”
<USP>
Note 1
U+0020 (SPACE) and U+00A0 (NO-BREAK SPACE) code points are part of <USP>.
Note 2
Other than for the code points listed in Table 35, ECMAScript WhiteSpace intentionally excludes all code points that have the Unicode “White_Space” property but which are not classified in general category “Space_Separator” (“Zs”).
Like white space code points, line terminator code points are used to improve source text readability and to separate tokens (indivisible lexical units) from each other. However, unlike white space code points, line terminators have some influence over the behaviour of the syntactic grammar. In general, line terminators may occur between any two tokens, but there are a few places where they are forbidden by the syntactic grammar. Line terminators also affect the process of automatic semicolon insertion (12.10). A line terminator cannot occur within any token except a StringLiteral, Template, or TemplateSubstitutionTail. <LF> and <CR> line terminators cannot occur within a StringLiteral token except as part of a LineContinuation.
Line terminators are included in the set of white space code points that are matched by the \s class in regular expressions.
The ECMAScript line terminator code points are listed in Table 36.
Table 36: Line Terminator Code Points
Code Point
Unicode Name
Abbreviation
U+000A
LINE FEED (LF)
<LF>
U+000D
CARRIAGE RETURN (CR)
<CR>
U+2028
LINE SEPARATOR
<LS>
U+2029
PARAGRAPH SEPARATOR
<PS>
Only the Unicode code points in Table 36 are treated as line terminators. Other new line or line breaking Unicode code points are not treated as line terminators but are treated as white space if they meet the requirements listed in Table 35. The sequence <CR><LF> is commonly used as a line terminator. It should be considered a single SourceCharacter for the purpose of reporting line numbers.
Comments can be either single or multi-line. Multi-line comments cannot nest.
Because a single-line comment can contain any Unicode code point except a LineTerminator code point, and because of the general rule that a token is always as long as possible, a single-line comment always consists of all code points from the // marker to the end of the line. However, the LineTerminator at the end of the line is not considered to be part of the single-line comment; it is recognized separately by the lexical grammar and becomes part of the stream of input elements for the syntactic grammar. This point is very important, because it implies that the presence or absence of single-line comments does not affect the process of automatic semicolon insertion (see 12.10).
Comments behave like white space and are discarded except that, if a MultiLineComment contains a line terminator code point, then the entire comment is considered to be a LineTerminator for purposes of parsing by the syntactic grammar.
IdentifierName and ReservedWord are tokens that are interpreted according to the Default Identifier Syntax given in Unicode Standard Annex #31, Identifier and Pattern Syntax, with some small modifications. ReservedWord is an enumerated subset of IdentifierName. The syntactic grammar defines Identifier as an IdentifierName that is not a ReservedWord. The Unicode identifier grammar is based on character properties specified by the Unicode Standard. The Unicode code points in the specified categories in the latest version of the Unicode Standard must be treated as in those categories by all conforming ECMAScript implementations. ECMAScript implementations may recognize identifier code points defined in later editions of the Unicode Standard.
Note 1
This standard specifies specific code point additions: U+0024 (DOLLAR SIGN) and U+005F (LOW LINE) are permitted anywhere in an IdentifierName.
The sets of code points with Unicode properties “ID_Start” and “ID_Continue” include, respectively, the code points with Unicode properties “Other_ID_Start” and “Other_ID_Continue”.
12.7.1 Identifier Names
Unicode escape sequences are permitted in an IdentifierName, where they contribute a single Unicode code point equal to the IdentifierCodePoint of the UnicodeEscapeSequence. The \ preceding the UnicodeEscapeSequence does not contribute any code points. A UnicodeEscapeSequence cannot be used to contribute a code point to an IdentifierName that would otherwise be invalid. In other words, if a \UnicodeEscapeSequence sequence were replaced by the SourceCharacter it contributes, the result must still be a valid IdentifierName that has the exact same sequence of SourceCharacter elements as the original IdentifierName. All interpretations of IdentifierName within this specification are based upon their actual code points regardless of whether or not an escape sequence was used to contribute any particular code point.
Two IdentifierNames that are canonically equivalent according to the Unicode Standard are not equal unless, after replacement of each UnicodeEscapeSequence, they are represented by the exact same sequence of code points.
The syntax-directed operation IdentifierCodePoints takes no arguments and returns a List of code points. It is defined piecewise over the following productions:
Return the code point whose numeric value is the MV of CodePoint.
12.7.2 Keywords and Reserved Words
A keyword is a token that matches IdentifierName, but also has a syntactic use; that is, it appears literally, in a fixed width font, in some syntactic production. The keywords of ECMAScript include if, while, async, await, and many others.
A reserved word is an IdentifierName that cannot be used as an identifier. Many keywords are reserved words, but some are not, and some are reserved only in certain contexts. if and while are reserved words. await is reserved only inside async functions and modules. async is not reserved; it can be used as a variable name or statement label without restriction.
This specification uses a combination of grammatical productions and early error rules to specify which names are valid identifiers and which are reserved words. All tokens in the ReservedWord list below, except for await and yield, are unconditionally reserved. Exceptions for await and yield are specified in 13.1, using parameterized syntactic productions. Lastly, several early error rules restrict the set of valid identifiers. See 13.1.1, 14.3.1.1, 14.7.5.1, and 15.7.1. In summary, there are five categories of identifier names:
Those that are always allowed as identifiers, and are not keywords, such as Math, window, toString, and _;
Those that are never allowed as identifiers, namely the ReservedWords listed below except await and yield;
Those that are contextually allowed as identifiers, namely await and yield;
Those that are contextually disallowed as identifiers, in strict mode code: let, static, implements, interface, package, private, protected, and public;
Those that are always allowed as identifiers, but also appear as keywords within certain syntactic productions, at places where Identifier is not allowed: as, async, from, get, meta, of, set, and target.
The term conditional keyword, or contextual keyword, is sometimes used to refer to the keywords that fall in the last three categories, and thus can be used as identifiers in some contexts and as keywords in others.
Per 5.1.5, keywords in the grammar match literal sequences of specific SourceCharacter elements. A code point in a keyword cannot be expressed by a \UnicodeEscapeSequence.
enum is not currently used as a keyword in this specification. It is a future reserved word, set aside for use as a keyword in future language extensions.
Similarly, implements, interface, package, private, protected, and public are future reserved words in strict mode code.
The syntax-directed operation NumericValue takes no arguments and returns a Number or a BigInt. It is defined piecewise over the following productions:
A string literal is 0 or more Unicode code points enclosed in single or double quotes. Unicode code points may also be represented by an escape sequence. All code points may appear literally in a string literal except for the closing quote code points, U+005C (REVERSE SOLIDUS), U+000D (CARRIAGE RETURN), and U+000A (LINE FEED). Any code points may appear in the form of an escape sequence. String literals evaluate to ECMAScript String values. When generating these String values Unicode code points are UTF-16 encoded as defined in 11.1.1. Code points belonging to the Basic Multilingual Plane are encoded as a single code unit element of the string. All other code points are encoded as two code unit elements of the string.
<LF> and <CR> cannot appear in a string literal, except as part of a LineContinuation to produce the empty code points sequence. The proper way to include either in the String value of a string literal is to use an escape sequence such as \n or \u000A.
It is possible for string literals to precede a Use Strict Directive that places the enclosing code in strict mode, and implementations must take care to enforce the above rules for such literals. For example, the following source text contains a Syntax Error:
A string literal stands for a value of the String type. SV produces String values for string literals through recursive application on the various parts of the string literal. As part of this process, some Unicode code points within the string literal are interpreted as having a mathematical value, as described below or in 12.9.3.
A regular expression literal is an input element that is converted to a RegExp object (see 22.2) each time the literal is evaluated. Two regular expression literals in a program evaluate to regular expression objects that never compare as === to each other even if the two literals' contents are identical. A RegExp object may also be created at runtime by new RegExp or calling the RegExp constructor as a function (see 22.2.4).
The productions below describe the syntax for a regular expression literal and are used by the input element scanner to find the end of the regular expression literal. The source text comprising the RegularExpressionBody and the RegularExpressionFlags are subsequently parsed again using the more stringent ECMAScript Regular Expression grammar (22.2.1).
An implementation may extend the ECMAScript Regular Expression grammar defined in 22.2.1, but it must not extend the RegularExpressionBody and RegularExpressionFlags productions defined below or the productions used by these productions.
Regular expression literals may not be empty; instead of representing an empty regular expression literal, the code unit sequence // starts a single-line comment. To specify an empty regular expression, use: /(?:)/.
12.9.5.1 Static Semantics: BodyText
The syntax-directed operation BodyText takes no arguments and returns source text. It is defined piecewise over the following productions:
The syntax-directed operation TV takes no arguments and returns a String or undefined. A template literal component is interpreted by TV as a value of the String type. TV is used to construct the indexed components of a template object (colloquially, the template values). In TV, escape sequences are replaced by the UTF-16 code unit(s) of the Unicode code point represented by the escape sequence.
The syntax-directed operation TRV takes no arguments and returns a String. A template literal component is interpreted by TRV as a value of the String type. TRV is used to construct the raw components of a template object (colloquially, the template raw values). TRV is similar to TV with the difference being that in TRV, escape sequences are interpreted as they appear in the literal.
The TRV of HexDigit::one of0123456789abcdefABCDEF is the result of performing UTF16EncodeCodePoint on the single code point matched by this production.
Most ECMAScript statements and declarations must be terminated with a semicolon. Such semicolons may always appear explicitly in the source text. For convenience, however, such semicolons may be omitted from the source text in certain situations. These situations are described by saying that semicolons are automatically inserted into the source code token stream in those situations.
12.10.1 Rules of Automatic Semicolon Insertion
In the following rules, “token” means the actual recognized lexical token determined using the current lexical goal symbol as described in clause 12.
There are three basic rules of semicolon insertion:
When, as the source text is parsed from left to right, a token (called the offending token) is encountered that is not allowed by any production of the grammar, then a semicolon is automatically inserted before the offending token if one or more of the following conditions is true:
The offending token is separated from the previous token by at least one LineTerminator.
The offending token is }.
The previous token is ) and the inserted semicolon would then be parsed as the terminating semicolon of a do-while statement (14.7.2).
When, as the source text is parsed from left to right, the end of the input stream of tokens is encountered and the parser is unable to parse the input token stream as a single instance of the goal nonterminal, then a semicolon is automatically inserted at the end of the input stream.
When, as the source text is parsed from left to right, a token is encountered that is allowed by some production of the grammar, but the production is a restricted production and the token would be the first token for a terminal or nonterminal immediately following the annotation “[no LineTerminator here]” within the restricted production (and therefore such a token is called a restricted token), and the restricted token is separated from the previous token by at least one LineTerminator, then a semicolon is automatically inserted before the restricted token.
However, there is an additional overriding condition on the preceding rules: a semicolon is never inserted automatically if the semicolon would then be parsed as an empty statement or if that semicolon would become one of the two semicolons in the header of a for statement (see 14.7.4).
Note
The following are the only restricted productions in the grammar:
The practical effect of these restricted productions is as follows:
When a ++ or -- token is encountered where the parser would treat it as a postfix operator, and at least one LineTerminator occurred between the preceding token and the ++ or -- token, then a semicolon is automatically inserted before the ++ or -- token.
When a continue, break, return, throw, or yield token is encountered and a LineTerminator is encountered before the next token, a semicolon is automatically inserted after the continue, break, return, throw, or yield token.
When arrow function parameter(s) are followed by a LineTerminator before a => token, a semicolon is automatically inserted and the punctuator causes a syntax error.
When an async token is followed by a LineTerminator before a function or IdentifierName or ( token, a semicolon is automatically inserted and the async token is not treated as part of the same expression or class element as the following tokens.
When an async token is followed by a LineTerminator before a * token, a semicolon is automatically inserted and the punctuator causes a syntax error.
The resulting practical advice to ECMAScript programmers is:
A postfix ++ or -- operator should be on the same line as its operand.
An Expression in a return or throw statement or an AssignmentExpression in a yield expression should start on the same line as the return, throw, or yield token.
A LabelIdentifier in a break or continue statement should be on the same line as the break or continue token.
The end of an arrow function's parameter(s) and its => should be on the same line.
The async token preceding an asynchronous function or method should be on the same line as the immediately following token.
12.10.2 Examples of Automatic Semicolon Insertion
This section is non-normative.
The source
{ 12 } 3
is not a valid sentence in the ECMAScript grammar, even with the automatic semicolon insertion rules. In contrast, the source
{ 12 } 3
is also not a valid ECMAScript sentence, but is transformed by automatic semicolon insertion into the following:
{ 1
;2 ;} 3;
which is a valid ECMAScript sentence.
The source
for (a; b
)
is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion because the semicolon is needed for the header of a for statement. Automatic semicolon insertion never inserts one of the two semicolons in the header of a for statement.
The source
return
a + b
is transformed by automatic semicolon insertion into the following:
return;
a + b;
Note 1
The expression a + b is not treated as a value to be returned by the return statement, because a LineTerminator separates it from the token return.
The source
a = b
++c
is transformed by automatic semicolon insertion into the following:
a = b;
++c;
Note 2
The token ++ is not treated as a postfix operator applying to the variable b, because a LineTerminator occurs between b and ++.
The source
if (a > b)
else c = d
is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion before the else token, even though no production of the grammar applies at that point, because an automatically inserted semicolon would then be parsed as an empty statement.
The source
a = b + c
(d + e).print()
is not transformed by automatic semicolon insertion, because the parenthesized expression that begins the second line can be interpreted as an argument list for a function call:
a = b + c(d + e).print()
In the circumstance that an assignment statement must begin with a left parenthesis, it is a good idea for the programmer to provide an explicit semicolon at the end of the preceding statement rather than to rely on automatic semicolon insertion.
12.10.3 Interesting Cases of Automatic Semicolon Insertion
This section is non-normative.
ECMAScript programs can be written in a style with very few semicolons by relying on automatic semicolon insertion. As described above, semicolons are not inserted at every newline, and automatic semicolon insertion can depend on multiple tokens across line terminators.
As new syntactic features are added to ECMAScript, additional grammar productions could be added that cause lines relying on automatic semicolon insertion preceding them to change grammar productions when parsed.
For the purposes of this section, a case of automatic semicolon insertion is considered interesting if it is a place where a semicolon may or may not be inserted, depending on the source text which precedes it. The rest of this section describes a number of interesting cases of automatic semicolon insertion in this version of ECMAScript.
12.10.3.1 Interesting Cases of Automatic Semicolon Insertion in Statement Lists
In a StatementList, many StatementListItems end in semicolons, which may be omitted using automatic semicolon insertion. As a consequence of the rules above, at the end of a line ending an expression, a semicolon is required if the following line begins with any of the following:
An opening parenthesis ((). Without a semicolon, the two lines together are treated as a CallExpression.
An opening square bracket ([). Without a semicolon, the two lines together are treated as property access, rather than an ArrayLiteral or ArrayAssignmentPattern.
A template literal (`). Without a semicolon, the two lines together are interpreted as a tagged Template (13.3.11), with the previous expression as the MemberExpression.
Unary + or -. Without a semicolon, the two lines together are interpreted as a usage of the corresponding binary operator.
A RegExp literal. Without a semicolon, the two lines together may be parsed instead as the /MultiplicativeOperator, for example if the RegExp has flags.
12.10.3.2 Cases of Automatic Semicolon Insertion and “[no LineTerminator here]”
This section is non-normative.
ECMAScript contains grammar productions which include “[no LineTerminator here]”. These productions are sometimes a means to have optional operands in the grammar. Introducing a LineTerminator in these locations would change the grammar production of a source text by using the grammar production without the optional operand.
The rest of this section describes a number of productions using “[no LineTerminator here]” in this version of ECMAScript.
12.10.3.2.1 List of Grammar Productions with Optional Operands and “[no LineTerminator here]”
yield and await are permitted as BindingIdentifier in the grammar, and prohibited with static semantics below, to prohibit automatic semicolon insertion in cases such as
It is a Syntax Error if IsStrict(this phrase) is true and the StringValue of IdentifierName is one of "implements", "interface", "let", "package", "private", "protected", "public", "static", or "yield".
An ArrayLiteral is an expression describing the initialization of an Array, using a list, of zero or more expressions each of which represents an array element, enclosed in square brackets. The elements need not be literals; they are evaluated each time the array initializer is evaluated.
Array elements may be elided at the beginning, middle or end of the element list. Whenever a comma in the element list is not preceded by an AssignmentExpression (i.e., a comma at the beginning or after another comma), the missing array element contributes to the length of the Array and increases the index of subsequent elements. Elided array elements are not defined. If an element is elided at the end of an array, that element does not contribute to the length of the Array.
CreateDataPropertyOrThrow is used to ensure that own properties are defined for the array even if the standard built-in Array prototype object has been modified in a manner that would preclude the creation of new own properties using [[Set]].
An object initializer is an expression describing the initialization of an Object, written in a form resembling a literal. It is a list of zero or more pairs of property keys and associated values, enclosed in curly brackets. The values need not be literals; they are evaluated each time the object initializer is evaluated.
In certain contexts, ObjectLiteral is used as a cover grammar for a more restricted secondary grammar. The CoverInitializedName production is necessary to fully cover these secondary grammars. However, use of this production results in an early Syntax Error in normal contexts where an actual ObjectLiteral is expected.
It is a Syntax Error if any source text is matched by this production.
Note 1
This production exists so that ObjectLiteral can serve as a cover grammar for ObjectAssignmentPattern. It cannot occur in an actual object initializer.
The syntax-directed operation PropertyNameList takes no arguments and returns a List of Strings. It is defined piecewise over the following productions:
The abstract operation IsValidRegularExpressionLiteral takes argument literal (a RegularExpressionLiteralParse Node) and returns a Boolean. It determines if its argument is a valid regular expression literal. It performs the following steps when called:
Set patternText to the sequence of code points resulting from interpreting each of the 16-bit elements of stringValue as a Unicode BMP code point. UTF-16 decoding is not applied to the elements.
Let parseResult be ParsePattern(patternText, u, v).
If parseResult is a Parse Node, return true; else return false.
The syntax-directed operation TemplateStrings takes argument raw (a Boolean) and returns a List of either Strings or undefined. It is defined piecewise over the following productions:
This operation returns undefined if raw is false and templateToken contains a NotEscapeSequence. In all other cases, it returns a String.
13.2.8.4 GetTemplateObject ( templateLiteral )
The abstract operation GetTemplateObject takes argument templateLiteral (a Parse Node) and returns an Array. It performs the following steps when called:
Append the Record { [[Site]]: templateLiteral, [[Array]]: template } to realm.[[TemplateMap]].
Return template.
Note 1
The creation of a template object cannot result in an abrupt completion.
Note 2
Each TemplateLiteral in the program code of a realm is associated with a unique template object that is used in the evaluation of tagged Templates (13.2.8.6). The template objects are frozen and the same template object is used each time a specific tagged Template is evaluated. Whether template objects are created lazily upon first evaluation of the TemplateLiteral or eagerly prior to first evaluation is an implementation choice that is not observable to ECMAScript code.
Note 3
Future editions of this specification may define additional non-enumerable properties of template objects.
This algorithm does not apply GetValue to Evaluation of Expression. The principal motivation for this is so that operators such as delete and typeof may be applied to parenthesized expressions.
Let propertyNameReference be ? Evaluation of expression.
Let propertyNameValue be ? GetValue(propertyNameReference).
NOTE: In most cases, ToPropertyKey will be performed on propertyNameValue immediately after this step. However, in the case of a[b] = c, it will not be performed until after evaluation of c.
Return the Reference Record { [[Base]]: baseValue, [[ReferencedName]]: propertyNameValue, [[Strict]]: strict, [[ThisValue]]: empty }.
The abstract operation EvaluatePropertyAccessWithIdentifierKey takes arguments baseValue (an ECMAScript language value), identifierName (an IdentifierNameParse Node), and strict (a Boolean) and returns a Reference Record. It performs the following steps when called:
Let propertyNameString be the StringValue of identifierName.
Return the Reference Record { [[Base]]: baseValue, [[ReferencedName]]: propertyNameString, [[Strict]]: strict, [[ThisValue]]: empty }.
NOTE: In most cases, ToPropertyKey will be performed on propertyNameValue immediately after this step. However, in the case of super[b] = c, it will not be performed until after evaluation of c.
The abstract operation ContinueDynamicImport takes arguments promiseCapability (a PromiseCapability Record) and moduleCompletion (either a normal completion containing a Module Record or a throw completion) and returns unused. It completes the process of a dynamic import originally started by an import() call, resolving or rejecting the promise returned by that call as appropriate. It performs the following steps when called:
Let linkAndEvaluateClosure be a new Abstract Closure with no parameters that captures module, promiseCapability, and onRejected and performs the following steps when called:
Let fulfilledClosure be a new Abstract Closure with no parameters that captures module and promiseCapability and performs the following steps when called:
A tagged template is a function call where the arguments of the call are derived from a TemplateLiteral (13.2.8). The actual arguments include a template object (13.2.8.4) and the values produced by evaluating the expressions embedded within the TemplateLiteral.
The host-defined abstract operation HostFinalizeImportMeta takes arguments importMeta (an Object) and moduleRecord (a Module Record) and returns unused. It allows hosts to perform any extraordinary operations to prepare the object returned from import.meta.
Most hosts will be able to simply define HostGetImportMetaProperties, and leave HostFinalizeImportMeta with its default behaviour. However, HostFinalizeImportMeta provides an "escape hatch" for hosts which need to directly manipulate the object before it is exposed to ECMAScript code.
The default implementation of HostFinalizeImportMeta is to return unused.
When a delete operator occurs within strict mode code, a SyntaxError exception is thrown if its UnaryExpression is a direct reference to a variable, function argument, or function name. In addition, if a delete operator occurs within strict mode code and the property to be deleted has the attribute { [[Configurable]]: false } (or otherwise cannot be deleted), a TypeError exception is thrown.
Note 2
The object that may be created in step 4.c is not accessible outside of the above abstract operation and the ordinary object[[Delete]] internal method. An implementation might choose to avoid the actual creation of that object.
The result of evaluating a relational operator is always of type Boolean, reflecting whether the relationship named by the operator holds between its two operands.
The abstract operation InstanceofOperator takes arguments V (an ECMAScript language value) and target (an ECMAScript language value) and returns either a normal completion containing a Boolean or a throw completion. It implements the generic algorithm for determining if V is an instance of target either by consulting target's %Symbol.hasInstance% method or, if absent, determining whether the value of target's "prototype" property is present in V's prototype chain. It performs the following steps when called:
Steps 4 and 5 provide compatibility with previous editions of ECMAScript that did not use a %Symbol.hasInstance% method to define the instanceof operator semantics. If an object does not define or inherit %Symbol.hasInstance% it uses the default instanceof semantics.
13.11 Equality Operators
Note
The result of evaluating an equality operator is always of type Boolean, reflecting whether the relationship named by the operator holds between its two operands.
If r is true, return false. Otherwise, return true.
Note 1
Given the above definition of equality:
String comparison can be forced by: `${a}` == `${b}`.
Numeric comparison can be forced by: +a == +b.
Boolean comparison can be forced by: !a == !b.
Note 2
The equality operators maintain the following invariants:
A != B is equivalent to !(A == B).
A == B is equivalent to B == A, except in the order of evaluation of A and B.
Note 3
The equality operator is not always transitive. For example, there might be two distinct String objects, each representing the same String value; each String object would be considered equal to the String value by the == operator, but the two String objects would not be equal to each other. For example:
new String("a") == "a" and "a" == new String("a") are both true.
new String("a") == new String("a") is false.
Note 4
Comparison of Strings uses a simple equality test on sequences of code unit values. There is no attempt to use the more complex, semantically oriented definitions of character or string equality and collating order defined in the Unicode specification. Therefore Strings values that are canonically equal according to the Unicode Standard could test as unequal. In effect this algorithm assumes that both Strings are already in normalized form.
The value produced by a && or || operator is not necessarily of type Boolean. The value produced will always be the value of one of the two operand expressions.
The grammar for a ConditionalExpression in ECMAScript is slightly different from that in C and Java, which each allow the second subexpression to be an Expression but restrict the third expression to be a ConditionalExpression. The motivation for this difference in ECMAScript is to allow an assignment expression to be governed by either arm of a conditional and to eliminate the confusing and fairly useless case of a comma expression as the centre expression.
When this expression occurs within strict mode code, it is a runtime error if lRef in step 1.d, 2, 2, 2, 2 is an unresolvable reference. If it is, a ReferenceError exception is thrown. Additionally, it is a runtime error if the lRef in step 8, 6, 6, 6 is a reference to a data property with the attribute value { [[Writable]]: false }, to an accessor property with the attribute value { [[Set]]: undefined }, or to a non-existent property of an object for which the IsExtensible predicate returns the value false. In these cases a TypeError exception is thrown.
No hint is provided in the calls to ToPrimitive in steps 1.a and 1.b. All standard objects except Dates handle the absence of a hint as if number were given; Dates handle the absence of a hint as if string were given. Exotic objects may handle the absence of a hint in some other manner.
Note 2
Step 1.c differs from step 3 of the IsLessThan algorithm, by using the logical-or operation instead of the logical-and operation.
The abstract operation EvaluateStringOrNumericBinaryExpression takes arguments leftOperand (a Parse Node), opText (a sequence of Unicode code points), and rightOperand (a Parse Node) and returns either a normal completion containing either a String, a BigInt, or a Number, or an abrupt completion. It performs the following steps when called:
Left to right evaluation order is maintained by evaluating a DestructuringAssignmentTarget that is not a destructuring pattern prior to accessing the iterator or evaluating the Initializer.
The value of a StatementList is the value of the last value-producing item in the StatementList. For example, the following calls to the eval function all return the value 1:
The abstract operation BlockDeclarationInstantiation takes arguments code (a Parse Node) and env (a Declarative Environment Record) and returns unused. code is the Parse Node corresponding to the body of the block. env is the Environment Record in which bindings are to be created.
When undefined is passed for environment it indicates that a PutValue operation should be used to assign the initialization value. This is the case for formal parameter lists of non-strict functions. In that case the formal parameter bindings are preinitialized in order to deal with the possibility of multiple parameters with the same name.
It is defined piecewise over the following productions:
The lookahead-restriction [lookahead ≠ else] resolves the classic "dangling else" problem in the usual way. That is, when the choice of associated if is otherwise ambiguous, the else is associated with the nearest (innermost) of the candidate ifs
The abstract operation LoopContinues takes arguments completion (a Completion Record) and labelSet (a List of Strings) and returns a Boolean. It performs the following steps when called:
The abstract operation CreatePerIterationEnvironment takes argument perIterationBindings (a List of Strings) and returns either a normal completion containingunused or a throw completion. It performs the following steps when called:
undefined is passed for environment to indicate that a PutValue operation should be used to assign the initialization value. This is the case for var statements and the formal parameter lists of some non-strict functions (see 10.2.11). In those cases a lexical binding is hoisted and preinitialized prior to evaluation of its initializer.
It is defined piecewise over the following productions:
The abstract operation ForIn/OfBodyEvaluation takes arguments lhs (a Parse Node), stmt (a StatementParse Node), iteratorRecord (an Iterator Record), iterationKind (enumerate or iterate), lhsKind (assignment, var-binding, or lexical-binding), and labelSet (a List of Strings) and optional argument iteratorKind (sync or async) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It performs the following steps when called:
If iteratorKind is not present, set iteratorKind to sync.
The abstract operation EnumerateObjectProperties takes argument O (an Object) and returns an Iterator. It performs the following steps when called:
Return an Iterator object (27.1.1.2) whose next method iterates over all the String-valued keys of enumerable properties of O. The iterator object is never directly accessible to ECMAScript code. The mechanics and order of enumerating the properties is not specified but must conform to the rules specified below.
The iterator's throw and return methods are null and are never invoked. The iterator's next method processes object properties to determine whether the property key should be returned as an iterator value. Returned property keys do not include keys that are Symbols. Properties of the target object may be deleted during enumeration. A property that is deleted before it is processed by the iterator's next method is ignored. If new properties are added to the target object during enumeration, the newly added properties are not guaranteed to be processed in the active enumeration. A property name will be returned by the iterator's next method at most once in any enumeration.
Enumerating the properties of the target object includes enumerating properties of its prototype, and the prototype of the prototype, and so on, recursively; but a property of a prototype is not processed if it has the same name as a property that has already been processed by the iterator's next method. The values of [[Enumerable]] attributes are not considered when determining if a property of a prototype object has already been processed. The enumerable property names of prototype objects must be obtained by invoking EnumerateObjectProperties passing the prototype object as the argument. EnumerateObjectProperties must obtain the own property keys of the target object by calling its [[OwnPropertyKeys]] internal method. Property attributes of the target object must be obtained by calling its [[GetOwnProperty]] internal method.
the value of the [[Prototype]] internal slot of O or an object in its prototype chain changes,
a property is removed from O or an object in its prototype chain,
a property is added to an object in O's prototype chain, or
the value of the [[Enumerable]] attribute of a property of O or an object in its prototype chain changes.
Note 1
ECMAScript implementations are not required to implement the algorithm in 14.7.5.10.2.1 directly. They may choose any implementation whose behaviour will not deviate from that algorithm unless one of the constraints in the previous paragraph is violated.
The following is an informative definition of an ECMAScript generator function that conforms to these rules:
function* EnumerateObjectProperties(obj) {
const visited = newSet();
for (const key ofReflect.ownKeys(obj)) {
if (typeof key === "symbol") continue;
const desc = Reflect.getOwnPropertyDescriptor(obj, key);
if (desc) {
visited.add(key);
if (desc.enumerable) yield key;
}
}
const proto = Reflect.getPrototypeOf(obj);
if (proto === null) return;
for (const protoKey ofEnumerateObjectProperties(proto)) {
if (!visited.has(protoKey)) yield protoKey;
}
}
Note 2
The list of exotic objects for which implementations are not required to match CreateForInIterator was chosen because implementations historically differed in behaviour for those cases, and agreed in all others.
14.7.5.10 For-In Iterator Objects
A For-In Iterator is an object that represents a specific iteration over some specific object. For-In Iterator objects are never directly accessible to ECMAScript code; they exist solely to illustrate the behaviour of EnumerateObjectProperties.
14.7.5.10.1 CreateForInIterator ( object )
The abstract operation CreateForInIterator takes argument object (an Object) and returns a For-In Iterator. It is used to create a For-In Iterator object which iterates over the own and inherited enumerable string properties of object in a specific order. It performs the following steps when called:
14.7.5.10.3 Properties of For-In Iterator Instances
For-In Iterator instances are ordinary objects that inherit properties from the %ForInIteratorPrototype% intrinsic object. For-In Iterator instances are initially created with the internal slots listed in Table 38.
Table 38: Internal Slots of For-In Iterator Instances
Internal Slot
Type
Description
[[Object]]
an Object
The Object value whose properties are being iterated.
[[ObjectWasVisited]]
a Boolean
true if the iterator has invoked [[OwnPropertyKeys]] on [[Object]], false otherwise.
It is a Syntax Error if this ContinueStatement is not nested, directly or indirectly (but not crossing function or static initialization block boundaries), within an IterationStatement.
It is a Syntax Error if this BreakStatement is not nested, directly or indirectly (but not crossing function or static initialization block boundaries), within an IterationStatement or a SwitchStatement.
A return statement causes a function to cease execution and, in most cases, returns a value to the caller. If Expression is omitted, the return value is undefined. Otherwise, the return value is the value of Expression. A return statement may not actually return a value to the caller depending on surrounding context. For example, in a try block, a return statement's Completion Record may be replaced with another Completion Record during evaluation of the finally block.
The with statement adds an Object Environment Record for a computed object to the lexical environment of the running execution context. It then executes a statement using this augmented lexical environment. Finally, it restores the original lexical environment.
No matter how control leaves the embedded Statement, whether normally or by some form of abrupt completion or exception, the LexicalEnvironment is always restored to its former state.
This operation does not execute C's StatementList (if any). The CaseBlock algorithm uses its return value to determine which StatementList to start executing.
A Statement may be prefixed by a label. Labelled statements are only used in conjunction with labelled break and continue statements. ECMAScript has no goto statement. A Statement can be part of a LabelledStatement, which itself can be part of a LabelledStatement, and so on. The labels introduced this way are collectively referred to as the “current label set” when describing the semantics of individual statements.
The abstract operation IsLabelledFunction takes argument stmt (a StatementParse Node) and returns a Boolean. It performs the following steps when called:
The try statement encloses a block of code in which an exceptional condition can occur, such as a runtime error or a throw statement. The catch clause provides the exception-handling code. When a catch clause catches an exception, its CatchParameter is bound to that exception.
Evaluating a DebuggerStatement may allow an implementation to cause a breakpoint when run under a debugger. If a debugger is not present or active this statement has no observable effect.
Various ECMAScript language elements cause the creation of ECMAScript function objects (10.2). Evaluation of such functions starts with the execution of their [[Call]] internal method (10.2.1).
The ExpectedArgumentCount of a FormalParameterList is the number of FormalParameters to the left of either the rest parameter or the first FormalParameter with an Initializer. A FormalParameter without an initializer is allowed after the first parameter with an initializer but such parameters are considered to be optional with undefined as their default value.
The syntax-directed operation FunctionBodyContainsUseStrict takes no arguments and returns a Boolean. It is defined piecewise over the following productions:
A "prototype" property is automatically created for every function defined using a FunctionDeclaration or FunctionExpression, to allow for the possibility that the function will be used as a constructor.
The syntax-directed operation ConciseBodyContainsUseStrict takes no arguments and returns a Boolean. It is defined piecewise over the following productions:
An ArrowFunction does not define local bindings for arguments, super, this, or new.target. Any reference to arguments, super, this, or new.target within an ArrowFunction must resolve to a binding in a lexically enclosing environment. Typically this will be the Function Environment of an immediately enclosing function. Even though an ArrowFunction may contain references to super, the function object created in step 5 is not made into a method by performing MakeMethod. An ArrowFunction that references super is always contained within a non-ArrowFunction and the necessary state to implement super is accessible via the env that is captured by the function object of the ArrowFunction.
YieldExpression cannot be used within the FormalParameters of a generator function because any expressions that are part of FormalParameters are evaluated before the resulting Generator is in a resumable state.
Let G be ? OrdinaryCreateFromConstructor(functionObject, "%GeneratorFunction.prototype.prototype%", « [[GeneratorState]], [[GeneratorContext]], [[GeneratorBrand]] »).
Let innerResult be ? Call(throw, iterator, « received.[[Value]] »).
If generatorKind is async, set innerResult to ? Await(innerResult).
NOTE: Exceptions from the inner iterator throw method are propagated. Normal completions from an inner throw method are processed similarly to an inner next.
NOTE: If iterator does not have a throw method, this throw is going to terminate the yield* loop. But first we need to give iterator a chance to clean up.
Let closeCompletion be Completion Record { [[Type]]: normal, [[Value]]: empty, [[Target]]: empty }.
If generatorKind is async, perform ? AsyncIteratorClose(iteratorRecord, closeCompletion).
It is a Syntax Error if the PrivateBoundIdentifiers of ClassElementList contains any duplicate entries, unless the name is used once for a getter and once for a setter and in no other entries, and the getter and setter are either both static or both non-static.
The syntax-directed operation ClassElementKind takes no arguments and returns constructor-method, non-constructor-method, or empty. It is defined piecewise over the following productions:
The syntax-directed operation AllPrivateIdentifiersValid takes argument names (a List of Strings) and returns a Boolean.
Every grammar production alternative in this specification which is not listed below implicitly has the following default definition of AllPrivateIdentifiersValid:
The syntax-directed operation PrivateBoundIdentifiers takes no arguments and returns a List of Strings. It is defined piecewise over the following productions:
Every grammar production alternative in this specification which is not listed below implicitly has the following default definition of ContainsArguments:
For ease of specification, private methods and accessors are included alongside private fields in the [[PrivateElements]] slot of class instances. However, any given object has either all or none of the private methods and accessors defined by a given class. This feature has been designed so that implementations may choose to implement private methods and accessors using a strategy which does not require tracking each method or accessor individually.
For example, an implementation could directly associate instance private methods with their corresponding Private Name and track, for each object, which class constructors have run with that object as their this value. Looking up an instance private method on an object then consists of checking that the class constructor which defines the method has been used to initialize the object, then returning the method associated with the Private Name.
This differs from private fields: because field initializers can throw during class instantiation, an individual object may have some proper subset of the private fields of a given class, and so private fields must in general be tracked individually.
It is defined piecewise over the following productions:
NOTE: This branch behaves similarly to constructor(...args) { super(...args); }. The most notable distinction is that while the aforementioned ECMAScript source text observably calls the %Symbol.iterator% method on %Array.prototype%, this function does not.
Let func be ! F.[[GetPrototypeOf]]().
If IsConstructor(func) is false, throw a TypeError exception.
await is parsed as a keyword of an AwaitExpression when the [Await] parameter is present. The [Await] parameter is present in the top level of the following contexts, although the parameter may be absent in some contexts depending on the nonterminals, such as FunctionBody:
When Script is the syntactic goal symbol, await may be parsed as an identifier when the [Await] parameter is absent. This includes the following contexts:
The syntax-directed operation AsyncConciseBodyContainsUseStrict takes no arguments and returns a Boolean. It is defined piecewise over the following productions:
Tail Position calls are only defined in strict mode code because of a common non-standard language extension (see 10.2.4) that enables observation of the chain of caller contexts.
call is a Parse Node that represents a specific range of source text. When the following algorithms compare call to another Parse Node, it is a test of whether they represent the same source text.
Note 2
A potential tail position call that is immediately followed by return GetValue of the call result is also a possible tail position call. A function call cannot return a Reference Record, so such a GetValue operation will always return the same value as the actual function call result.
It is defined piecewise over the following productions:
The abstract operation PrepareForTailCall takes no arguments and returns unused. It performs the following steps when called:
Assert: The current execution context will not subsequently be used for the evaluation of any ECMAScript code or built-in functions. The invocation of Call subsequent to the invocation of this abstract operation will create and push a new execution context before performing any such evaluation.
Discard all resources associated with the current execution context.
Return unused.
A tail position call must either release any transient internal resources associated with the currently executing function execution context before invoking the target function or reuse those resources in support of the target function.
Note
For example, a tail position call should only grow an implementation's activation record stack by the amount that the size of the target function's activation record exceeds the size of the calling function's activation record. If the target function's activation record is smaller, then the total size of the stack should decrease.
A map from the specifier strings imported by this script to the resolved Module Record. The list does not contain two different Records with the same [[Specifier]].
[[HostDefined]]
anything (default value is empty)
Field reserved for use by host environments that need to associate additional information with a script.
The abstract operation ParseScript takes arguments sourceText (ECMAScript source text), realm (a Realm Record), and hostDefined (anything) and returns a Script Record or a non-empty List of SyntaxError objects. It creates a Script Record based upon the result of parsing sourceText as a Script. It performs the following steps when called:
An implementation may parse script source text and analyse it for Early Error conditions prior to evaluation of ParseScript for that script source text. However, the reporting of any errors must be deferred until the point where this specification actually performs ParseScript upon that source text.
When an execution context is established for evaluating scripts, declarations are instantiated in the current global environment. Each global binding declared in the code is instantiated.
If declaredFunctionNames does not contain vn, then
Let vnDefinable be ? env.CanDeclareGlobalVar(vn).
If vnDefinable is false, throw a TypeError exception.
If declaredVarNames does not contain vn, then
Append vn to declaredVarNames.
NOTE: No abnormal terminations occur after this algorithm step if the global object is an ordinary object. However, if the global object is a Proxy exotic object it may exhibit behaviours that cause abnormal terminations in some of the following steps.
NOTE: Annex B.3.2.2 adds additional steps at this point.
Early errors specified in 16.1.1 prevent name conflicts between function/var declarations and let/const/class declarations as well as redeclaration of let/const/class bindings for declaration contained within a single Script. However, such conflicts and redeclarations that span more than one Script are detected as runtime errors during GlobalDeclarationInstantiation. If any such errors are detected, no bindings are instantiated for the script. However, if the global object is defined using Proxy exotic objects then the runtime tests for conflicting declarations may be unreliable resulting in an abrupt completion and some global declarations not being instantiated. If this occurs, the code for the Script is not evaluated.
Unlike explicit var or function declarations, properties that are directly created on the global object result in global bindings that may be shadowed by let/const/class declarations.
The duplicate ExportedNames rule implies that multiple export defaultExportDeclaration items within a ModuleBody is a Syntax Error. Additional error conditions relating to conflicting or duplicate declarations are checked during module linking prior to evaluation of a Module. If any such errors are detected the Module is not evaluated.
The abstract operation ImportedLocalNames takes argument importEntries (a List of ImportEntry Records) and returns a List of Strings. It creates a List of all of the local name bindings defined by importEntries. It performs the following steps when called:
A Module Record encapsulates structural information about the imports and exports of a single module. This information is used to link the imports and exports of sets of connected modules. A Module Record includes four fields that are only used when evaluating a module.
For specification purposes Module Record values are values of the Record specification type and can be thought of as existing in a simple object-oriented hierarchy where Module Record is an abstract class with both abstract and concrete subclasses. This specification defines the abstract subclass named Cyclic Module Record and its concrete subclass named Source Text Module Record. Other specifications and implementations may define additional Module Record subclasses corresponding to alternative module definition facilities that they defined.
Module Record defines the fields listed in Table 40. All Module Definition subclasses include at least those fields. Module Record also defines the abstract method list in Table 41. All Module definition subclasses must provide concrete implementations of these abstract methods.
Prepares the module for linking by recursively loading all its dependencies, and returns a promise.
GetExportedNames([exportStarSet])
Return a list of all names that are either directly or indirectly exported from this module.
LoadRequestedModules must have completed successfully prior to invoking this method.
ResolveExport(exportName [, resolveSet])
Return the binding of a name exported by this module. Bindings are represented by a ResolvedBinding Record, of the form { [[Module]]: Module Record, [[BindingName]]: String | namespace }. If the export is a Module Namespace Object without a direct binding in any module, [[BindingName]] will be set to namespace. Return null if the name cannot be resolved, or ambiguous if multiple bindings were found.
Each time this operation is called with a specific exportName, resolveSet pair as arguments it must return the same result.
LoadRequestedModules must have completed successfully prior to invoking this method.
Link()
Prepare the module for evaluation by transitively resolving all module dependencies and creating a Module Environment Record.
LoadRequestedModules must have completed successfully prior to invoking this method.
Evaluate()
Returns a promise for the evaluation of this module and its dependencies, resolving on successful evaluation or if it has already been evaluated successfully, and rejecting for an evaluation error or if it has already been evaluated unsuccessfully. If the promise is rejected, hosts are expected to handle the promise rejection and rethrow the evaluation error.
Link must have completed successfully prior to invoking this method.
16.2.1.5 Cyclic Module Records
A Cyclic Module Record is used to represent information about a module that can participate in dependency cycles with other modules that are subclasses of the Cyclic Module Record type. Module Records that are not subclasses of the Cyclic Module Record type must not participate in dependency cycles with Source Text Module Records.
new, unlinked, linking, linked, evaluating, evaluating-async, or evaluated
Initially new. Transitions to unlinked, linking, linked, evaluating, possibly evaluating-async, evaluated (in that order) as the module progresses throughout its lifecycle. evaluating-async indicates this module is queued to execute on completion of its asynchronous dependencies or it is a module whose [[HasTLA]] field is true that has been executed and is pending top-level completion.
Auxiliary field used during Link and Evaluate only. If [[Status]] is either linking or evaluating, this non-negative number records the point at which the module was first visited during the depth-first traversal of the dependency graph.
Auxiliary field used during Link and Evaluate only. If [[Status]] is either linking or evaluating, this is either the module's own [[DFSIndex]] or that of an "earlier" module in the same strongly connected component.
A List of all the ModuleSpecifier strings used by the module represented by this record to request the importation of a module. The List is in source text occurrence order.
A map from the specifier strings used by the module represented by this record to request the importation of a module to the resolved Module Record. The list does not contain two different Records with the same [[Specifier]].
The first visited module of the cycle, the root DFS ancestor of the strongly connected component. For a module not in a cycle, this would be the module itself. Once Evaluate has completed, a module's [[DFSAncestorIndex]] is the [[DFSIndex]] of its [[CycleRoot]].
[[HasTLA]]
a Boolean
Whether this module is individually asynchronous (for example, if it's a Source Text Module Record containing a top-level await). Having an asynchronous dependency does not mean this field is true. This field must not change after the module is parsed.
[[AsyncEvaluation]]
a Boolean
Whether this module is either itself asynchronous or has an asynchronous dependency. Note: The order in which this field is set is used to order queued executions, see 16.2.1.5.3.4.
If this module is the [[CycleRoot]] of some cycle, and Evaluate() was called on some module in that cycle, this field contains the PromiseCapability Record for that entire evaluation. It is used to settle the Promise object that is returned from the Evaluate() abstract method. This field will be empty for any dependencies of that module, unless a top-level Evaluate() has been initiated for some of those dependencies.
If this module or a dependency has [[HasTLA]]true, and execution is in progress, this tracks the parent importers of this module for the top-level execution job. These parent modules will not start executing before this module has successfully completed execution.
If this module has any asynchronous dependencies, this tracks the number of asynchronous dependency modules remaining to execute for this module. A module with asynchronous dependencies will be executed when this field reaches 0 and there are no execution errors.
Evaluate the module's code within its execution context. If this module has true in [[HasTLA]], then a PromiseCapability Record is passed as an argument, and the method is expected to resolve or reject the given capability. In this case, the method must not throw an exception, but instead reject the PromiseCapability Record if necessary.
A GraphLoadingState Record is a Record that contains information about the loading process of a module graph. It's used to continue loading after a call to HostLoadImportedModule. Each GraphLoadingState Record has the fields defined in Table 44:
It is a list of the Cyclic Module Records that have been already loaded by the current loading process, to avoid infinite loops with circular dependencies.
The LoadRequestedModules concrete method of a Cyclic Module Recordmodule takes optional argument hostDefined (anything) and returns a Promise. It populates the [[LoadedModules]] of all the Module Records in the dependency graph of module (most of the work is done by the auxiliary function InnerModuleLoading). It takes an optional hostDefined parameter that is passed to the HostLoadImportedModule hook. It performs the following steps when called:
If hostDefined is not present, let hostDefined be empty.
Let state be the GraphLoadingState Record { [[IsLoading]]: true, [[PendingModulesCount]]: 1, [[Visited]]: « », [[PromiseCapability]]: pc, [[HostDefined]]: hostDefined }.
The hostDefined parameter can be used to pass additional information necessary to fetch the imported modules. It is used, for example, by HTML to set the correct fetch destination for <link rel="preload" as="..."> tags.
import() expressions never set the hostDefined parameter.
16.2.1.5.1.1 InnerModuleLoading ( state, module )
The abstract operation InnerModuleLoading takes arguments state (a GraphLoadingState Record) and module (a Module Record) and returns unused. It is used by LoadRequestedModules to recursively perform the actual loading process for module's dependency graph. It performs the following steps when called:
The Link concrete method of a Cyclic Module Recordmodule takes no arguments and returns either a normal completion containingunused or a throw completion. On success, Link transitions this module's [[Status]] from unlinked to linked. On failure, an exception is thrown and this module's [[Status]] remains unlinked. (Most of the work is done by the auxiliary function InnerModuleLinking.) It performs the following steps when called:
Assert: module.[[Status]] is one of unlinked, linked, evaluating-async, or evaluated.
16.2.1.5.2.1 InnerModuleLinking ( module, stack, index )
The abstract operation InnerModuleLinking takes arguments module (a Module Record), stack (a List of Cyclic Module Records), and index (a non-negative integer) and returns either a normal completion containing a non-negative integer or a throw completion. It is used by Link to perform the actual linking process for module, as well as recursively on all other modules in the dependency graph. The stack and index parameters, as well as a module's [[DFSIndex]] and [[DFSAncestorIndex]] fields, keep track of the depth-first search (DFS) traversal. In particular, [[DFSAncestorIndex]] is used to discover strongly connected components (SCCs), such that all modules in an SCC transition to linked together. It performs the following steps when called:
If requiredModule and module are the same Module Record, set done to true.
Return index.
16.2.1.5.3 Evaluate ( )
The Evaluate concrete method of a Cyclic Module Recordmodule takes no arguments and returns a Promise. Evaluate transitions this module's [[Status]] from linked to either evaluating-async or evaluated. The first time it is called on a module in a given strongly connected component, Evaluate creates and returns a Promise which resolves when the module has finished evaluating. This Promise is stored in the [[TopLevelCapability]] field of the [[CycleRoot]] for the component. Future invocations of Evaluate on any module in the component return the same Promise. (Most of the work is done by the auxiliary function InnerModuleEvaluation.) It performs the following steps when called:
Assert: This call to Evaluate is not happening at the same time as another call to Evaluate within the surrounding agent.
Assert: module.[[Status]] is one of linked, evaluating-async, or evaluated.
If module.[[Status]] is either evaluating-async or evaluated, set module to module.[[CycleRoot]].
If module.[[TopLevelCapability]] is not empty, then
16.2.1.5.3.1 InnerModuleEvaluation ( module, stack, index )
The abstract operation InnerModuleEvaluation takes arguments module (a Module Record), stack (a List of Cyclic Module Records), and index (a non-negative integer) and returns either a normal completion containing a non-negative integer or a throw completion. It is used by Evaluate to perform the actual evaluation process for module, as well as recursively on all other modules in the dependency graph. The stack and index parameters, as well as module's [[DFSIndex]] and [[DFSAncestorIndex]] fields, are used the same way as in InnerModuleLinking. It performs the following steps when called:
If requiredModule.[[AsyncEvaluation]] is false, set requiredModule.[[Status]] to evaluated.
Otherwise, set requiredModule.[[Status]] to evaluating-async.
If requiredModule and module are the same Module Record, set done to true.
Set requiredModule.[[CycleRoot]] to module.
Return index.
Note 1
A module is evaluating while it is being traversed by InnerModuleEvaluation. A module is evaluated on execution completion or evaluating-async during execution if its [[HasTLA]] field is true or if it has asynchronous dependencies.
Note 2
Any modules depending on a module of an asynchronous cycle when that cycle is not evaluating will instead depend on the execution of the root of the cycle via [[CycleRoot]]. This ensures that the cycle state can be treated as a single strongly connected component through its root module state.
16.2.1.5.3.2 ExecuteAsyncModule ( module )
The abstract operation ExecuteAsyncModule takes argument module (a Cyclic Module Record) and returns unused. It performs the following steps when called:
Assert: module.[[Status]] is either evaluating or evaluating-async.
The abstract operation GatherAvailableAncestors takes arguments module (a Cyclic Module Record) and execList (a List of Cyclic Module Records) and returns unused. It performs the following steps when called:
When an asynchronous execution for a root module is fulfilled, this function determines the list of modules which are able to synchronously execute together on this completion, populating them in execList.
The abstract operation AsyncModuleExecutionFulfilled takes argument module (a Cyclic Module Record) and returns unused. It performs the following steps when called:
Let sortedExecList be a List whose elements are the elements of execList, in the order in which they had their [[AsyncEvaluation]] fields set to true in InnerModuleEvaluation.
Assert: All elements of sortedExecList have their [[AsyncEvaluation]] field set to true, [[PendingAsyncDependencies]] field set to 0, and [[EvaluationError]] field set to empty.
The abstract operation AsyncModuleExecutionRejected takes arguments module (a Cyclic Module Record) and error (an ECMAScript language value) and returns unused. It performs the following steps when called:
NOTE: module.[[AsyncEvaluation]] is set to false for symmetry with AsyncModuleExecutionFulfilled. In InnerModuleEvaluation, the value of a module's [[AsyncEvaluation]] internal slot is unused when its [[EvaluationError]] internal slot is not empty.
This non-normative section gives a series of examples of the linking and evaluation of a few common module graphs, with a specific focus on how errors can occur.
First consider the following simple module graph:
Figure 2: A simple module graph
Let's first assume that there are no error conditions. When a host first calls A.LoadRequestedModules(), this will complete successfully by assumption, and recursively load the dependencies of B and C as well (respectively, C and none), and then set A.[[Status]] = B.[[Status]] = C.[[Status]] = unlinked. Then, when the host calls A.Link(), it will complete successfully (again by assumption) such that A.[[Status]] = B.[[Status]] = C.[[Status]] = linked. These preparatory steps can be performed at any time. Later, when the host is ready to incur any possible side effects of the modules, it can call A.Evaluate(), which will complete successfully, returning a Promise resolving to undefined (again by assumption), recursively having evaluated first C and then B. Each module's [[Status]] at this point will be evaluated.
Consider then cases involving linking errors, after a successful call to A.LoadRequestedModules(). If InnerModuleLinking of C succeeds but, thereafter, fails for B, for example because it imports something that C does not provide, then the original A.Link() will fail, and both A and B's [[Status]] remain unlinked. C's [[Status]] has become linked, though.
Finally, consider a case involving evaluation errors after a successful call to Link(). If InnerModuleEvaluation of C succeeds but, thereafter, fails for B, for example because B contains code that throws an exception, then the original A.Evaluate() will fail, returning a rejected Promise. The resulting exception will be recorded in both A and B's [[EvaluationError]] fields, and their [[Status]] will become evaluated. C will also become evaluated but, in contrast to A and B, will remain without an [[EvaluationError]], as it successfully completed evaluation. Storing the exception ensures that any time a host tries to reuse A or B by calling their Evaluate() method, it will encounter the same exception. (Hosts are not required to reuse Cyclic Module Records; similarly, hosts are not required to expose the exception objects thrown by these methods. However, the specification enables such uses.)
Now consider a different type of error condition:
Figure 3: A module graph with an unresolvable module
In this scenario, module A declares a dependency on some other module, but no Module Record exists for that module, i.e. HostLoadImportedModule calls FinishLoadingImportedModule with an exception when asked for it. This could occur for a variety of reasons, such as the corresponding resource not existing, or the resource existing but ParseModule returning some errors when trying to parse the resulting source text. Hosts can choose to expose the cause of failure via the completion they pass to FinishLoadingImportedModule. In any case, this exception causes a loading failure, which results in A's [[Status]] remaining new.
The difference here between loading, linking and evaluation errors is due to the following characteristic:
Evaluation must be only performed once, as it can cause side effects; it is thus important to remember whether evaluation has already been performed, even if unsuccessfully. (In the error case, it makes sense to also remember the exception because otherwise subsequent Evaluate() calls would have to synthesize a new one.)
Linking, on the other hand, is side-effect-free, and thus even if it fails, it can be retried at a later time with no issues.
Loading closely interacts with the host, and it may be desirable for some of them to allow users to retry failed loads (for example, if the failure is caused by temporarily bad network conditions).
Now, consider a module graph with a cycle:
Figure 4: A cyclic module graph
Here we assume that the entry point is module A, so that the host proceeds by calling A.LoadRequestedModules(), which performs InnerModuleLoading on A. This in turn calls InnerModuleLoading on B and C. Because of the cycle, this again triggers InnerModuleLoading on A, but at this point it is a no-op since A's dependencies loading has already been triggered during this LoadRequestedModules process. When all the modules in the graph have been successfully loaded, their [[Status]] transitions from new to unlinked at the same time.
Then the host proceeds by calling A.Link(), which performs InnerModuleLinking on A. This in turn calls InnerModuleLinking on B. Because of the cycle, this again triggers InnerModuleLinking on A, but at this point it is a no-op since A.[[Status]] is already linking. B.[[Status]] itself remains linking when control gets back to A and InnerModuleLinking is triggered on C. After this returns with C.[[Status]] being linked, both A and B transition from linking to linked together; this is by design, since they form a strongly connected component. It's possible to transition the status of modules in the same SCC at the same time because during this phase the module graph is traversed with a depth-first search.
An analogous story occurs for the evaluation phase of a cyclic module graph, in the success case.
Now consider a case where A has a linking error; for example, it tries to import a binding from C that does not exist. In that case, the above steps still occur, including the early return from the second call to InnerModuleLinking on A. However, once we unwind back to the original InnerModuleLinking on A, it fails during InitializeEnvironment, namely right after C.ResolveExport(). The thrown SyntaxError exception propagates up to A.Link, which resets all modules that are currently on its stack (these are always exactly the modules that are still linking). Hence both A and B become unlinked. Note that C is left as linked.
Alternatively, consider a case where A has an evaluation error; for example, its source code throws an exception. In that case, the evaluation-time analogue of the above steps still occurs, including the early return from the second call to InnerModuleEvaluation on A. However, once we unwind back to the original InnerModuleEvaluation on A, it fails by assumption. The exception thrown propagates up to A.Evaluate(), which records the error in all modules that are currently on its stack (i.e., the modules that are still evaluating) as well as via [[AsyncParentModules]], which form a chain for modules which contain or depend on top-level await through the whole dependency graph through the AsyncModuleExecutionRejected algorithm. Hence both A and B become evaluated and the exception is recorded in both A and B's [[EvaluationError]] fields, while C is left as evaluated with no [[EvaluationError]].
Lastly, consider a module graph with a cycle, where all modules complete asynchronously:
Figure 5: An asynchronous cyclic module graph
Loading and linking happen as before, and all modules end up with [[Status]] set to linked.
Calling A.Evaluate() calls InnerModuleEvaluation on A, B, and D, which all transition to evaluating. Then InnerModuleEvaluation is called on A again, which is a no-op because it is already evaluating. At this point, D.[[PendingAsyncDependencies]] is 0, so ExecuteAsyncModule(D) is called and we call D.ExecuteModule with a new PromiseCapability tracking the asynchronous execution of D. We unwind back to the InnerModuleEvaluation on B, setting B.[[PendingAsyncDependencies]] to 1 and B.[[AsyncEvaluation]] to true. We unwind back to the original InnerModuleEvaluation on A, setting A.[[PendingAsyncDependencies]] to 1. In the next iteration of the loop over A's dependencies, we call InnerModuleEvaluation on C and thus on D (again a no-op) and E. As E has no dependencies and is not part of a cycle, we call ExecuteAsyncModule(E) in the same manner as D and E is immediately removed from the stack. We unwind once more to the original InnerModuleEvaluation on A, setting C.[[AsyncEvaluation]] to true. Now we finish the loop over A's dependencies, set A.[[AsyncEvaluation]] to true, and remove the entire strongly connected component from the stack, transitioning all of the modules to evaluating-async at once. At this point, the fields of the modules are as given in Table 45.
Table 45: Module fields after the initial Evaluate() call
Field
Module
A
B
C
D
E
[[DFSIndex]]
0
1
3
2
4
[[DFSAncestorIndex]]
0
0
0
0
4
[[Status]]
evaluating-async
evaluating-async
evaluating-async
evaluating-async
evaluating-async
[[AsyncEvaluation]]
true
true
true
true
true
[[AsyncParentModules]]
« »
« A »
« A »
« B, C »
« C »
[[PendingAsyncDependencies]]
2 (B and C)
1 (D)
2 (D and E)
0
0
Let us assume that E finishes executing first. When that happens, AsyncModuleExecutionFulfilled is called, E.[[Status]] is set to evaluated and C.[[PendingAsyncDependencies]] is decremented to become 1. The fields of the updated modules are as given in Table 46.
Table 46: Module fields after module E finishes executing
Fields / Modules
C
E
[[DFSIndex]]
3
4
[[DFSAncestorIndex]]
0
4
[[Status]]
evaluating-async
evaluated
[[AsyncEvaluation]]
true
true
[[AsyncParentModules]]
« A »
« C »
[[PendingAsyncDependencies]]
1 (D)
0
D is next to finish (as it was the only module that was still executing). When that happens, AsyncModuleExecutionFulfilled is called again and D.[[Status]] is set to evaluated. Then B.[[PendingAsyncDependencies]] is decremented to become 0, ExecuteAsyncModule is called on B, and it starts executing. C.[[PendingAsyncDependencies]] is also decremented to become 0, and C starts executing (potentially in parallel to B if B contains an await). The fields of the updated modules are as given in Table 47.
Table 47: Module fields after module D finishes executing
Fields / Module
B
C
D
[[DFSIndex]]
1
3
2
[[DFSAncestorIndex]]
0
0
0
[[Status]]
evaluating-async
evaluating-async
evaluated
[[AsyncEvaluation]]
true
true
true
[[AsyncParentModules]]
« A »
« A »
« B, C »
[[PendingAsyncDependencies]]
0
0
0
Let us assume that C finishes executing next. When that happens, AsyncModuleExecutionFulfilled is called again, C.[[Status]] is set to evaluated and A.[[PendingAsyncDependencies]] is decremented to become 1. The fields of the updated modules are as given in Table 48.
Table 48: Module fields after module C finishes executing
Fields / Modules
A
C
[[DFSIndex]]
0
3
[[DFSAncestorIndex]]
0
0
[[Status]]
evaluating-async
evaluated
[[AsyncEvaluation]]
true
true
[[AsyncParentModules]]
« »
« A »
[[PendingAsyncDependencies]]
1 (B)
0
Then, B finishes executing. When that happens, AsyncModuleExecutionFulfilled is called again and B.[[Status]] is set to evaluated. A.[[PendingAsyncDependencies]] is decremented to become 0, so ExecuteAsyncModule is called and it starts executing. The fields of the updated modules are as given in Table 49.
Table 49: Module fields after module B finishes executing
Fields / Modules
A
B
[[DFSIndex]]
0
1
[[DFSAncestorIndex]]
0
0
[[Status]]
evaluating-async
evaluated
[[AsyncEvaluation]]
true
true
[[AsyncParentModules]]
« »
« A »
[[PendingAsyncDependencies]]
0
0
Finally, A finishes executing. When that happens, AsyncModuleExecutionFulfilled is called again and A.[[Status]] is set to evaluated. At this point, the Promise in A.[[TopLevelCapability]] (which was returned from A.Evaluate()) is resolved, and this concludes the handling of this module graph. The fields of the updated module are as given in Table 50.
Table 50: Module fields after module A finishes executing
Fields / Modules
A
[[DFSIndex]]
0
[[DFSAncestorIndex]]
0
[[Status]]
evaluated
[[AsyncEvaluation]]
true
[[AsyncParentModules]]
« »
[[PendingAsyncDependencies]]
0
Alternatively, consider a failure case where C fails execution and returns an error before B has finished executing. When that happens, AsyncModuleExecutionRejected is called, which sets C.[[Status]] to evaluated and C.[[EvaluationError]] to the error. It then propagates this error to all of the AsyncParentModules by performing AsyncModuleExecutionRejected on each of them. The fields of the updated modules are as given in Table 51.
Table 51: Module fields after module C finishes with an error
Fields / Modules
A
C
[[DFSIndex]]
0
3
[[DFSAncestorIndex]]
0
0
[[Status]]
evaluated
evaluated
[[AsyncEvaluation]]
true
true
[[AsyncParentModules]]
« »
« A »
[[PendingAsyncDependencies]]
1 (B)
0
[[EvaluationError]]
empty
C's evaluation error
A will be rejected with the same error as C since C will call AsyncModuleExecutionRejected on A with C's error. A.[[Status]] is set to evaluated. At this point the Promise in A.[[TopLevelCapability]] (which was returned from A.Evaluate()) is rejected. The fields of the updated module are as given in Table 52.
Table 52: Module fields after module A is rejected
Then, B finishes executing without an error. When that happens, AsyncModuleExecutionFulfilled is called again and B.[[Status]] is set to evaluated. GatherAvailableAncestors is called on B. However, A.[[CycleRoot]] is A which has an evaluation error, so it will not be added to the returned sortedExecList and AsyncModuleExecutionFulfilled will return without further processing. Any future importer of B will resolve the rejection of B.[[CycleRoot]].[[EvaluationError]] from the evaluation error from C that was set on the cycle root A. The fields of the updated modules are as given in Table 53.
Table 53: Module fields after module B finishes executing in an erroring graph
A Source Text Module Record is used to represent information about a module that was defined from ECMAScript source text (11) that was parsed using the goal symbolModule. Its fields contain digested information about the names that are imported and exported by the module, and its concrete methods use these digests to link and evaluate the module.
A List of ExportEntry records derived from the code of this module that correspond to reexported imports that occur within the module or exports from export * as namespace declarations.
A List of ExportEntry records derived from the code of this module that correspond to export * declarations that occur within the module, not including export * as namespace declarations.
An ImportEntry Record is a Record that digests information about a single declarative import. Each ImportEntry Record has the fields defined in Table 55:
The name under which the desired binding is exported by the module identified by [[ModuleRequest]]. The value namespace-object indicates that the import request is for the target module's namespace object.
[[LocalName]]
a String
The name that is used to locally access the imported value from within the importing module.
Note 1
Table 56 gives examples of ImportEntry records fields used to represent the syntactic import forms:
An ExportEntry Record is a Record that digests information about a single declarative export. Each ExportEntry Record has the fields defined in Table 57:
The name under which the desired binding is exported by the module identified by [[ModuleRequest]]. null if the ExportDeclaration does not have a ModuleSpecifier. all is used for export * as ns from "mod" declarations. all-but-default is used for export * from "mod" declarations.
[[LocalName]]
a String or null
The name that is used to locally access the exported value from within the importing module. null if the exported value is not locally accessible from within the module.
Note 2
Table 58 gives examples of the ExportEntry record fields used to represent the syntactic export forms:
An implementation may parse module source text and analyse it for Early Error conditions prior to the evaluation of ParseModule for that module source text. However, the reporting of any errors must be deferred until the point where this specification actually performs ParseModule upon that source text.
The ResolveExport concrete method of a Source Text Module Recordmodule takes argument exportName (a String) and optional argument resolveSet (a List of Records with fields [[Module]] (a Module Record) and [[ExportName]] (a String)) and returns a ResolvedBinding Record, null, or ambiguous.
ResolveExport attempts to resolve an imported binding to the actual defining module and local binding name. The defining module may be the module represented by the Module Record this method was invoked on or some other module that is imported by that module. The parameter resolveSet is used to detect unresolved circular import/export paths. If a pair consisting of specific Module Record and exportName is reached that is already in resolveSet, an import circularity has been encountered. Before recursively calling ResolveExport, a pair consisting of module and exportName is added to resolveSet.
If a defining module is found, a ResolvedBinding Record { [[Module]], [[BindingName]] } is returned. This record identifies the resolved binding of the originally requested export, unless this is the export of a namespace with no local binding. In this case, [[BindingName]] will be set to namespace. If no definition was found or the request is found to be circular, null is returned. If the request is found to be ambiguous, ambiguous is returned.
Assert: There is more than one * import that includes the requested name.
If resolution.[[Module]] and starResolution.[[Module]] are not the same Module Record, return ambiguous.
If resolution.[[BindingName]] is not starResolution.[[BindingName]] and either resolution.[[BindingName]] or starResolution.[[BindingName]] is namespace, return ambiguous.
If resolution.[[BindingName]]is a String, starResolution.[[BindingName]]is a String, and resolution.[[BindingName]] is not starResolution.[[BindingName]], return ambiguous.
The abstract operation GetImportedModule takes arguments referrer (a Cyclic Module Record) and specifier (a String) and returns a Module Record. It performs the following steps when called:
Assert: Exactly one element of referrer.[[LoadedModules]] is a Record whose [[Specifier]] is specifier, since LoadRequestedModules has completed successfully on referrer prior to invoking this abstract operation.
Let record be the Record in referrer.[[LoadedModules]] whose [[Specifier]] is specifier.
If this operation is called multiple times with the same (referrer, specifier) pair and it performs FinishLoadingImportedModule(referrer, specifier, payload, result) where result is a normal completion, then it must perform FinishLoadingImportedModule(referrer, specifier, payload, result) with the same result each time.
The actual process performed is host-defined, but typically consists of performing whatever I/O operations are necessary to load the appropriate Module Record. Multiple different (referrer, specifier) pairs may map to the same Module Record instance. The actual mapping semantics is host-defined but typically a normalization process is applied to specifier as part of the mapping process. A typical normalization process would include actions such as expansion of relative and abbreviated path specifiers.
16.2.1.9 FinishLoadingImportedModule ( referrer, specifier, payload, result )
The abstract operation GetModuleNamespace takes argument module (an instance of a concrete subclass of Module Record) and returns a Module Namespace Object or empty. It retrieves the Module Namespace Object representing module's exports, lazily creating it the first time it was requested, and storing it in module.[[Namespace]] for future retrieval. It performs the following steps when called:
GetModuleNamespace never throws. Instead, unresolvable names are simply excluded from the namespace at this point. They will lead to a real linking error later unless they are all ambiguous star exports that are not explicitly requested anywhere.
Return a List whose sole element is a new ExportEntry Record { [[ModuleRequest]]: null, [[ImportName]]: null, [[LocalName]]: localName, [[ExportName]]: "default" }.
Return a List whose sole element is a new ExportEntry Record { [[ModuleRequest]]: null, [[ImportName]]: null, [[LocalName]]: localName, [[ExportName]]: "default" }.
Let entry be the ExportEntry Record { [[ModuleRequest]]: null, [[ImportName]]: null, [[LocalName]]: "*default*", [[ExportName]]: "default" }.
Return « entry ».
Note
"*default*" is used within this specification as a synthetic name for anonymous default export values. See this note for more details.
16.2.3.5 Static Semantics: ExportEntriesForModule
The syntax-directed operation ExportEntriesForModule takes argument module (a String or null) and returns a List of ExportEntry Records. It is defined piecewise over the following productions:
Return a List whose sole element is a new ExportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: importName, [[LocalName]]: localName, [[ExportName]]: sourceName }.
Return a List whose sole element is a new ExportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: importName, [[LocalName]]: localName, [[ExportName]]: exportName }.
An implementation must report most errors at the time the relevant ECMAScript language construct is evaluated. An early error is an error that can be detected and reported prior to the evaluation of any construct in the Script containing the error. The presence of an early error prevents the evaluation of the construct. An implementation must report early errors in a Script as part of parsing that Script in ParseScript. Early errors in a Module are reported at the point when the Module would be evaluated and the Module is never initialized. Early errors in eval code are reported at the time eval is called and prevent evaluation of the eval code. All errors that are not early errors are runtime errors.
An implementation must report as an early error any occurrence of a condition that is listed in a “Static Semantics: Early Errors” subclause of this specification.
An implementation shall not treat other kinds of errors as early errors even if the compiler can prove that a construct cannot execute without error under any circumstances. An implementation may issue an early warning in such a case, but it should not report the error until the relevant construct is actually executed.
An implementation shall report all errors as specified, except for the following:
Except as restricted in 17.1, a host or implementation may extend Script syntax, Module syntax, and regular expression pattern or flag syntax. To permit this, all operations (such as calling eval, using a regular expression literal, or using the Function or RegExp constructor) that are allowed to throw SyntaxError are permitted to exhibit host-defined behaviour instead of throwing SyntaxError when they encounter a host-defined extension to the script syntax or regular expression pattern or flag syntax.
Except as restricted in 17.1, a host or implementation may provide additional types, values, objects, properties, and functions beyond those described in this specification. This may cause constructs (such as looking up a variable in the global scope) to have host-defined behaviour instead of throwing an error (such as ReferenceError).
17.1 Forbidden Extensions
An implementation must not extend this specification in the following ways:
If an implementation extends any function object with an own property named "caller" the value of that property, as observed using [[Get]] or [[GetOwnProperty]], must not be a strict function object. If it is an accessor property, the function that is the value of the property's [[Get]] attribute must never return a strict function when called.
Neither mapped nor unmapped arguments objects may be created with an own property named "caller".
The behaviour of built-in methods which are specified in ECMA-402, such as those named toLocaleString, must not be extended except as specified in ECMA-402.
The RegExp pattern grammars in 22.2.1 and B.1.2 must not be extended to recognize any of the source characters A-Z or a-z as IdentityEscape[+UnicodeMode] when the [UnicodeMode] grammar parameter is present.
The Syntactic Grammar must not be extended in any manner that allows the token : to immediately follow source text that is matched by the BindingIdentifier nonterminal symbol.
There are certain built-in objects available whenever an ECMAScript Script or Module begins execution. One, the global object, is part of the global environment of the executing program. Others are accessible as initial properties of the global object or indirectly as properties of accessible built-in objects.
Unless specified otherwise, a built-in object that is callable as a function is a built-in function object with the characteristics described in 10.3. Unless specified otherwise, the [[Extensible]] internal slot of a built-in object initially has the value true. Every built-in function object has a [[Realm]] internal slot whose value is the Realm Record of the realm for which the object was initially created.
Many built-in objects are functions: they can be invoked with arguments. Some of them furthermore are constructors: they are functions intended for use with the new operator. For each built-in function, this specification describes the arguments required by that function and the properties of that function object. For each built-in constructor, this specification furthermore describes properties of the prototype object of that constructor and properties of specific object instances returned by a new expression that invokes that constructor.
Unless otherwise specified in the description of a particular function, if a built-in function or constructor is given fewer arguments than the function is specified to require, the function or constructor shall behave exactly as if it had been given sufficient additional arguments, each such argument being the undefined value. Such missing arguments are considered to be “not present” and may be identified in that manner by specification algorithms. In the description of a particular function, the terms “this value” and “NewTarget” have the meanings given in 10.3.
Unless otherwise specified in the description of a particular function, if a built-in function or constructor described is given more arguments than the function is specified to allow, the extra arguments are evaluated by the call and then ignored by the function. However, an implementation may define implementation specific behaviour relating to such arguments as long as the behaviour is not the throwing of a TypeError exception that is predicated simply on the presence of an extra argument.
Note 1
Implementations that add additional capabilities to the set of built-in functions are encouraged to do so by adding new functions rather than adding new parameters to existing functions.
Unless otherwise specified every built-in function and every built-in constructor has the Function prototype object, which is the initial value of the expression Function.prototype (20.2.3), as the value of its [[Prototype]] internal slot.
Unless otherwise specified every built-in prototype object has the Object prototype object, which is the initial value of the expression Object.prototype (20.1.3), as the value of its [[Prototype]] internal slot, except the Object prototype object itself.
If this specification defines a built-in constructor's behaviour via algorithm steps, then that is its behaviour for the purposes of both [[Call]] and [[Construct]]. If such an algorithm needs to distinguish the two cases, it checks whether NewTarget is undefined, which indicates a [[Call]] invocation.
Built-in function objects that are not identified as constructors do not implement the [[Construct]] internal method unless otherwise specified in the description of a particular function.
Built-in function objects that are not constructors do not have a "prototype" property unless otherwise specified in the description of a particular function.
Each built-in function defined in this specification is created by calling the CreateBuiltinFunction abstract operation (10.3.4). The values of the length and name parameters are the initial values of the "length" and "name" properties as discussed below. The values of the prefix parameter are similarly discussed below.
Every built-in function object, including constructors, has a "length" property whose value is a non-negative integral Number. Unless otherwise specified, this value is the number of required parameters shown in the subclause heading for the function description. Optional parameters and rest parameters are not included in the parameter count.
Note 2
For example, the function object that is the initial value of the "map" property of the Array prototype object is described under the subclause heading «Array.prototype.map (callback [ , thisArg])» which shows the two named arguments callback and thisArg, the latter being optional; therefore the value of the "length" property of that function object is 1𝔽.
Unless otherwise specified, the "length" property of a built-in function object has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
Every built-in function object, including constructors, has a "name" property whose value is a String. Unless otherwise specified, this value is the name that is given to the function in this specification. Functions that are identified as anonymous functions use the empty String as the value of the "name" property. For functions that are specified as properties of objects, the name value is the property name string used to access the function. Functions that are specified as get or set accessor functions of built-in properties have "get" or "set" (respectively) passed to the prefix parameter when calling CreateBuiltinFunction.
The value of the "name" property is explicitly specified for each built-in functions whose property keyis a Symbol value. If such an explicitly specified value starts with the prefix "get " or "set " and the function for which it is specified is a get or set accessor function of a built-in property, the value without the prefix is passed to the name parameter, and the value "get" or "set" (respectively) is passed to the prefix parameter when calling CreateBuiltinFunction.
Unless otherwise specified, the "name" property of a built-in function object has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
Every other data property described in clauses 19 through 28 and in Annex B.2 has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true } unless otherwise specified.
Every accessor property described in clauses 19 through 28 and in Annex B.2 has the attributes { [[Enumerable]]: false, [[Configurable]]: true } unless otherwise specified. If only a get accessor function is described, the set accessor function is the default value, undefined. If only a set accessor is described the get accessor is the default value, undefined.
does not have a [[Construct]] internal method; it cannot be used as a constructor with the new operator.
does not have a [[Call]] internal method; it cannot be invoked as a function.
has a [[Prototype]] internal slot whose value is host-defined.
may have host-defined properties in addition to the properties defined in this specification. This may include a property whose value is the global object itself.
19.1 Value Properties of the Global Object
19.1.1 globalThis
The initial value of the "globalThis" property of the global object in a Realm Recordrealm is realm.[[GlobalEnv]].[[GlobalThisValue]].
This property has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }.
19.1.2 Infinity
The value of Infinity is +∞𝔽 (see 6.1.6.1). This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
19.1.3 NaN
The value of NaN is NaN (see 6.1.6.1). This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
19.1.4 undefined
The value of undefined is undefined (see 6.1.1). This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
NOTE: If direct is true, runningContext will be the execution context that performed the direct eval. If direct is false, runningContext will be the execution context for the invocation of the eval function.
The eval code cannot instantiate variable or function bindings in the variable environment of the calling context that invoked the eval if either the code of the calling context or the eval code is strict mode code. Instead such bindings are instantiated in a new VariableEnvironment that is only accessible to the eval code. Bindings introduced by let, const, or class declarations are always instantiated in a new LexicalEnvironment.
19.2.1.2 HostEnsureCanCompileStrings ( calleeRealm, parameterStrings, bodyString, direct )
The host-defined abstract operation HostEnsureCanCompileStrings takes arguments calleeRealm (a Realm Record), parameterStrings (a List of Strings), bodyString (a String), and direct (a Boolean) and returns either a normal completion containingunused or a throw completion. It allows host environments to block certain ECMAScript functions which allow developers to interpret and evaluate strings as ECMAScript code.
parameterStrings represents the strings that, when using one of the function constructors, will be concatenated together to build the parameters list. bodyString represents the function body or the string passed to an eval call.
direct signifies whether the evaluation is a direct eval.
The default implementation of HostEnsureCanCompileStrings is to return NormalCompletion(unused).
Let trimmedPrefix be the longest prefix of trimmed that satisfies the syntax of a StrDecimalLiteral, which might be trimmed itself. If there is no such prefix, return NaN.
This function may interpret only a leading portion of string as a Number value; it ignores any code units that cannot be interpreted as part of the notation of a decimal literal, and no indication is given that any such code units were ignored.
19.2.5 parseInt ( string, radix )
This function produces an integral Number dictated by interpretation of the contents of string according to the specified radix. Leading white space in string is ignored. If radix coerces to 0 (such as when it is undefined), it is assumed to be 10 except when the number representation begins with "0x" or "0X", in which case it is assumed to be 16. If radix is 16, the number representation may optionally begin with "0x" or "0X".
If S is not empty and the first code unit of S is the code unit 0x002D (HYPHEN-MINUS), set sign to -1.
If S is not empty and the first code unit of S is either the code unit 0x002B (PLUS SIGN) or the code unit 0x002D (HYPHEN-MINUS), set S to the substring of S from index 1.
If S contains a code unit that is not a radix-R digit, let end be the index within S of the first such code unit; otherwise, let end be the length of S.
Let mathInt be the integer value that is represented by Z in radix-R notation, using the letters A through Z and a through z for digits with values 10 through 35. (However, if R = 10 and Z contains more than 20 significant digits, every significant digit after the 20th may be replaced by a 0 digit, at the option of the implementation; and if R is not one of 2, 4, 8, 10, 16, or 32, then mathInt may be an implementation-approximatedinteger representing the integer value denoted by Z in radix-R notation.)
This function may interpret only a leading portion of string as an integer value; it ignores any code units that cannot be interpreted as part of the notation of an integer, and no indication is given that any such code units were ignored.
19.2.6 URI Handling Functions
Uniform Resource Identifiers, or URIs, are Strings that identify resources (e.g. web pages or files) and transport protocols by which to access them (e.g. HTTP or FTP) on the Internet. The ECMAScript language itself does not provide any support for using URIs except for functions that encode and decode URIs as described in this section. encodeURI and decodeURI are intended to work with complete URIs; they assume that any reserved characters are intended to have special meaning (e.g., as delimiters) and so are not encoded. encodeURIComponent and decodeURIComponent are intended to work with the individual components of a URI; they assume that any reserved characters represent text and must be encoded to avoid special meaning when the component is part of a complete URI.
Note 1
The set of reserved characters is based upon RFC 2396 and does not reflect changes introduced by the more recent RFC 3986.
Note 2
Many implementations of ECMAScript provide additional functions and methods that manipulate web pages; these functions are beyond the scope of this standard.
19.2.6.1 decodeURI ( encodedURI )
This function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the sort that might be introduced by the encodeURI function is replaced with the UTF-16 encoding of the code point that it represents. Escape sequences that could not have been introduced by encodeURI are not replaced.
This function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the sort that might be introduced by the encodeURIComponent function is replaced with the UTF-16 encoding of the code point that it represents.
It is the %decodeURIComponent% intrinsic object.
It performs the following steps when called:
Let componentString be ? ToString(encodedURIComponent).
This function computes a new version of a UTF-16 encoded (6.1.4) URI in which each instance of certain code points is replaced by one, two, three, or four escape sequences representing the UTF-8 encoding of the code point.
This function computes a new version of a UTF-16 encoded (6.1.4) URI in which each instance of certain code points is replaced by one, two, three, or four escape sequences representing the UTF-8 encoding of the code point.
The abstract operation Encode takes arguments string (a String) and extraUnescaped (a String) and returns either a normal completion containing a String or a throw completion. It performs URI encoding and escaping, interpreting string as a sequence of UTF-16 encoded code points as described in 6.1.4. If a character is identified as unreserved in RFC 2396 or appears in extraUnescaped, it is not escaped. It performs the following steps when called:
Because percent-encoding is used to represent individual octets, a single code point may be expressed as multiple consecutive escape sequences (one for each of its 8-bit UTF-8 code units).
19.2.6.6 Decode ( string, preserveEscapeSet )
The abstract operation Decode takes arguments string (a String) and preserveEscapeSet (a String) and returns either a normal completion containing a String or a throw completion. It performs URI unescaping and decoding, preserving any escape sequences that correspond to Basic Latin characters in preserveEscapeSet. It performs the following steps when called:
Let len be the length of string.
Let R be the empty String.
Let k be 0.
Repeat, while k < len,
Let C be the code unit at index k within string.
Let S be C.
If C is the code unit 0x0025 (PERCENT SIGN), then
If k + 3 > len, throw a URIError exception.
Let escape be the substring of string from k to k + 3.
RFC 3629 prohibits the decoding of invalid UTF-8 octet sequences. For example, the invalid sequence 0xC0 0x80 must not decode into the code unit 0x0000. Implementations of the Decode algorithm are required to throw a URIError when encountering such invalid sequences.
19.2.6.7 ParseHexOctet ( string, position )
The abstract operation ParseHexOctet takes arguments string (a String) and position (a non-negative integer) and returns either a non-negative integer or a non-empty List of SyntaxError objects. It parses a sequence of two hexadecimal characters at the specified position in string into an unsigned 8-bit integer. It performs the following steps when called:
callback should be a function that accepts two arguments. groupBy calls callback once for each element in items, in ascending order, and constructs a new object. Each value returned by callback is coerced to a property key. For each such property key, the result object has a property whose key is that property key and whose value is an array containing all the elements for which the callback return value coerced to that key.
callback is called with two arguments: the value of the element and the index of the element.
The return value of groupBy is an object that does not inherit from %Object.prototype%.
This function performs the following steps when called:
Let groups be ? GroupBy(items, callback, property).
The ordering of steps 1 and 2 is chosen to ensure that any exception that would have been thrown by step 1 in previous editions of this specification will continue to be thrown even if the this value is undefined or null.
20.1.3.3 Object.prototype.isPrototypeOf ( V )
This method performs the following steps when called:
The ordering of steps 1 and 2 preserves the behaviour specified by previous editions of this specification for the case where V is not an object and the this value is undefined or null.
20.1.3.4 Object.prototype.propertyIsEnumerable ( V )
This method performs the following steps when called:
This method does not consider objects in the prototype chain.
Note 2
The ordering of steps 1 and 2 is chosen to ensure that any exception that would have been thrown by step 1 in previous editions of this specification will continue to be thrown even if the this value is undefined or null.
The optional parameters to this method are not used but are intended to correspond to the parameter pattern used by ECMA-402 toLocaleString methods. Implementations that do not include ECMA-402 support must not use those parameter positions for other purposes.
Note 1
This method provides a generic toLocaleString implementation for objects that have no locale-sensitive toString behaviour. Array, Number, Date, and %TypedArray% provide their own locale-sensitive toLocaleString methods.
Note 2
ECMA-402 intentionally does not provide an alternative to this default implementation.
20.1.3.6 Object.prototype.toString ( )
This method performs the following steps when called:
If the this value is undefined, return "[object Undefined]".
If the this value is null, return "[object Null]".
Historically, this method was occasionally used to access the String value of the [[Class]] internal slot that was used in previous editions of this specification as a nominal type tag for various built-in objects. The above definition of toString preserves compatibility for legacy code that uses toString as a test for those specific kinds of built-in objects. It does not provide a reliable type testing mechanism for other kinds of built-in or program defined objects. In addition, programs can use %Symbol.toStringTag% in ways that will invalidate the reliability of such legacy type tests.
20.1.3.7 Object.prototype.valueOf ( )
This method performs the following steps when called:
Object.prototype.__proto__ is an accessor property with attributes { [[Enumerable]]: false, [[Configurable]]: true }. The [[Get]] and [[Set]] attributes are defined as follows:
20.1.3.8.1 get Object.prototype.__proto__
The value of the [[Get]] attribute is a built-in function that requires no arguments. It performs the following steps when called:
is the initial value of the "Function" property of the global object.
creates and initializes a new function object when called as a function rather than as a constructor. Thus the function call Function(…) is equivalent to the object creation expression new Function(…) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Function behaviour must include a super call to the Function constructor to create and initialize a subclass instance with the internal slots necessary for built-in function behaviour. All ECMAScript syntactic forms for defining function objects create instances of Function. There is no syntactic means to create instances of Function subclasses except for the built-in GeneratorFunction, AsyncFunction, and AsyncGeneratorFunction subclasses.
20.2.1.1 Function ( ...parameterArgs, bodyArg )
The last argument (if any) specifies the body (executable code) of a function; any preceding arguments specify formal parameters.
This function performs the following steps when called:
It is permissible but not necessary to have one argument for each formal parameter to be specified. For example, all three of the following expressions produce the same result:
The abstract operation CreateDynamicFunction takes arguments constructor (a constructor), newTarget (a constructor), kind (normal, generator, async, or async-generator), parameterArgs (a List of ECMAScript language values), and bodyArg (an ECMAScript language value) and returns either a normal completion containing an ECMAScript function object or a throw completion. constructor is the constructor function that is performing this action. newTarget is the constructor that new was initially applied to. parameterArgs and bodyArg reflect the argument values that were passed to constructor. It performs the following steps when called:
If newTarget is undefined, set newTarget to constructor.
If body is a List of errors, throw a SyntaxError exception.
NOTE: The parameters and body are parsed separately to ensure that each is valid alone. For example, new Function("/*", "*/ ) {") does not evaluate to a function.
NOTE: If this step is reached, sourceText must have the syntax of exprSym (although the reverse implication does not hold). The purpose of the next two steps is to enforce any Early Error rules which apply to exprSym directly.
NOTE: Functions whose kind is async are not constructable and do not have a [[Construct]] internal method or a "prototype" property.
Return F.
Note
CreateDynamicFunction defines a "prototype" property on any function it creates whose kind is not async to provide for the possibility that the function will be used as a constructor.
has a "name" property whose value is the empty String.
Note
The Function prototype object is specified to be a function object to ensure compatibility with ECMAScript code that was created prior to the ECMAScript 2015 specification.
The thisArg value is passed without modification as the this value. This is a change from Edition 3, where an undefined or nullthisArg is replaced with the global object and ToObject is applied to all other values and that result is passed as the this value. Even though the thisArg is passed without modification, non-strict functions still perform these transformations upon entry to the function.
Note 2
If func is either an arrow function or a bound function exotic object, then the thisArg will be ignored by the function [[Call]] in step 6.
Function objects created using Function.prototype.bind are exotic objects. They also do not have a "prototype" property.
Note 2
If Target is either an arrow function or a bound function exotic object, then the thisArg passed to this method will not be used by subsequent calls to F.
The thisArg value is passed without modification as the this value. This is a change from Edition 3, where an undefined or nullthisArg is replaced with the global object and ToObject is applied to all other values and that result is passed as the this value. Even though the thisArg is passed without modification, non-strict functions still perform these transformations upon entry to the function.
Note 2
If func is either an arrow function or a bound function exotic object, then the thisArg will be ignored by the function [[Call]] in step 4.
20.2.3.4 Function.prototype.constructor
The initial value of Function.prototype.constructor is %Function%.
20.2.3.5 Function.prototype.toString ( )
This method performs the following steps when called:
Let func be the this value.
If funcis an Object, func has a [[SourceText]] internal slot, func.[[SourceText]] is a sequence of Unicode code points, and HostHasSourceTextAvailable(func) is true, then
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
Note
This is the default implementation of %Symbol.hasInstance% that most functions inherit. %Symbol.hasInstance% is called by the instanceof operator to determine whether a value is an instance of a specific constructor. An expression such as
v instanceof F
evaluates as
F[%Symbol.hasInstance%](v)
A constructor function can control which objects are recognized as its instances by instanceof by exposing a different %Symbol.hasInstance% method on the function.
This property is non-writable and non-configurable to prevent tampering that could be used to globally expose the target function of a bound function.
The value of the "name" property of this method is "[Symbol.hasInstance]".
20.2.4 Function Instances
Every Function instance is an ECMAScript function object and has the internal slots listed in Table 30. Function objects created using the Function.prototype.bind method (20.2.3.2) have the internal slots listed in Table 31.
Function instances have the following properties:
20.2.4.1 length
The value of the "length" property is an integral Number that indicates the typical number of arguments expected by the function. However, the language permits the function to be invoked with some other number of arguments. The behaviour of a function when invoked on a number of arguments other than the number specified by its "length" property depends on the function. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
20.2.4.2 name
The value of the "name" property is a String that is descriptive of the function. The name has no semantic significance but is typically a variable or property name that is used to refer to the function at its point of definition in ECMAScript source text. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
Anonymous functions objects that do not have a contextual name associated with them by this specification use the empty String as the value of the "name" property.
20.2.4.3 prototype
Function instances that can be used as a constructor have a "prototype" property. Whenever such a Function instance is created another ordinary object is also created and is the initial value of the function's "prototype" property. Unless otherwise specified, the value of the "prototype" property is used to initialize the [[Prototype]] internal slot of the object created when that function is invoked as a constructor.
This property has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.
The host-defined abstract operation HostHasSourceTextAvailable takes argument func (a function object) and returns a Boolean. It allows host environments to prevent the source text from being provided for func.
An implementation of HostHasSourceTextAvailable must conform to the following requirements:
It must be deterministic with respect to its parameters. Each time it is called with a specific func as its argument, it must return the same result.
The default implementation of HostHasSourceTextAvailable is to return true.
is the initial value of the "Boolean" property of the global object.
creates and initializes a new Boolean object when called as a constructor.
performs a type conversion when called as a function rather than as a constructor.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Boolean behaviour must include a super call to the Boolean constructor to create and initialize the subclass instance with a [[BooleanData]] internal slot.
20.3.1.1 Boolean ( value )
This function performs the following steps when called:
Boolean instances are ordinary objects that inherit properties from the Boolean prototype object. Boolean instances have a [[BooleanData]] internal slot. The [[BooleanData]] internal slot is the Boolean value represented by this Boolean object.
For each element e of the GlobalSymbolRegistry List, do
If e.[[Key]] is stringKey, return e.[[Symbol]].
Assert: GlobalSymbolRegistry does not currently contain an entry for stringKey.
Let newSymbol be a new Symbol whose [[Description]] is stringKey.
Append the Record { [[Key]]: stringKey, [[Symbol]]: newSymbol } to the GlobalSymbolRegistry List.
Return newSymbol.
The GlobalSymbolRegistry is an append-only List that is globally available. It is shared by all realms. Prior to the evaluation of any ECMAScript code, it is initialized as a new empty List. Elements of the GlobalSymbolRegistry are Records with the structure defined in Table 59.
The initial value of Symbol.prototype.constructor is %Symbol%.
20.4.3.2 get Symbol.prototype.description
Symbol.prototype.description is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
The initial value of the %Symbol.toStringTag% property is the String value "Symbol".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
20.4.4 Properties of Symbol Instances
Symbol instances are ordinary objects that inherit properties from the Symbol prototype object. Symbol instances have a [[SymbolData]] internal slot. The [[SymbolData]] internal slot is the Symbol value represented by this Symbol object.
20.4.5 Abstract Operations for Symbols
20.4.5.1 KeyForSymbol ( sym )
The abstract operation KeyForSymbol takes argument sym (a Symbol) and returns a String or undefined. If sym is in the GlobalSymbolRegistry (see 20.4.2.2) the String used to register sym will be returned. It performs the following steps when called:
For each element e of the GlobalSymbolRegistry List, do
If SameValue(e.[[Symbol]], sym) is true, return e.[[Key]].
Assert: GlobalSymbolRegistry does not currently contain an entry for sym.
Return undefined.
20.5 Error Objects
Instances of Error objects are thrown as exceptions when runtime errors occur. The Error objects may also serve as base objects for user-defined exception classes.
When an ECMAScript implementation detects a runtime error, it throws a new instance of one of the NativeError objects defined in 20.5.5 or a new instance of the AggregateError object defined in 20.5.7.
is the initial value of the "Error" property of the global object.
creates and initializes a new Error object when called as a function rather than as a constructor. Thus the function call Error(…) is equivalent to the object creation expression new Error(…) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Error behaviour must include a super call to the Error constructor to create and initialize subclass instances with an [[ErrorData]] internal slot.
20.5.1.1 Error ( message [ , options ] )
This function performs the following steps when called:
If NewTarget is undefined, let newTarget be the active function object; else let newTarget be NewTarget.
If msg is undefined, set msg to the empty String; otherwise set msg to ? ToString(msg).
If name is the empty String, return msg.
If msg is the empty String, return name.
Return the string-concatenation of name, the code unit 0x003A (COLON), the code unit 0x0020 (SPACE), and msg.
20.5.4 Properties of Error Instances
Error instances are ordinary objects that inherit properties from the Error prototype object and have an [[ErrorData]] internal slot whose value is undefined. The only specified uses of [[ErrorData]] is to identify Error, AggregateError, and NativeError instances as Error objects within Object.prototype.toString.
20.5.5 Native Error Types Used in This Standard
A new instance of one of the NativeError objects below or of the AggregateError object is thrown when a runtime error is detected. All NativeError objects share the same structure, as described in 20.5.6.
Indicates that one of the global URI handling functions was used in a way that is incompatible with its definition.
20.5.6NativeError Object Structure
Each of these objects has the structure described below, differing only in the name used as the constructor name and in the "name" property of the prototype object.
For each error object, references to NativeError in the definition should be replaced with the appropriate error object name from 20.5.5.
creates and initializes a new NativeError object when called as a function rather than as a constructor. A call of the object as a function is equivalent to calling it as a constructor with the same arguments. Thus the function call NativeError(…) is equivalent to the object creation expression new NativeError(…) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified NativeError behaviour must include a super call to the NativeErrorconstructor to create and initialize subclass instances with an [[ErrorData]] internal slot.
20.5.6.1.1NativeError ( message [ , options ] )
Each NativeError function performs the following steps when called:
If NewTarget is undefined, let newTarget be the active function object; else let newTarget be NewTarget.
The actual value of the string passed in step 2 is either "%EvalError.prototype%", "%RangeError.prototype%", "%ReferenceError.prototype%", "%SyntaxError.prototype%", "%TypeError.prototype%", or "%URIError.prototype%" corresponding to which NativeErrorconstructor is being defined.
20.5.6.2 Properties of the NativeError Constructors
The initial value of the "constructor" property of the prototype for a given NativeErrorconstructor is the constructor itself.
20.5.6.3.2NativeError.prototype.message
The initial value of the "message" property of the prototype for a given NativeErrorconstructor is the empty String.
20.5.6.3.3NativeError.prototype.name
The initial value of the "name" property of the prototype for a given NativeErrorconstructor is the String value consisting of the name of the constructor (the name used instead of NativeError).
20.5.6.4 Properties of NativeError Instances
NativeError instances are ordinary objects that inherit properties from their NativeError prototype object and have an [[ErrorData]] internal slot whose value is undefined. The only specified use of [[ErrorData]] is by Object.prototype.toString (20.1.3.6) to identify Error, AggregateError, or NativeError instances.
is the initial value of the "AggregateError" property of the global object.
creates and initializes a new AggregateError object when called as a function rather than as a constructor. Thus the function call AggregateError(…) is equivalent to the object creation expression new AggregateError(…) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified AggregateError behaviour must include a super call to the AggregateError constructor to create and initialize subclass instances with an [[ErrorData]] internal slot.
The initial value of AggregateError.prototype.constructor is %AggregateError%.
20.5.7.3.2 AggregateError.prototype.message
The initial value of AggregateError.prototype.message is the empty String.
20.5.7.3.3 AggregateError.prototype.name
The initial value of AggregateError.prototype.name is "AggregateError".
20.5.7.4 Properties of AggregateError Instances
AggregateError instances are ordinary objects that inherit properties from their AggregateError prototype object and have an [[ErrorData]] internal slot whose value is undefined. The only specified use of [[ErrorData]] is by Object.prototype.toString (20.1.3.6) to identify Error, AggregateError, or NativeError instances.
20.5.8 Abstract Operations for Error Objects
20.5.8.1 InstallErrorCause ( O, options )
The abstract operation InstallErrorCause takes arguments O (an Object) and options (an ECMAScript language value) and returns either a normal completion containingunused or a throw completion. It is used to create a "cause" property on O when a "cause" property is present on options. It performs the following steps when called:
is the initial value of the "Number" property of the global object.
creates and initializes a new Number object when called as a constructor.
performs a type conversion when called as a function rather than as a constructor.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Number behaviour must include a super call to the Number constructor to create and initialize the subclass instance with a [[NumberData]] internal slot.
21.1.1.1 Number ( value )
This function performs the following steps when called:
The value of Number.EPSILON is the Number value for the magnitude of the difference between 1 and the smallest value greater than 1 that is representable as a Number value, which is approximately 2.2204460492503130808472633361816 × 10-16.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
21.1.2.2 Number.isFinite ( number )
This function performs the following steps when called:
This function differs from the global isNaN function (19.2.3) in that it does not convert its argument to a Number before determining whether it is NaN.
Due to rounding behaviour necessitated by precision limitations of IEEE 754-2019, the Number value for every integer greater than Number.MAX_SAFE_INTEGER is shared with at least one other integer. Such large-magnitude integers are therefore not safe, and are not guaranteed to be exactly representable as Number values or even to be distinguishable from each other. For example, both 9007199254740992 and 9007199254740993 evaluate to the Number value 9007199254740992𝔽.
The value of Number.MAX_SAFE_INTEGER is 9007199254740991𝔽 (𝔽(253 - 1)).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
21.1.2.7 Number.MAX_VALUE
The value of Number.MAX_VALUE is the largest positive finite value of the Number type, which is approximately 1.7976931348623157 × 10308.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
21.1.2.8 Number.MIN_SAFE_INTEGER
Note
Due to rounding behaviour necessitated by precision limitations of IEEE 754-2019, the Number value for every integer less than Number.MIN_SAFE_INTEGER is shared with at least one other integer. Such large-magnitude integers are therefore not safe, and are not guaranteed to be exactly representable as Number values or even to be distinguishable from each other. For example, both -9007199254740992 and -9007199254740993 evaluate to the Number value -9007199254740992𝔽.
The value of Number.MIN_SAFE_INTEGER is -9007199254740991𝔽 (𝔽(-(253 - 1))).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
21.1.2.9 Number.MIN_VALUE
The value of Number.MIN_VALUE is the smallest positive value of the Number type, which is approximately 5 × 10-324.
In the IEEE 754-2019 double precision binary representation, the smallest possible value is a denormalized number. If an implementation does not support denormalized values, the value of Number.MIN_VALUE must be the smallest non-zero positive value that can actually be represented by the implementation.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
21.1.2.10 Number.NaN
The value of Number.NaN is NaN.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
21.1.2.11 Number.NEGATIVE_INFINITY
The value of Number.NEGATIVE_INFINITY is -∞𝔽.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
21.1.2.12 Number.parseFloat ( string )
The initial value of the "parseFloat" property is %parseFloat%.
21.1.2.13 Number.parseInt ( string, radix )
The initial value of the "parseInt" property is %parseInt%.
21.1.2.14 Number.POSITIVE_INFINITY
The value of Number.POSITIVE_INFINITY is +∞𝔽.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
Unless explicitly stated otherwise, the methods of the Number prototype object defined below are not generic and the this value passed to them must be either a Number value or an object that has a [[NumberData]] internal slot that has been initialized to a Number value.
The phrase “this Number value” within the specification of a method refers to the result returned by calling the abstract operation ThisNumberValue with the this value of the method invocation passed as the argument.
21.1.3.1 Number.prototype.constructor
The initial value of Number.prototype.constructor is %Number%.
This method returns a String containing this Number value represented in decimal exponential notation with one digit before the significand's decimal point and fractionDigits digits after the significand's decimal point. If fractionDigits is undefined, it includes as many significand digits as necessary to uniquely specify the Number (just like in ToString except that in this case the Number is always output in exponential notation).
Let m be the String value consisting of f + 1 occurrences of the code unit 0x0030 (DIGIT ZERO).
Let e be 0.
Else,
If fractionDigits is not undefined, then
Let e and n be integers such that 10f ≤ n < 10f + 1 and for which n × 10e - f - x is as close to zero as possible. If there are two such sets of e and n, pick the e and n for which n × 10e - f is larger.
Else,
Let e, n, and ff be integers such that ff ≥ 0, 10ff ≤ n < 10ff + 1, 𝔽(n × 10e - ff) is 𝔽(x), and ff is as small as possible. Note that the decimal representation of n has ff + 1 digits, n is not divisible by 10, and the least significant digit of n is not necessarily uniquely determined by these criteria.
Set f to ff.
Let m be the String value consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
For implementations that provide more accurate conversions than required by the rules above, it is recommended that the following alternative version of step 10.b.i be used as a guideline:
Let e, n, and f be integers such that f ≥ 0, 10f ≤ n < 10f + 1, 𝔽(n × 10e - f) is 𝔽(x), and f is as small as possible. If there are multiple possibilities for n, choose the value of n for which 𝔽(n × 10e - f) is closest in value to 𝔽(x). If there are two such possible values of n, choose the one that is even.
This method returns a String containing this Number value represented in decimal fixed-point notation with fractionDigits digits after the decimal point. If fractionDigits is undefined, 0 is assumed.
Let n be an integer for which n / 10f - x is as close to zero as possible. If there are two such n, pick the larger n.
If n = 0, let m be "0". Otherwise, let m be the String value consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
If f ≠ 0, then
Let k be the length of m.
If k ≤ f, then
Let z be the String value consisting of f + 1 - k occurrences of the code unit 0x0030 (DIGIT ZERO).
The output of toFixed may be more precise than toString for some values because toString only prints enough significant digits to distinguish the number from adjacent Number values. For example,
(1000000000000000128).toString() returns "1000000000000000100", while (1000000000000000128).toFixed(0) returns "1000000000000000128".
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of this method is used:
This method produces a String value that represents this Number value formatted according to the conventions of the host environment's current locale. This method is implementation-defined, and it is permissible, but not encouraged, for it to return the same thing as toString.
The meanings of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
This method returns a String containing this Number value represented either in decimal exponential notation with one digit before the significand's decimal point and precision - 1 digits after the significand's decimal point or in decimal fixed notation with precision significant digits. If precision is undefined, it calls ToString instead.
Let m be the String value consisting of p occurrences of the code unit 0x0030 (DIGIT ZERO).
Let e be 0.
Else,
Let e and n be integers such that 10p - 1 ≤ n < 10p and for which n × 10e - p + 1 - x is as close to zero as possible. If there are two such sets of e and n, pick the e and n for which n × 10e - p + 1 is larger.
Let m be the String value consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
Set m to the string-concatenation of the first e + 1 code units of m, the code unit 0x002E (FULL STOP), and the remaining p - (e + 1) code units of m.
Else,
Set m to the string-concatenation of the code unit 0x0030 (DIGIT ZERO), the code unit 0x002E (FULL STOP), -(e + 1) occurrences of the code unit 0x0030 (DIGIT ZERO), and the String m.
The optional radix should be an integral Number value in the inclusive interval from 2𝔽 to 36𝔽. If radix is undefined then 10𝔽 is used as the value of radix.
This method performs the following steps when called:
This method is not generic; it throws a TypeError exception if its this value is not a Number or a Number object. Therefore, it cannot be transferred to other kinds of objects for use as a method.
Number instances are ordinary objects that inherit properties from the Number prototype object. Number instances also have a [[NumberData]] internal slot. The [[NumberData]] internal slot is the Number value represented by this Number object.
is the initial value of the "BigInt" property of the global object.
performs a type conversion when called as a function rather than as a constructor.
is not intended to be used with the new operator or to be subclassed. It may be used as the value of an extends clause of a class definition but a super call to the BigInt constructor will cause an exception.
21.2.1.1 BigInt ( value )
This function performs the following steps when called:
If NewTarget is not undefined, throw a TypeError exception.
The abstract operation NumberToBigInt takes argument number (a Number) and returns either a normal completion containing a BigInt or a throw completion. It performs the following steps when called:
If number is not an integral Number, throw a RangeError exception.
The phrase “this BigInt value” within the specification of a method refers to the result returned by calling the abstract operation ThisBigIntValue with the this value of the method invocation passed as the argument.
21.2.3.1 BigInt.prototype.constructor
The initial value of BigInt.prototype.constructor is %BigInt%.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of this method is used:
This method produces a String value that represents this BigInt value formatted according to the conventions of the host environment's current locale. This method is implementation-defined, and it is permissible, but not encouraged, for it to return the same thing as toString.
The meanings of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
21.2.3.3 BigInt.prototype.toString ( [ radix ] )
Note
The optional radix should be an integral Number value in the inclusive interval from 2𝔽 to 36𝔽. If radix is undefined then 10𝔽 is used as the value of radix.
This method performs the following steps when called:
This method is not generic; it throws a TypeError exception if its this value is not a BigInt or a BigInt object. Therefore, it cannot be transferred to other kinds of objects for use as a method.
The initial value of the %Symbol.toStringTag% property is the String value "BigInt".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
21.2.4 Properties of BigInt Instances
BigInt instances are ordinary objects that inherit properties from the BigInt prototype object. BigInt instances also have a [[BigIntData]] internal slot. The [[BigIntData]] internal slot is the BigInt value represented by this BigInt object.
21.3 The Math Object
The Math object:
is %Math%.
is the initial value of the "Math" property of the global object.
does not have a [[Construct]] internal method; it cannot be used as a constructor with the new operator.
does not have a [[Call]] internal method; it cannot be invoked as a function.
Note
In this specification, the phrase “the Number value forx” has a technical meaning defined in 6.1.6.1.
21.3.1 Value Properties of the Math Object
21.3.1.1 Math.E
The Number value fore, the base of the natural logarithms, which is approximately 2.7182818284590452354.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
21.3.1.2 Math.LN10
The Number value for the natural logarithm of 10, which is approximately 2.302585092994046.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
21.3.1.3 Math.LN2
The Number value for the natural logarithm of 2, which is approximately 0.6931471805599453.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
21.3.1.4 Math.LOG10E
The Number value for the base-10 logarithm of e, the base of the natural logarithms; this value is approximately 0.4342944819032518.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
Note
The value of Math.LOG10E is approximately the reciprocal of the value of Math.LN10.
21.3.1.5 Math.LOG2E
The Number value for the base-2 logarithm of e, the base of the natural logarithms; this value is approximately 1.4426950408889634.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
Note
The value of Math.LOG2E is approximately the reciprocal of the value of Math.LN2.
21.3.1.6 Math.PI
The Number value for π, the ratio of the circumference of a circle to its diameter, which is approximately 3.1415926535897932.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
21.3.1.7 Math.SQRT1_2
The Number value for the square root of ½, which is approximately 0.7071067811865476.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
Note
The value of Math.SQRT1_2 is approximately the reciprocal of the value of Math.SQRT2.
21.3.1.8 Math.SQRT2
The Number value for the square root of 2, which is approximately 1.4142135623730951.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
21.3.1.9 Math [ %Symbol.toStringTag% ]
The initial value of the %Symbol.toStringTag% property is the String value "Math".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
21.3.2 Function Properties of the Math Object
Note
The behaviour of the functions acos, acosh, asin, asinh, atan, atanh, atan2, cbrt, cos, cosh, exp, expm1, hypot, log, log1p, log2, log10, pow, random, sin, sinh, tan, and tanh is not precisely specified here except to require specific results for certain argument values that represent boundary cases of interest. For other argument values, these functions are intended to compute approximations to the results of familiar mathematical functions, but some latitude is allowed in the choice of approximation algorithms. The general intent is that an implementer should be able to use the same mathematical library for ECMAScript on a given hardware platform that is available to C programmers on that platform.
Although the choice of algorithms is left to the implementation, it is recommended (but not specified by this standard) that implementations use the approximation algorithms for IEEE 754-2019 arithmetic contained in fdlibm, the freely distributable mathematical library from Sun Microsystems (http://www.netlib.org/fdlibm).
21.3.2.1 Math.abs ( x )
This function returns the absolute value of x; the result has the same magnitude as x but has positive sign.
This function returns the inverse tangent of the quotient y / x of the arguments y and x, where the signs of y and x are used to determine the quadrant of the result. Note that it is intentional and traditional for the two-argument inverse tangent function that the argument named y be first and the argument named x be second. The result is expressed in radians and is in the inclusive interval from -π to +π.
This function returns the result of subtracting 1 from the exponential function of x (e raised to the power of x, where e is the base of the natural logarithms). The result is computed in a way that is accurate even when the value of x is close to 0.
This function returns the greatest (closest to +∞) integral Number value that is not greater than x. If x is already an integral Number, the result is x.
Implementations should take care to avoid the loss of precision from overflows and underflows that are prone to occur in naive implementations when this function is called with two or more arguments.
21.3.2.19 Math.imul ( x, y )
This function performs the following steps when called:
This function returns a Number value with positive sign, greater than or equal to +0𝔽 but strictly less than 1𝔽, chosen randomly or pseudo randomly with approximately uniform distribution over that range, using an implementation-defined algorithm or strategy.
Each Math.random function created for distinct realms must produce a distinct sequence of values from successive calls.
21.3.2.28 Math.round ( x )
This function returns the Number value that is closest to x and is integral. If two integral Numbers are equally close to x, then the result is the Number value that is closer to +∞. If x is already integral, the result is x.
Return the integral Number closest to n, preferring the Number closer to +∞ in the case of a tie.
Note 1
Math.round(3.5) returns 4, but Math.round(-3.5) returns -3.
Note 2
The value of Math.round(x) is not always the same as the value of Math.floor(x + 0.5). When x is -0𝔽 or x is less than +0𝔽 but greater than or equal to -0.5𝔽, Math.round(x) returns -0𝔽, but Math.floor(x + 0.5) returns +0𝔽. Math.round(x) may also differ from the value of Math.floor(x + 0.5)because of internal rounding when computing x + 0.5.
21.3.2.29 Math.sign ( x )
This function returns the sign of x, indicating whether x is positive, negative, or zero.
If n is not finite or n is either +0𝔽 or -0𝔽, return n.
If n < 1𝔽 and n > +0𝔽, return +0𝔽.
If n < -0𝔽 and n > -1𝔽, return -0𝔽.
Return the integral Number nearest n in the direction of +0𝔽.
21.4 Date Objects
21.4.1 Overview of Date Objects and Definitions of Abstract Operations
The following abstract operations operate on time values (defined in 21.4.1.1). Note that, in every case, if any argument to one of these functions is NaN, the result will be NaN.
21.4.1.1 Time Values and Time Range
Time measurement in ECMAScript is analogous to time measurement in POSIX, in particular sharing definition in terms of the proleptic Gregorian calendar, an epoch of midnight at the beginning of 1 January 1970 UTC, and an accounting of every day as comprising exactly 86,400 seconds (each of which is 1000 milliseconds long).
An ECMAScript time value is a Number, either a finiteintegral Number representing an instant in time to millisecond precision or NaN representing no specific instant. A time value that is a multiple of 24 × 60 × 60 × 1000 = 86,400,000 (i.e., is 86,400,000 × d for some integerd) represents the instant at the start of the UTC day that follows the epoch by d whole UTC days (preceding the epoch for negative d). Every other finite time value t is defined relative to the greatest preceding time value s that is such a multiple, and represents the instant that occurs within the same UTC day as s but follows it by (t - s) milliseconds.
Time values do not account for UTC leap seconds—there are no time values representing instants within positive leap seconds, and there are time values representing instants removed from the UTC timeline by negative leap seconds. However, the definition of time values nonetheless yields piecewise alignment with UTC, with discontinuities only at leap second boundaries and zero difference outside of leap seconds.
A Number can exactly represent all integers from -9,007,199,254,740,992 to 9,007,199,254,740,992 (21.1.2.8 and 21.1.2.6). A time value supports a slightly smaller range of -8,640,000,000,000,000 to 8,640,000,000,000,000 milliseconds. This yields a supported time value range of exactly -100,000,000 days to 100,000,000 days relative to midnight at the beginning of 1 January 1970 UTC.
The exact moment of midnight at the beginning of 1 January 1970 UTC is represented by the time value +0𝔽.
Note
In the proleptic Gregorian calendar, leap years are precisely those which are both divisible by 4 and either divisible by 400 or not divisible by 100.
The 400 year cycle of the proleptic Gregorian calendar contains 97 leap years. This yields an average of 365.2425 days per year, which is 31,556,952,000 milliseconds. Therefore, the maximum range a Number could represent exactly with millisecond precision is approximately -285,426 to 285,426 years relative to 1970. The smaller range supported by a time value as specified in this section is approximately -273,790 to 273,790 years relative to 1970.
21.4.1.2 Time-related Constants
These constants are referenced by algorithms in the following sections.
The abstract operation Day takes argument t (a finitetime value) and returns an integral Number. It returns the day number of the day in which t falls. It performs the following steps when called:
The abstract operation TimeWithinDay takes argument t (a finitetime value) and returns an integral Number in the interval from +0𝔽 (inclusive) to msPerDay (exclusive). It returns the number of milliseconds since the start of the day in which t falls. It performs the following steps when called:
The abstract operation DaysInYear takes argument y (an integral Number) and returns 365𝔽 or 366𝔽. It returns the number of days in year y. Leap years have 366 days; all other years have 365. It performs the following steps when called:
The abstract operation DayFromYear takes argument y (an integral Number) and returns an integral Number. It returns the day number of the first day of year y. It performs the following steps when called:
NOTE: In the following steps, numYears1, numYears4, numYears100, and numYears400 represent the number of years divisible by 1, 4, 100, and 400, respectively, that occur between the epoch and the start of year y. The number is negative if y is before the epoch.
The abstract operation TimeFromYear takes argument y (an integral Number) and returns a time value. It returns the time value of the start of year y. It performs the following steps when called:
The abstract operation YearFromTime takes argument t (a finitetime value) and returns an integral Number. It returns the year in which t falls. It performs the following steps when called:
The abstract operation InLeapYear takes argument t (a finitetime value) and returns +0𝔽 or 1𝔽. It returns 1𝔽 if t is within a leap year and +0𝔽 otherwise. It performs the following steps when called:
The abstract operation MonthFromTime takes argument t (a finitetime value) and returns an integral Number in the inclusive interval from +0𝔽 to 11𝔽. It returns a Number identifying the month in which t falls. A month value of +0𝔽 specifies January; 1𝔽 specifies February; 2𝔽 specifies March; 3𝔽 specifies April; 4𝔽 specifies May; 5𝔽 specifies June; 6𝔽 specifies July; 7𝔽 specifies August; 8𝔽 specifies September; 9𝔽 specifies October; 10𝔽 specifies November; and 11𝔽 specifies December. Note that MonthFromTime(+0𝔽) = +0𝔽, corresponding to Thursday, 1 January 1970. It performs the following steps when called:
The abstract operation DateFromTime takes argument t (a finitetime value) and returns an integral Number in the inclusive interval from 1𝔽 to 31𝔽. It returns the day of the month in which t falls. It performs the following steps when called:
The abstract operation WeekDay takes argument t (a finitetime value) and returns an integral Number in the inclusive interval from +0𝔽 to 6𝔽. It returns a Number identifying the day of the week in which t falls. A weekday value of +0𝔽 specifies Sunday; 1𝔽 specifies Monday; 2𝔽 specifies Tuesday; 3𝔽 specifies Wednesday; 4𝔽 specifies Thursday; 5𝔽 specifies Friday; and 6𝔽 specifies Saturday. Note that WeekDay(+0𝔽) = 4𝔽, corresponding to Thursday, 1 January 1970. It performs the following steps when called:
The abstract operation HourFromTime takes argument t (a finitetime value) and returns an integral Number in the inclusive interval from +0𝔽 to 23𝔽. It returns the hour of the day in which t falls. It performs the following steps when called:
The abstract operation MinFromTime takes argument t (a finitetime value) and returns an integral Number in the inclusive interval from +0𝔽 to 59𝔽. It returns the minute of the hour in which t falls. It performs the following steps when called:
The abstract operation SecFromTime takes argument t (a finitetime value) and returns an integral Number in the inclusive interval from +0𝔽 to 59𝔽. It returns the second of the minute in which t falls. It performs the following steps when called:
The abstract operation msFromTime takes argument t (a finitetime value) and returns an integral Number in the inclusive interval from +0𝔽 to 999𝔽. It returns the millisecond of the second in which t falls. It performs the following steps when called:
Time zones in ECMAScript are represented by time zone identifiers, which are Strings composed entirely of code units in the inclusive interval from 0x0000 to 0x007F.
Time zones supported by an ECMAScript implementation may be available named time zones, represented by the [[Identifier]] field of the Time Zone Identifier Records returned by AvailableNamedTimeZoneIdentifiers, or offset time zones, represented by Strings for which IsTimeZoneOffsetString returns true.
A primary time zone identifier is the preferred identifier for an available named time zone.
A non-primary time zone identifier is an identifier for an available named time zone that is not a primary time zone identifier.
An available named time zone identifier is either a primary time zone identifier or a non-primary time zone identifier.
Each available named time zone identifier is associated with exactly one available named time zone.
Each available named time zone is associated with exactly one primary time zone identifier and zero or more non-primary time zone identifiers.
ECMAScript implementations must support an available named time zone with the identifier "UTC", which must be the primary time zone identifier for the UTC time zone.
In addition, implementations may support any number of other available named time zones.
Implementations that follow the requirements for time zones as described in the ECMA-402 Internationalization API specification are called time zone aware.
Time zone aware implementations must support available named time zones corresponding to the Zone and Link names of the IANA Time Zone Database, and only such names.
In time zone aware implementations, a primary time zone identifier is a Zone name, and a non-primary time zone identifier is a Link name, respectively, in the IANA Time Zone Database except as specifically overridden by AvailableNamedTimeZoneIdentifiers as specified in the ECMA-402 specification.
Implementations that do not support the entire IANA Time Zone Database are still recommended to use IANA Time Zone Database names as identifiers to represent time zones.
When the input represents a local time occurring more than once because of a negative time zone transition (e.g. when daylight saving time ends or the time zone offset is decreased due to a time zone rule change), the returned List will have more than one element and will be sorted by ascending numerical value.
When the input represents a local time skipped because of a positive time zone transition (e.g. when daylight saving time begins or the time zone offset is increased due to a time zone rule change), the returned List will be empty.
Otherwise, the returned List will have one element.
The default implementation of GetNamedTimeZoneEpochNanoseconds, to be used for ECMAScript implementations that do not include local political rules for any time zones, performs the following steps when called:
1:30 AM on 5 November 2017 in America/New_York is repeated twice, so GetNamedTimeZoneEpochNanoseconds("America/New_York", 2017, 11, 5, 1, 30, 0, 0, 0, 0) would return a List of length 2 in which the first element represents 05:30 UTC (corresponding with 01:30 US Eastern Daylight Time at UTC offset -04:00) and the second element represents 06:30 UTC (corresponding with 01:30 US Eastern Standard Time at UTC offset -05:00).
2:30 AM on 12 March 2017 in America/New_York does not exist, so GetNamedTimeZoneEpochNanoseconds("America/New_York", 2017, 3, 12, 2, 30, 0, 0, 0, 0) would return an empty List.
The implementation-defined abstract operation GetNamedTimeZoneOffsetNanoseconds takes arguments timeZoneIdentifier (a String) and epochNanoseconds (a BigInt) and returns an integer.
The returned integer represents the offset from UTC of the named time zone identified by timeZoneIdentifier, at the instant corresponding with epochNanoseconds relative to the epoch, both in nanoseconds.
The default implementation of GetNamedTimeZoneOffsetNanoseconds, to be used for ECMAScript implementations that do not include local political rules for any time zones, performs the following steps when called:
Time zone aware implementations, including all implementations that implement the ECMA-402 Internationalization API, must implement the AvailableNamedTimeZoneIdentifiers abstract operation as specified in the ECMA-402 specification.
For implementations that are not time zone aware, AvailableNamedTimeZoneIdentifiers performs the following steps when called:
If the implementation does not include local political rules for any time zones, then
The implementation-defined abstract operation SystemTimeZoneIdentifier takes no arguments and returns a String.
It returns a String representing the host environment's current time zone, which is either a String representing a UTC offset for which IsTimeZoneOffsetString returns true, or a primary time zone identifier.
It performs the following steps when called:
If the implementation only supports the UTC time zone, return "UTC".
To ensure the level of functionality that implementations commonly provide in the methods of the Date object, it is recommended that SystemTimeZoneIdentifier return an IANA time zone name corresponding to the host environment's time zone setting, if such a thing exists.
GetNamedTimeZoneEpochNanoseconds and GetNamedTimeZoneOffsetNanoseconds must reflect the local political rules for standard time and daylight saving time in that time zone, if such rules exist.
For example, if the host environment is a browser on a system where the user has chosen US Eastern Time as their time zone, SystemTimeZoneIdentifier returns "America/New_York".
21.4.1.25 LocalTime ( t )
The abstract operation LocalTime takes argument t (a finitetime value) and returns an integral Number.
It converts t from UTC to local time.
The local political rules for standard time and daylight saving time in effect at t should be used to determine the result in the way specified in this section.
It performs the following steps when called:
Two different input time valuestUTC are converted to the same local time tlocal at a negative time zone transition when there are repeated times (e.g. the daylight saving time ends or the time zone adjustment is decreased.).
LocalTime(UTC(tlocal)) is not necessarily always equal to tlocal. Correspondingly, UTC(LocalTime(tUTC)) is not necessarily always equal to tUTC.
21.4.1.26 UTC ( t )
The abstract operation UTC takes argument t (a Number) and returns a time value.
It converts t from local time to a UTC time value.
The local political rules for standard time and daylight saving time in effect at t should be used to determine the result in the way specified in this section.
It performs the following steps when called:
NOTE: The following steps ensure that when t represents local time repeating multiple times at a negative time zone transition (e.g. when the daylight saving time ends or the time zone offset is decreased due to a time zone rule change) or skipped local time at a positive time zone transition (e.g. when the daylight saving time starts or the time zone offset is increased due to a time zone rule change), t is interpreted using the time zone offset before the transition.
If possibleInstants is not empty, then
Let disambiguatedInstant be possibleInstants[0].
Else,
NOTE: t represents a local time skipped at a positive time zone transition (e.g. due to daylight saving time starting or a time zone rule change increasing the UTC offset).
Input t is nominally a time value but may be any Number value.
The algorithm must not limit t to the time value range, so that inputs corresponding with a boundary of the time value range can be supported regardless of local UTC offset.
For example, the maximum time value is 8.64 × 1015, corresponding with "+275760-09-13T00:00:00Z".
In an environment where the local time zone offset is ahead of UTC by 1 hour at that instant, it is represented by the larger input of 8.64 × 1015 + 3.6 × 106, corresponding with "+275760-09-13T01:00:00+01:00".
1:30 AM on 5 November 2017 in America/New_York is repeated twice (fall backward), but it must be interpreted as 1:30 AM UTC-04 instead of 1:30 AM UTC-05.
In UTC(TimeClip(MakeDate(MakeDay(2017, 10, 5), MakeTime(1, 30, 0, 0)))), the value of offsetMs is -4 × msPerHour.
2:30 AM on 12 March 2017 in America/New_York does not exist, but it must be interpreted as 2:30 AM UTC-05 (equivalent to 3:30 AM UTC-04).
In UTC(TimeClip(MakeDate(MakeDay(2017, 2, 12), MakeTime(2, 30, 0, 0)))), the value of offsetMs is -5 × msPerHour.
Note 2
UTC(LocalTime(tUTC)) is not necessarily always equal to tUTC. Correspondingly, LocalTime(UTC(tlocal)) is not necessarily always equal to tlocal.
21.4.1.27 MakeTime ( hour, min, sec, ms )
The abstract operation MakeTime takes arguments hour (a Number), min (a Number), sec (a Number), and ms (a Number) and returns a Number. It calculates a number of milliseconds. It performs the following steps when called:
The arithmetic in MakeTime is floating-point arithmetic, which is not associative, so the operations must be performed in the correct order.
21.4.1.28 MakeDay ( year, month, date )
The abstract operation MakeDay takes arguments year (a Number), month (a Number), and date (a Number) and returns a Number. It calculates a number of days. It performs the following steps when called:
If year is not finite, month is not finite, or date is not finite, return NaN.
The abstract operation MakeDate takes arguments day (a Number) and time (a Number) and returns a Number. It calculates a number of milliseconds. It performs the following steps when called:
If day is not finite or time is not finite, return NaN.
The abstract operation MakeFullYear takes argument year (a Number) and returns an integral Number or NaN. It returns the full year associated with the integer part of year, interpreting any value in the inclusive interval from 0 to 99 as a count of years since the start of 1900. For alignment with the proleptic Gregorian calendar, "full year" is defined as the signed count of complete years since the start of year 0 (1 B.C.). It performs the following steps when called:
The abstract operation TimeClip takes argument time (a Number) and returns a Number. It calculates a number of milliseconds. It performs the following steps when called:
ECMAScript defines a string interchange format for date-times based upon a simplification of the ISO 8601 calendar date extended format. The format is as follows: YYYY-MM-DDTHH:mm:ss.sssZ
Where the elements are as follows:
YYYY
is the year in the proleptic Gregorian calendar as four decimal digits from 0000 to 9999, or as an expanded year of "+" or "-" followed by six decimal digits.
-
"-" (hyphen) appears literally twice in the string.
MM
is the month of the year as two decimal digits from 01 (January) to 12 (December).
DD
is the day of the month as two decimal digits from 01 to 31.
T
"T" appears literally in the string, to indicate the beginning of the time element.
HH
is the number of complete hours that have passed since midnight as two decimal digits from 00 to 24.
:
":" (colon) appears literally twice in the string.
mm
is the number of complete minutes since the start of the hour as two decimal digits from 00 to 59.
ss
is the number of complete seconds since the start of the minute as two decimal digits from 00 to 59.
.
"." (dot) appears literally in the string.
sss
is the number of complete milliseconds since the start of the second as three decimal digits.
Z
is the UTC offset representation specified as "Z" (for UTC with no offset) or as either "+" or "-" followed by a time expression HH:mm (a subset of the time zone offset string format for indicating local time ahead of or behind UTC, respectively)
This format includes date-only forms:
YYYY
YYYY-MM
YYYY-MM-DD
It also includes “date-time” forms that consist of one of the above date-only forms immediately followed by one of the following time forms with an optional UTC offset representation appended:
THH:mm
THH:mm:ss
THH:mm:ss.sss
A string containing out-of-bounds or nonconforming elements is not a valid instance of this format.
Note 1
As every day both starts and ends with midnight, the two notations 00:00 and 24:00 are available to distinguish the two midnights that can be associated with one date. This means that the following two notations refer to exactly the same point in time: 1995-02-04T24:00 and 1995-02-05T00:00. This interpretation of the latter form as "end of a calendar day" is consistent with ISO 8601, even though that specification reserves it for describing time intervals and does not permit it within representations of single points in time.
Note 2
There exists no international standard that specifies abbreviations for civil time zones like CET, EST, etc. and sometimes the same abbreviation is even used for two very different time zones. For this reason, both ISO 8601 and this format specify numeric representations of time zone offsets.
21.4.1.32.1 Expanded Years
Covering the full time value range of approximately 273,790 years forward or backward from 1 January 1970 (21.4.1.1) requires representing years before 0 or after 9999. ISO 8601 permits expansion of the year representation, but only by mutual agreement of the partners in information interchange. In the simplified ECMAScript format, such an expanded year representation shall have 6 digits and is always prefixed with a + or - sign. The year 0 is considered positive and must be prefixed with a + sign. The representation of the year 0 as -000000 is invalid. Strings matching the Date Time String Format with expanded years representing instants in time outside the range of a time value are treated as unrecognizable by Date.parse and cause that function to return NaN without falling back to implementation-specific behaviour or heuristics.
Note
Examples of date-time values with expanded years:
-271821-04-20T00:00:00Z
271822 B.C.
-000001-01-01T00:00:00Z
2 B.C.
+000000-01-01T00:00:00Z
1 B.C.
+000001-01-01T00:00:00Z
1 A.D.
+001970-01-01T00:00:00Z
1970 A.D.
+002009-12-15T00:00:00Z
2009 A.D.
+275760-09-13T00:00:00Z
275760 A.D.
21.4.1.33 Time Zone Offset String Format
ECMAScript defines a string interchange format for UTC offsets, derived from ISO 8601.
The format is described by the following grammar.
The abstract operation IsTimeZoneOffsetString takes argument offsetString (a String) and returns a Boolean. The return value indicates whether offsetString conforms to the grammar given by UTCOffset. It performs the following steps when called:
The abstract operation ParseTimeZoneOffsetString takes argument offsetString (a String) and returns an integer. The return value is the UTC offset, as a number of nanoseconds, that corresponds to the String offsetString. It performs the following steps when called:
If parsedSign is the single code point U+002D (HYPHEN-MINUS), then
Let sign be -1.
Else,
Let sign be 1.
NOTE: Applications of StringToNumber below do not lose precision, since each of the parsed values is guaranteed to be a sufficiently short string of decimal digits.
is the initial value of the "Date" property of the global object.
creates and initializes a new Date when called as a constructor.
returns a String representing the current time (UTC) when called as a function rather than as a constructor.
is a function whose behaviour differs based upon the number and types of its arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Date behaviour must include a super call to the Date constructor to create and initialize the subclass instance with a [[DateValue]] internal slot.
21.4.2.1 Date ( ...values )
This function performs the following steps when called:
If NewTarget is undefined, then
Let now be the time value (UTC) identifying the current time.
This function returns the time value designating the UTC date and time of the occurrence of the call to it.
21.4.3.2 Date.parse ( string )
This function applies the ToString operator to its argument. If ToString results in an abrupt completion the Completion Record is immediately returned. Otherwise, this function interprets the resulting String as a date and time; it returns a Number, the UTC time value corresponding to the date and time. The String may be interpreted as a local time, a UTC time, or a time in some other time zone, depending on the contents of the String. The function first attempts to parse the String according to the format described in Date Time String Format (21.4.1.32), including expanded years. If the String does not conform to that format the function may fall back to any implementation-specific heuristics or implementation-specific date formats. Strings that are unrecognizable or contain out-of-bounds format element values shall cause this function to return NaN.
If the String conforms to the Date Time String Format, substitute values take the place of absent format elements. When the MM or DD elements are absent, "01" is used. When the HH, mm, or ss elements are absent, "00" is used. When the sss element is absent, "000" is used. When the UTC offset representation is absent, date-only forms are interpreted as a UTC time and date-time forms are interpreted as a local time.
If x is any Date whose milliseconds amount is zero within a particular implementation of ECMAScript, then all of the following expressions should produce the same numeric value in that implementation, if all the properties referenced have their initial values:
is not required to produce the same Number value as the preceding three expressions and, in general, the value produced by this function is implementation-defined when given any String value that does not conform to the Date Time String Format (21.4.1.32) and that could not be produced in that implementation by the toString or toUTCString method.
This function differs from the Date constructor in two ways: it returns a time value as a Number, rather than creating a Date, and it interprets the arguments in UTC rather than as local time.
Unless explicitly defined otherwise, the methods of the Date prototype object defined below are not generic and the this value passed to them must be an object that has a [[DateValue]] internal slot that has been initialized to a time value.
21.4.4.1 Date.prototype.constructor
The initial value of Date.prototype.constructor is %Date%.
21.4.4.2 Date.prototype.getDate ( )
This method performs the following steps when called:
If month is not present, this method behaves as if month was present with the value getMonth(). If date is not present, it behaves as if date was present with the value getDate().
21.4.4.22 Date.prototype.setHours ( hour [ , min [ , sec [ , ms ] ] ] )
This method performs the following steps when called:
If min is not present, this method behaves as if min was present with the value getMinutes(). If sec is not present, it behaves as if sec was present with the value getSeconds(). If ms is not present, it behaves as if ms was present with the value getMilliseconds().
21.4.4.23 Date.prototype.setMilliseconds ( ms )
This method performs the following steps when called:
If sec is not present, this method behaves as if sec was present with the value getSeconds(). If ms is not present, this behaves as if ms was present with the value getMilliseconds().
21.4.4.25 Date.prototype.setMonth ( month [ , date ] )
This method performs the following steps when called:
If month is not present, this method behaves as if month was present with the value getUTCMonth(). If date is not present, it behaves as if date was present with the value getUTCDate().
21.4.4.30 Date.prototype.setUTCHours ( hour [ , min [ , sec [ , ms ] ] ] )
This method performs the following steps when called:
If min is not present, this method behaves as if min was present with the value getUTCMinutes(). If sec is not present, it behaves as if sec was present with the value getUTCSeconds(). If ms is not present, it behaves as if ms was present with the value getUTCMilliseconds().
21.4.4.31 Date.prototype.setUTCMilliseconds ( ms )
This method performs the following steps when called:
If sec is not present, this method behaves as if sec was present with the value getUTCSeconds(). If ms is not present, it behaves as if ms was present with the value return by getUTCMilliseconds().
21.4.4.33 Date.prototype.setUTCMonth ( month [ , date ] )
This method performs the following steps when called:
If tv corresponds with a year that cannot be represented in the Date Time String Format, throw a RangeError exception.
Return a String representation of tv in the Date Time String Format on the UTC time scale, including all format elements and the UTC offset representation "Z".
21.4.4.37 Date.prototype.toJSON ( key )
This method provides a String representation of a Date for use by JSON.stringify (25.5.2).
This method is intentionally generic; it does not require that its this value be a Date. Therefore, it can be transferred to other kinds of objects for use as a method. However, it does require that any such object have a toISOString method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of this method is used:
This method returns a String value. The contents of the String are implementation-defined, but are intended to represent the “date” portion of the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of the host environment's current locale.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of this method is used:
This method returns a String value. The contents of the String are implementation-defined, but are intended to represent the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of the host environment's current locale.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of this method is used:
This method returns a String value. The contents of the String are implementation-defined, but are intended to represent the “time” portion of the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of the host environment's current locale.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
21.4.4.41 Date.prototype.toString ( )
This method performs the following steps when called:
For any Date d such that d.[[DateValue]] is evenly divisible by 1000, the result of Date.parse(d.toString()) = d.valueOf(). See 21.4.3.2.
Note 2
This method is not generic; it throws a TypeError exception if its this value is not a Date. Therefore, it cannot be transferred to other kinds of objects for use as a method.
21.4.4.41.1 TimeString ( tv )
The abstract operation TimeString takes argument tv (a Number, but not NaN) and returns a String. It performs the following steps when called:
Return the string-concatenation of weekday, the code unit 0x0020 (SPACE), month, the code unit 0x0020 (SPACE), day, the code unit 0x0020 (SPACE), yearSign, and paddedYear.
Table 61: Names of days of the week
Number
Name
+0𝔽
"Sun"
1𝔽
"Mon"
2𝔽
"Tue"
3𝔽
"Wed"
4𝔽
"Thu"
5𝔽
"Fri"
6𝔽
"Sat"
Table 62: Names of months of the year
Number
Name
+0𝔽
"Jan"
1𝔽
"Feb"
2𝔽
"Mar"
3𝔽
"Apr"
4𝔽
"May"
5𝔽
"Jun"
6𝔽
"Jul"
7𝔽
"Aug"
8𝔽
"Sep"
9𝔽
"Oct"
10𝔽
"Nov"
11𝔽
"Dec"
21.4.4.41.3 TimeZoneString ( tv )
The abstract operation TimeZoneString takes argument tv (an integral Number) and returns a String. It performs the following steps when called:
Let tzName be an implementation-defined string that is either the empty String or the string-concatenation of the code unit 0x0020 (SPACE), the code unit 0x0028 (LEFT PARENTHESIS), an implementation-defined timezone name, and the code unit 0x0029 (RIGHT PARENTHESIS).
Return the string-concatenation of offsetSign, offsetHour, offsetMin, and tzName.
21.4.4.41.4 ToDateString ( tv )
The abstract operation ToDateString takes argument tv (an integral Number or NaN) and returns a String. It performs the following steps when called:
This method returns a String value representing the instant in time corresponding to the this value. The format of the String is based upon "HTTP-date" from RFC 7231, generalized to support the full range of times supported by ECMAScript Dates.
Return the string-concatenation of weekday, ",", the code unit 0x0020 (SPACE), day, the code unit 0x0020 (SPACE), month, the code unit 0x0020 (SPACE), yearSign, paddedYear, the code unit 0x0020 (SPACE), and TimeString(tv).
21.4.4.44 Date.prototype.valueOf ( )
This method performs the following steps when called:
21.4.4.45 Date.prototype [ %Symbol.toPrimitive% ] ( hint )
This method is called by ECMAScript language operators to convert a Date to a primitive value. The allowed values for hint are "default", "number", and "string". Dates are unique among built-in ECMAScript object in that they treat "default" as being equivalent to "string", All other built-in ECMAScript objects treat "default" as being equivalent to "number".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
The value of the "name" property of this method is "[Symbol.toPrimitive]".
21.4.5 Properties of Date Instances
Date instances are ordinary objects that inherit properties from the Date prototype object. Date instances also have a [[DateValue]] internal slot. The [[DateValue]] internal slot is the time value represented by this Date.
is the initial value of the "String" property of the global object.
creates and initializes a new String object when called as a constructor.
performs a type conversion when called as a function rather than as a constructor.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified String behaviour must include a super call to the String constructor to create and initialize the subclass instance with a [[StringData]] internal slot.
22.1.1.1 String ( value )
This function performs the following steps when called:
This function may be called with a variable number of arguments. The first argument is template and the remainder of the arguments form the Listsubstitutions.
It performs the following steps when called:
Let substitutionCount be the number of elements in substitutions.
This function is intended for use as a tag function of a Tagged Template (13.3.11). When called as such, the first argument will be a well formed template object and the rest parameter will contain the substitution values.
Unless explicitly stated otherwise, the methods of the String prototype object defined below are not generic and the this value passed to them must be either a String value or an object that has a [[StringData]] internal slot that has been initialized to a String value.
This method returns a single element String containing the code unit at index pos within the String value resulting from converting this object to a String. If there is no element at that index, the result is the empty String. The result is a String value, not a String object.
If pos is an integral Number, then the result of x.charAt(pos) is equivalent to the result of x.substring(pos, pos + 1).
This method performs the following steps when called:
If position < 0 or position ≥ size, return the empty String.
Return the substring of S from position to position + 1.
Note 2
This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.3 String.prototype.charCodeAt ( pos )
Note 1
This method returns a Number (a non-negative integral Number less than 216) that is the numeric value of the code unit at index pos within the String resulting from converting this object to a String. If there is no element at that index, the result is NaN.
This method performs the following steps when called:
Return the Number value for the numeric value of the code unit at index position within the String S.
Note 2
This method is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.4 String.prototype.codePointAt ( pos )
Note 1
This method returns a non-negative integral Number less than or equal to 0x10FFFF𝔽 that is the numeric value of the UTF-16 encoded code point (6.1.4) starting at the string element at index pos within the String resulting from converting this object to a String. If there is no element at that index, the result is undefined. If a valid UTF-16 surrogate pair does not begin at pos, the result is the code unit at pos.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.5 String.prototype.concat ( ...args )
Note 1
When this method is called it returns the String value consisting of the code units of the this value (converted to a String) followed by the code units of each of the arguments converted to a String. The result is a String value, not a String object.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.6 String.prototype.constructor
The initial value of String.prototype.constructor is %String%.
If endPosition is undefined, let pos be len; else let pos be ? ToIntegerOrInfinity(endPosition).
Let end be the result of clampingpos between 0 and len.
Let searchLength be the length of searchStr.
If searchLength = 0, return true.
Let start be end - searchLength.
If start < 0, return false.
Let substring be the substring of S from start to end.
If substring is searchStr, return true.
Return false.
Note 1
This method returns true if the sequence of code units of searchString converted to a String is the same as the corresponding code units of this object (converted to a String) starting at endPosition - length(this). Otherwise it returns false.
Note 2
Throwing an exception if the first argument is a RegExp is specified in order to allow future editions to define extensions that allow such argument values.
Note 3
This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.8 String.prototype.includes ( searchString [ , position ] )
This method performs the following steps when called:
If searchString appears as a substring of the result of converting this object to a String, at one or more indices that are greater than or equal to position, this function returns true; otherwise, it returns false. If position is undefined, 0 is assumed, so as to search all of the String.
Note 2
Throwing an exception if the first argument is a RegExp is specified in order to allow future editions to define extensions that allow such argument values.
Note 3
This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.9 String.prototype.indexOf ( searchString [ , position ] )
Note 1
If searchString appears as a substring of the result of converting this object to a String, at one or more indices that are greater than or equal to position, then the smallest such index is returned; otherwise, -1𝔽 is returned. If position is undefined, +0𝔽 is assumed, so as to search all of the String.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.10 String.prototype.isWellFormed ( )
This method performs the following steps when called:
22.1.3.11 String.prototype.lastIndexOf ( searchString [ , position ] )
Note 1
If searchString appears as a substring of the result of converting this object to a String at one or more indices that are smaller than or equal to position, then the greatest such index is returned; otherwise, -1𝔽 is returned. If position is undefined, the length of the String value is assumed, so as to search all of the String.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of this method is used:
This method returns a Number other than NaN representing the result of an implementation-defined locale-sensitive String comparison of the this value (converted to a String S) with that (converted to a String thatValue). The result is intended to correspond with a sort order of String values according to conventions of the host environment's current locale, and will be negative when S is ordered before thatValue, positive when S is ordered after thatValue, and zero in all other cases (representing no relative ordering between S and thatValue).
Before performing the comparisons, this method performs the following steps to prepare the Strings:
The meaning of the optional second and third parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not assign any other interpretation to those parameter positions.
The actual return values are implementation-defined to permit encoding additional information in them, but this method, when considered as a method of two arguments, is required to be a consistent comparator defining a total ordering on the set of all Strings. This method is also required to recognize and honour canonical equivalence according to the Unicode Standard, including returning +0𝔽 when comparing distinguishable Strings that are canonically equivalent.
Note 1
This method itself is not directly suitable as an argument to Array.prototype.sort because the latter requires a function of two arguments.
Note 2
This method may rely on whatever language- and/or locale-sensitive comparison functionality is available to the ECMAScript environment from the host environment, and is intended to compare according to the conventions of the host environment's current locale. However, regardless of comparison capabilities, this method must recognize and honour canonical equivalence according to the Unicode Standard—for example, the following comparisons must all return +0𝔽:
// Å ANGSTROM SIGN vs.// Å LATIN CAPITAL LETTER A + COMBINING RING ABOVE"\u212B".localeCompare("A\u030A")
// Ω OHM SIGN vs.// Ω GREEK CAPITAL LETTER OMEGA"\u2126".localeCompare("\u03A9")
// ṩ LATIN SMALL LETTER S WITH DOT BELOW AND DOT ABOVE vs.// ṩ LATIN SMALL LETTER S + COMBINING DOT ABOVE + COMBINING DOT BELOW"\u1E69".localeCompare("s\u0307\u0323")
// ḍ̇ LATIN SMALL LETTER D WITH DOT ABOVE + COMBINING DOT BELOW vs.// ḍ̇ LATIN SMALL LETTER D WITH DOT BELOW + COMBINING DOT ABOVE"\u1E0B\u0323".localeCompare("\u1E0D\u0307")
// 가 HANGUL CHOSEONG KIYEOK + HANGUL JUNGSEONG A vs.// 가 HANGUL SYLLABLE GA"\u1100\u1161".localeCompare("\uAC00")
It is recommended that this method should not honour Unicode compatibility equivalents or compatibility decompositions as defined in the Unicode Standard, chapter 3, section 3.7.
Note 3
This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.13 String.prototype.match ( regexp )
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.14 String.prototype.matchAll ( regexp )
This method performs a regular expression match of the String representing the this value against regexp and returns an iterator. Each iteration result's value is an Array containing the results of the match, or null if the String did not match.
This method is intentionally generic, it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
Note 2
Similarly to String.prototype.split, String.prototype.matchAll is designed to typically act without mutating its inputs.
22.1.3.15 String.prototype.normalize ( [ form ] )
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
The abstract operation StringPad takes arguments S (a String), maxLength (a non-negative integer), fillString (a String), and placement (start or end) and returns a String. It performs the following steps when called:
Let stringLength be the length of S.
If maxLength ≤ stringLength, return S.
If fillString is the empty String, return S.
Let fillLen be maxLength - stringLength.
Let truncatedStringFiller be the String value consisting of repeated concatenations of fillString truncated to length fillLen.
If placement is start, return the string-concatenation of truncatedStringFiller and S.
The argument maxLength will be clamped such that it can be no smaller than the length of S.
Note 2
The argument fillString defaults to " " (the String value consisting of the code unit 0x0020 SPACE).
22.1.3.17.3 ToZeroPaddedDecimalString ( n, minLength )
The abstract operation ToZeroPaddedDecimalString takes arguments n (a non-negative integer) and minLength (a non-negative integer) and returns a String. It performs the following steps when called:
Let S be the String representation of n, formatted as a decimal number.
Return the String value that is made from n copies of S appended together.
Note 1
This method creates the String value consisting of the code units of the this value (converted to String) repeated count times.
Note 2
This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
The abstract operation GetSubstitution takes arguments matched (a String), str (a String), position (a non-negative integer), captures (a List of either Strings or undefined), namedCaptures (an Object or undefined), and replacementTemplate (a String) and returns either a normal completion containing a String or a throw completion. For the purposes of this abstract operation, a decimal digit is a code unit in the inclusive interval from 0x0030 (DIGIT ZERO) to 0x0039 (DIGIT NINE). It performs the following steps when called:
Repeat, while templateRemainder is not the empty String,
NOTE: The following steps isolate ref (a prefix of templateRemainder), determine refReplacement (its replacement), and then append that replacement to result.
If templateRemainder starts with "$$", then
Let ref be "$$".
Let refReplacement be "$".
Else if templateRemainder starts with "$`", then
Let ref be "$`".
Let refReplacement be the substring of str from 0 to position.
Else if templateRemainder starts with "$&", then
Let ref be "$&".
Let refReplacement be matched.
Else if templateRemainder starts with "$'" (0x0024 (DOLLAR SIGN) followed by 0x0027 (APOSTROPHE)), then
Let ref be "$'".
Let matchLength be the length of matched.
Let tailPos be position + matchLength.
Let refReplacement be the substring of str from min(tailPos, stringLength).
NOTE: tailPos can exceed stringLength only if this abstract operation was invoked by a call to the intrinsic %Symbol.replace% method of %RegExp.prototype% on an object whose "exec" property is not the intrinsic %RegExp.prototype.exec%.
Else if templateRemainder starts with "$" followed by 1 or more decimal digits, then
If templateRemainder starts with "$" followed by 2 or more decimal digits, let digitCount be 2. Otherwise, let digitCount be 1.
Let digits be the substring of templateRemainder from 1 to 1 + digitCount.
Let captureLen be the number of elements in captures.
If index > captureLen and digitCount = 2, then
NOTE: When a two-digit replacement pattern specifies an index exceeding the count of capturing groups, it is treated as a one-digit replacement pattern followed by a literal digit.
Set digitCount to 1.
Set digits to the substring of digits from 0 to 1.
This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.22 String.prototype.slice ( start, end )
This method returns a substring of the result of converting this object to a String, starting from index start and running to, but not including, index end (or through the end of the String if end is undefined). If start is negative, it is treated as sourceLength + start where sourceLength is the length of the String. If end is negative, it is treated as sourceLength + end where sourceLength is the length of the String. The result is a String value, not a String object.
This method is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
This method returns an Array into which substrings of the result of converting this object to a String have been stored. The substrings are determined by searching from left to right for occurrences of separator; these occurrences are not part of any String in the returned array, but serve to divide up the String value. The value of separator may be a String of any length or it may be an object, such as a RegExp, that has a %Symbol.split% method.
The value of separator may be an empty String. In this case, separator does not match the empty substring at the beginning or end of the input String, nor does it match the empty substring at the end of the previous separator match. If separator is the empty String, the String is split up into individual code unit elements; the length of the result array equals the length of the String, and each substring contains one code unit.
If the this value is (or converts to) the empty String, the result depends on whether separator can match the empty String. If it can, the result array contains no elements. Otherwise, the result array contains one element, which is the empty String.
If separator is undefined, then the result array contains just one String, which is the this value (converted to a String). If limit is not undefined, then the output array is truncated so that it contains no more than limit elements.
Note 2
This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.24 String.prototype.startsWith ( searchString [ , position ] )
This method performs the following steps when called:
If position is undefined, let pos be 0; else let pos be ? ToIntegerOrInfinity(position).
Let start be the result of clampingpos between 0 and len.
Let searchLength be the length of searchStr.
If searchLength = 0, return true.
Let end be start + searchLength.
If end > len, return false.
Let substring be the substring of S from start to end.
If substring is searchStr, return true.
Return false.
Note 1
This method returns true if the sequence of code units of searchString converted to a String is the same as the corresponding code units of this object (converted to a String) starting at index position. Otherwise it returns false.
Note 2
Throwing an exception if the first argument is a RegExp is specified in order to allow future editions to define extensions that allow such argument values.
Note 3
This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.25 String.prototype.substring ( start, end )
This method returns a substring of the result of converting this object to a String, starting from index start and running to, but not including, index end of the String (or through the end of the String if end is undefined). The result is a String value, not a String object.
If either argument is NaN or negative, it is replaced with zero; if either argument is strictly greater than the length of the String, it is replaced with the length of the String.
If start is strictly greater than end, they are swapped.
This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of this method is used:
This method interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4.
It works exactly the same as toLowerCase except that it is intended to yield a locale-sensitive result corresponding with conventions of the host environment's current locale. There will only be a difference in the few cases (such as Turkish) where the rules for that language conflict with the regular Unicode case mappings.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
Note
This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of this method is used:
This method interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4.
It works exactly the same as toUpperCase except that it is intended to yield a locale-sensitive result corresponding with conventions of the host environment's current locale. There will only be a difference in the few cases (such as Turkish) where the rules for that language conflict with the regular Unicode case mappings.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
Note
This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.28 String.prototype.toLowerCase ( )
This method interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4.
The result must be derived according to the locale-insensitive case mappings in the Unicode Character Database (this explicitly includes not only the file UnicodeData.txt, but also all locale-insensitive mappings in the file SpecialCasing.txt that accompanies it).
Note 1
The case mapping of some code points may produce multiple code points. In this case the result String may not be the same length as the source String. Because both toUpperCase and toLowerCase have context-sensitive behaviour, the methods are not symmetrical. In other words, s.toUpperCase().toLowerCase() is not necessarily equal to s.toLowerCase().
Note 2
This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.29 String.prototype.toString ( )
This method performs the following steps when called:
For a String object, this method happens to return the same thing as the valueOf method.
22.1.3.30 String.prototype.toUpperCase ( )
This method interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4.
It behaves in exactly the same way as String.prototype.toLowerCase, except that the String is mapped using the toUppercase algorithm of the Unicode Default Case Conversion.
Note
This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.32.1 TrimString ( string, where )
The abstract operation TrimString takes arguments string (an ECMAScript language value) and where (start, end, or start+end) and returns either a normal completion containing a String or a throw completion. It interprets string as a sequence of UTF-16 encoded code points, as described in 6.1.4. It performs the following steps when called:
Let T be the String value that is a copy of S with both leading and trailing white space removed.
Return T.
The definition of white space is the union of WhiteSpace and LineTerminator. When determining whether a Unicode code point is in Unicode general category “Space_Separator” (“Zs”), code unit sequences are interpreted as UTF-16 encoded code point sequences as specified in 6.1.4.
22.1.3.33 String.prototype.trimEnd ( )
This method interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4.
This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.34 String.prototype.trimStart ( )
This method interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4.
This method is intentionally generic; it does not require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.35 String.prototype.valueOf ( )
This method performs the following steps when called:
The value of the "name" property of this method is "[Symbol.iterator]".
22.1.4 Properties of String Instances
String instances are String exotic objects and have the internal methods specified for such objects. String instances inherit properties from the String prototype object. String instances also have a [[StringData]] internal slot. The [[StringData]] internal slot is the String value represented by this String object.
String instances have a "length" property, and a set of enumerable properties with integer-indexed names.
22.1.4.1 length
The number of elements in the String value represented by this String object.
Once a String object is initialized, this property is unchanging. It has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
22.1.5 String Iterator Objects
A String Iterator is an object, that represents a specific iteration over some specific String instance object. There is not a named constructor for String Iterator objects. Instead, String iterator objects are created by calling certain methods of String instance objects.
22.1.5.1 The %StringIteratorPrototype% Object
The %StringIteratorPrototype% object:
has properties that are inherited by all String Iterator Objects.
The initial value of the %Symbol.toStringTag% property is the String value "String Iterator".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
22.2 RegExp (Regular Expression) Objects
A RegExp object contains a regular expression and the associated flags.
Note
The form and functionality of regular expressions is modelled after the regular expression facility in the Perl 5 programming language.
22.2.1 Patterns
The RegExp constructor applies the following grammar to the input pattern String. An error occurs if the grammar cannot interpret the String as an expansion of Pattern.
The abstract operation CountLeftCapturingParensWithin takes argument node (a Parse Node) and returns a non-negative integer. It returns the number of left-capturing parentheses in node. A left-capturing parenthesis is any ( pattern character that is matched by the ( terminal of the Atom::(GroupSpecifieroptDisjunction) production.
The abstract operation CountLeftCapturingParensBefore takes argument node (a Parse Node) and returns a non-negative integer. It returns the number of left-capturing parentheses within the enclosing pattern that occur to the left of node.
The abstract operation GroupSpecifiersThatMatch takes argument thisGroupName (a GroupNameParse Node) and returns a List of GroupSpecifierParse Nodes. It performs the following steps when called:
The syntax-directed operation RegExpIdentifierCodePoints takes no arguments and returns a List of code points. It is defined piecewise over the following productions:
The syntax-directed operation RegExpIdentifierCodePoint takes no arguments and returns a code point. It is defined piecewise over the following productions:
A regular expression pattern is converted into an Abstract Closure using the process described below. An implementation is encouraged to use more efficient algorithms than the ones listed below, as long as the results are the same. The Abstract Closure is used as the value of a RegExp object's [[RegExpMatcher]] internal slot.
A Pattern is a BMP pattern if its associated flags contain neither a u nor a v. Otherwise, it is a Unicode pattern. A BMP pattern matches against a String interpreted as consisting of a sequence of 16-bit values that are Unicode code points in the range of the Basic Multilingual Plane. A Unicode pattern matches against a String interpreted as consisting of Unicode code points encoded using UTF-16. In the context of describing the behaviour of a BMP pattern “character” means a single 16-bit Unicode BMP code point. In the context of describing the behaviour of a Unicode pattern “character” means a UTF-16 encoded code point (6.1.4). In either context, “character value” means the numeric value of the corresponding non-encoded code point.
The syntax and semantics of Pattern is defined as if the source text for the Pattern was a List of SourceCharacter values where each SourceCharacter corresponds to a Unicode code point. If a BMP pattern contains a non-BMP SourceCharacter the entire pattern is encoded using UTF-16 and the individual code units of that encoding are used as the elements of the List.
Note
For example, consider a pattern expressed in source text as the single non-BMP character U+1D11E (MUSICAL SYMBOL G CLEF). Interpreted as a Unicode pattern, it would be a single element (character) List consisting of the single code point U+1D11E. However, interpreted as a BMP pattern, it is first UTF-16 encoded to produce a two element List consisting of the code units 0xD834 and 0xDD1E.
Patterns are passed to the RegExp constructor as ECMAScript String values in which non-BMP characters are UTF-16 encoded. For example, the single character MUSICAL SYMBOL G CLEF pattern, expressed as a String value, is a String of length 2 whose elements were the code units 0xD834 and 0xDD1E. So no further translation of the string would be necessary to process it as a BMP pattern consisting of two pattern characters. However, to process it as a Unicode pattern UTF16SurrogatePairToCodePoint must be used in producing a List whose sole element is a single pattern character, the code point U+1D11E.
An implementation may not actually perform such translations to or from UTF-16, but the semantics of this specification requires that the result of pattern matching be as if such translations were performed.
22.2.2.1 Notation
The descriptions below use the following internal data structures:
A CharSetElement is one of the two following entities:
If rer.[[UnicodeSets]] is false, then a CharSetElement is a character in the sense of the Pattern Semantics above.
If rer.[[UnicodeSets]] is true, then a CharSetElement is a sequence whose elements are characters in the sense of the Pattern Semantics above. This includes the empty sequence, sequences of one character, and sequences of more than one character. For convenience, when working with CharSetElements of this kind, an individual character is treated interchangeably with a sequence of one character.
A CharSet is a mathematical set of CharSetElements.
A CaptureRange is a Record { [[StartIndex]], [[EndIndex]] } that represents the range of characters included in a capture, where [[StartIndex]] is an integer representing the start index (inclusive) of the range within Input, and [[EndIndex]] is an integer representing the end index (exclusive) of the range within Input. For any CaptureRange, these indices must satisfy the invariant that [[StartIndex]] ≤ [[EndIndex]].
A MatchState is a Record { [[Input]], [[EndIndex]], [[Captures]] } where [[Input]] is a List of characters representing the String being matched, [[EndIndex]] is an integer, and [[Captures]] is a List of values, one for each left-capturing parenthesis in the pattern. States are used to represent partial match states in the regular expression matching algorithms. The [[EndIndex]] is one plus the index of the last input character matched so far by the pattern, while [[Captures]] holds the results of capturing parentheses. The nth element of [[Captures]] is either a CaptureRange representing the range of characters captured by the nth set of capturing parentheses, or undefined if the nth set of capturing parentheses hasn't been reached yet. Due to backtracking, many States may be in use at any time during the matching process.
A MatchResult is either a MatchState or the special token failure that indicates that the match failed.
A MatcherContinuation is an Abstract Closure that takes one MatchState argument and returns a MatchResult result. The MatcherContinuation attempts to match the remaining portion (specified by the closure's captured values) of the pattern against Input, starting at the intermediate state given by its MatchState argument. If the match succeeds, the MatcherContinuation returns the final MatchState that it reached; if the match fails, the MatcherContinuation returns failure.
A Matcher is an Abstract Closure that takes two arguments—a MatchState and a MatcherContinuation—and returns a MatchResult result. A Matcher attempts to match a middle subpattern (specified by the closure's captured values) of the pattern against the MatchState's [[Input]], starting at the intermediate state given by its MatchState argument. The MatcherContinuation argument should be a closure that matches the rest of the pattern. After matching the subpattern of a pattern to obtain a new MatchState, the Matcher then calls MatcherContinuation on that new MatchState to test if the rest of the pattern can match as well. If it can, the Matcher returns the MatchState returned by MatcherContinuation; if not, the Matcher may try different choices at its choice points, repeatedly calling MatcherContinuation until it either succeeds or all possibilities have been exhausted.
22.2.2.1.1 RegExp Records
A RegExp Record is a Record value used to store information about a RegExp that is needed during compilation and possibly during matching.
Let cap be a List of rer.[[CapturingGroupsCount]]undefined values, indexed 1 through rer.[[CapturingGroupsCount]].
Let x be the MatchState { [[Input]]: Input, [[EndIndex]]: index, [[Captures]]: cap }.
Return m(x, c).
Note
A Pattern compiles to an Abstract Closure value. RegExpBuiltinExec can then apply this procedure to a List of characters and an offset within that List to determine whether the pattern would match starting at exactly that offset within the List, and, if it does match, what the values of the capturing parentheses would be. The algorithms in 22.2.2 are designed so that compiling a pattern may throw a SyntaxError exception; on the other hand, once the pattern is successfully compiled, applying the resulting Abstract Closure to find a match in a List of characters cannot throw an exception (except for any implementation-defined exceptions that can occur anywhere such as out-of-memory).
The | regular expression operator separates two alternatives. The pattern first tries to match the left Alternative (followed by the sequel of the regular expression); if it fails, it tries to match the right Disjunction (followed by the sequel of the regular expression). If the left Alternative, the right Disjunction, and the sequel all have choice points, all choices in the sequel are tried before moving on to the next choice in the left Alternative. If choices in the left Alternative are exhausted, the right Disjunction is tried instead of the left Alternative. Any capturing parentheses inside a portion of the pattern skipped by | produce undefined values instead of Strings. Thus, for example,
Consecutive Terms try to simultaneously match consecutive portions of Input. When direction is forward, if the left Alternative, the right Term, and the sequel of the regular expression all have choice points, all choices in the sequel are tried before moving on to the next choice in the right Term, and all choices in the right Term are tried before moving on to the next choice in the left Alternative. When direction is backward, the evaluation order of Alternative and Term are reversed.
The abstract operation RepeatMatcher takes arguments m (a Matcher), min (a non-negative integer), max (a non-negative integer or +∞), greedy (a Boolean), x (a MatchState), c (a MatcherContinuation), parenIndex (a non-negative integer), and parenCount (a non-negative integer) and returns a MatchResult. It performs the following steps when called:
If max = 0, return c(x).
Let d be a new MatcherContinuation with parameters (y) that captures m, min, max, greedy, x, c, parenIndex, and parenCount and performs the following steps when called:
If min = 0 and y.[[EndIndex]] = x.[[EndIndex]], return failure.
If min = 0, let min2 be 0; otherwise let min2 be min - 1.
If max = +∞, let max2 be +∞; otherwise let max2 be max - 1.
Return RepeatMatcher(m, min2, max2, greedy, y, c, parenIndex, parenCount).
Let cap be a copy of x.[[Captures]].
For each integerk in the inclusive interval from parenIndex + 1 to parenIndex + parenCount, set cap[k] to undefined.
Let Input be x.[[Input]].
Let e be x.[[EndIndex]].
Let xr be the MatchState { [[Input]]: Input, [[EndIndex]]: e, [[Captures]]: cap }.
If min ≠ 0, return m(xr, d).
If greedy is false, then
Let z be c(x).
If z is not failure, return z.
Return m(xr, d).
Let z be m(xr, d).
If z is not failure, return z.
Return c(x).
Note 1
An Atom followed by a Quantifier is repeated the number of times specified by the Quantifier. A Quantifier can be non-greedy, in which case the Atom pattern is repeated as few times as possible while still matching the sequel, or it can be greedy, in which case the Atom pattern is repeated as many times as possible while still matching the sequel. The Atom pattern is repeated rather than the input character sequence that it matches, so different repetitions of the Atom can match different input substrings.
Note 2
If the Atom and the sequel of the regular expression all have choice points, the Atom is first matched as many (or as few, if non-greedy) times as possible. All choices in the sequel are tried before moving on to the next choice in the last repetition of Atom. All choices in the last (nth) repetition of Atom are tried before moving on to the next choice in the next-to-last (n - 1)st repetition of Atom; at which point it may turn out that more or fewer repetitions of Atom are now possible; these are exhausted (again, starting with either as few or as many as possible) before moving on to the next choice in the (n - 1)st repetition of Atom and so on.
Compare
/a[a-z]{2,4}/.exec("abcdefghi")
which returns "abcde" with
/a[a-z]{2,4}?/.exec("abcdefghi")
which returns "abc".
Consider also
/(aa|aabaac|ba|b|c)*/.exec("aabaac")
which, by the choice point ordering above, returns the array
["aaba", "ba"]
and not any of:
["aabaac", "aabaac"]
["aabaac", "c"]
The above ordering of choice points can be used to write a regular expression that calculates the greatest common divisor of two numbers (represented in unary notation). The following example calculates the gcd of 10 and 15:
Step 4 of the RepeatMatcher clears Atom's captures each time Atom is repeated. We can see its behaviour in the regular expression
/(z)((a+)?(b+)?(c))*/.exec("zaacbbbcac")
which returns the array
["zaacbbbcac", "z", "ac", "a", undefined, "c"]
and not
["zaacbbbcac", "z", "ac", "a", "bbb", "c"]
because each iteration of the outermost * clears all captured Strings contained in the quantified Atom, which in this case includes capture Strings numbered 2, 3, 4, and 5.
Note 4
Step 2.b of the RepeatMatcher states that once the minimum number of repetitions has been satisfied, any more expansions of Atom that match the empty character sequence are not considered for further repetitions. This prevents the regular expression engine from falling into an infinite loop on patterns such as:
/(a*)*/.exec("b")
or the slightly more complicated:
/(a*)b\1+/.exec("baaaac")
which returns the array
["b", ""]
22.2.2.3.2 EmptyMatcher ( )
The abstract operation EmptyMatcher takes no arguments and returns a Matcher. It performs the following steps when called:
Return a new Matcher with parameters (x, c) that captures nothing and performs the following steps when called:
The abstract operation MatchTwoAlternatives takes arguments m1 (a Matcher) and m2 (a Matcher) and returns a Matcher. It performs the following steps when called:
Return a new Matcher with parameters (x, c) that captures m1 and m2 and performs the following steps when called:
The abstract operation MatchSequence takes arguments m1 (a Matcher), m2 (a Matcher), and direction (forward or backward) and returns a Matcher. It performs the following steps when called:
If direction is forward, then
Return a new Matcher with parameters (x, c) that captures m1 and m2 and performs the following steps when called:
If e = 0, or if rer.[[Multiline]] is true and the character Input[e - 1] is matched by LineTerminator, then
Return c(x).
Return failure.
Note 2
Even when the y flag is used with a pattern, ^ always matches only at the beginning of Input, or (if rer.[[Multiline]] is true) at the beginning of a line.
Let z be the MatchState { [[Input]]: Input, [[EndIndex]]: xe, [[Captures]]: cap }.
Return c(z).
Note 3
The form (?=Disjunction) specifies a zero-width positive lookahead. In order for it to succeed, the pattern inside Disjunction must match at the current position, but the current position is not advanced before matching the sequel. If Disjunction can match at the current position in several ways, only the first one is tried. Unlike other regular expression operators, there is no backtracking into a (?= form (this unusual behaviour is inherited from Perl). This only matters when the Disjunction contains capturing parentheses and the sequel of the pattern contains backreferences to those captures.
For example,
/(?=(a+))/.exec("baaabac")
matches the empty String immediately after the first b and therefore returns the array:
["", "aaa"]
To illustrate the lack of backtracking into the lookahead, consider:
The form (?!Disjunction) specifies a zero-width negative lookahead. In order for it to succeed, the pattern inside Disjunction must fail to match at the current position. The current position is not advanced before matching the sequel. Disjunction can contain capturing parentheses, but backreferences to them only make sense from within Disjunction itself. Backreferences to these capturing parentheses from elsewhere in the pattern always return undefined because the negative lookahead must fail for the pattern to succeed. For example,
/(.*?)a(?!(a+)b\2c)\2(.*)/.exec("baaabaac")
looks for an a not immediately followed by some positive number n of a's, a b, another n a's (specified by the first \2) and a c. The second \2 is outside the negative lookahead, so it matches against undefined and therefore always succeeds. The whole expression returns the array:
The abstract operation IsWordChar takes arguments rer (a RegExp Record), Input (a List of characters), and e (an integer) and returns a Boolean. It performs the following steps when called:
Let InputLength be the number of elements in Input.
The syntax-directed operation CompileQuantifier takes no arguments and returns a Record with fields [[Min]] (a non-negative integer), [[Max]] (a non-negative integer or +∞), and [[Greedy]] (a Boolean). It is defined piecewise over the following productions:
The syntax-directed operation CompileQuantifierPrefix takes no arguments and returns a Record with fields [[Min]] (a non-negative integer) and [[Max]] (a non-negative integer or +∞). It is defined piecewise over the following productions:
If rer.[[UnicodeSets]] is false, or if every CharSetElement of cs consists of a single character (including if cs is empty), return CharacterSetMatcher(rer, cs, cc.[[Invert]], direction).
Let r be the CaptureRange { [[StartIndex]]: ye, [[EndIndex]]: xe }.
Set cap[parenIndex + 1] to r.
Let z be the MatchState { [[Input]]: Input, [[EndIndex]]: ye, [[Captures]]: cap }.
Return c(z).
Return m(x, d).
Note 2
Parentheses of the form (Disjunction) serve both to group the components of the Disjunction pattern together and to save the result of the match. The result can be used either in a backreference (\ followed by a non-zero decimal number), referenced in a replace String, or returned as part of an array from the regular expression matching Abstract Closure. To inhibit the capturing behaviour of parentheses, use the form (?:Disjunction) instead.
An escape sequence of the form \ followed by a non-zero decimal number n matches the result of the nth set of capturing parentheses (22.2.2.1). It is an error if the regular expression has fewer than n capturing parentheses. If the regular expression has n or more capturing parentheses but the nth one is undefined because it has not captured anything, then the backreference always succeeds.
If rer.[[UnicodeSets]] is false, or if every CharSetElement of cs consists of a single character (including if cs is empty), return CharacterSetMatcher(rer, cs, false, direction).
22.2.2.7.1 CharacterSetMatcher ( rer, A, invert, direction )
The abstract operation CharacterSetMatcher takes arguments rer (a RegExp Record), A (a CharSet), invert (a Boolean), and direction (forward or backward) and returns a Matcher. It performs the following steps when called:
If there exists a CharSetElement in A containing exactly one character a such that Canonicalize(rer, a) is cc, let found be true. Otherwise, let found be false.
If invert is false and found is false, return failure.
If invert is true and found is true, return failure.
Let cap be x.[[Captures]].
Let y be the MatchState { [[Input]]: Input, [[EndIndex]]: f, [[Captures]]: cap }.
Return c(y).
22.2.2.7.2 BackreferenceMatcher ( rer, n, direction )
The abstract operation BackreferenceMatcher takes arguments rer (a RegExp Record), n (a positive integer), and direction (forward or backward) and returns a Matcher. It performs the following steps when called:
If there exists an integeri in the interval from 0 (inclusive) to len (exclusive) such that Canonicalize(rer, Input[rs + i]) is not Canonicalize(rer, Input[g + i]), return failure.
Let y be the MatchState { [[Input]]: Input, [[EndIndex]]: f, [[Captures]]: cap }.
Return c(y).
22.2.2.7.3 Canonicalize ( rer, ch )
The abstract operation Canonicalize takes arguments rer (a RegExp Record) and ch (a character) and returns a character. It performs the following steps when called:
If the file CaseFolding.txt of the Unicode Character Database provides a simple or common case folding mapping for ch, return the result of applying that mapping to ch.
If the numeric value of ch ≥ 128 and the numeric value of cu < 128, return ch.
Return cu.
Note
In case-insignificant matches when HasEitherUnicodeFlag(rer) is true, all characters are implicitly case-folded using the simple mapping provided by the Unicode Standard immediately before they are compared. The simple mapping always maps to a single code point, so it does not map, for example, ß (U+00DF LATIN SMALL LETTER SHARP S) to ss or SS. It may however map code points outside the Basic Latin block to code points within it—for example, ſ (U+017F LATIN SMALL LETTER LONG S) case-folds to s (U+0073 LATIN SMALL LETTER S) and K (U+212A KELVIN SIGN) case-folds to k (U+006B LATIN SMALL LETTER K). Strings containing those code points are matched by regular expressions such as /[a-z]/ui.
In case-insignificant matches when HasEitherUnicodeFlag(rer) is false, the mapping is based on Unicode Default Case Conversion algorithm toUppercase rather than toCasefold, which results in some subtle differences. For example, Ω (U+2126 OHM SIGN) is mapped by toUppercase to itself but by toCasefold to ω (U+03C9 GREEK SMALL LETTER OMEGA) along with Ω (U+03A9 GREEK CAPITAL LETTER OMEGA), so "\u2126" is matched by /[ω]/ui and /[\u03A9]/ui but not by /[ω]/i or /[\u03A9]/i. Also, no code point outside the Basic Latin block is mapped to a code point within it, so strings such as "\u017F ſ" and "\u212A K" are not matched by /[a-z]/i.
22.2.2.8 Runtime Semantics: CompileCharacterClass
The syntax-directed operation CompileCharacterClass takes argument rer (a RegExp Record) and returns a Record with fields [[CharSet]] (a CharSet) and [[Invert]] (a Boolean). It is defined piecewise over the following productions:
ClassContents can expand into a single ClassAtom and/or ranges of two ClassAtom separated by dashes. In the latter case the ClassContents includes all characters between the first ClassAtom and the second ClassAtom, inclusive; an error occurs if either ClassAtom does not represent a single character (for example, if one is \w) or if the first ClassAtom's character value is strictly greater than the second ClassAtom's character value.
Note 3
Even if the pattern ignores case, the case of the two ends of a range is significant in determining which characters belong to the range. Thus, for example, the pattern /[E-F]/i matches only the letters E, F, e, and f, while the pattern /[E-f]/i matches all uppercase and lowercase letters in the Unicode Basic Latin block as well as the symbols [, \, ], ^, _, and `.
Note 4
A - character can be treated literally or it can denote a range. It is treated literally if it is the first or last character of ClassContents, the beginning or end limit of a range specification, or immediately follows a range specification.
Let c be the character whose character value is cv.
Return the CharSet containing the single character c.
Note 5
A ClassAtom can use any of the escape sequences that are allowed in the rest of the regular expression except for \b, \B, and backreferences. Inside a CharacterClass, \b means the backspace character, while \B and backreferences raise errors. Using a backreference inside a ClassAtom causes an error.
Assert: p is a binary Unicode property or binary property alias listed in the “Property name and aliases” column of Table 66, or a binary Unicode property of strings listed in the “Property name” column of Table 67.
Let A be the CharSet containing all CharSetElements whose character database definition includes the property p with value “True”.
The result will often consist of two or more ranges. When UnicodeSets is true and IgnoreCase is true, then MaybeSimpleCaseFolding(rer, [Ā-č]) will include only the odd-numbered code points of that range.
Return the CharSet containing the single character U+0008 (BACKSPACE).
22.2.2.9.1 CharacterRange ( A, B )
The abstract operation CharacterRange takes arguments A (a CharSet) and B (a CharSet) and returns a CharSet. It performs the following steps when called:
Assert: A and B each contain exactly one character.
Return the CharSet containing all characters with a character value in the inclusive interval from i to j.
22.2.2.9.2 HasEitherUnicodeFlag ( rer )
The abstract operation HasEitherUnicodeFlag takes argument rer (a RegExp Record) and returns a Boolean. It performs the following steps when called:
If rer.[[Unicode]] is true or rer.[[UnicodeSets]] is true, then
Return true.
Return false.
22.2.2.9.3 WordCharacters ( rer )
The abstract operation WordCharacters takes argument rer (a RegExp Record) and returns a CharSet. Returns a CharSet containing the characters considered "word characters" for the purposes of \b, \B, \w, and \W It performs the following steps when called:
Let extraWordChars be the CharSet containing all characters c such that c is not in basicWordChars but Canonicalize(rer, c) is in basicWordChars.
Assert: extraWordChars is empty unless HasEitherUnicodeFlag(rer) is true and rer.[[IgnoreCase]] is true.
Return the union of basicWordChars and extraWordChars.
22.2.2.9.4 AllCharacters ( rer )
The abstract operation AllCharacters takes argument rer (a RegExp Record) and returns a CharSet. Returns the set of “all characters” according to the regular expression flags. It performs the following steps when called:
If rer.[[UnicodeSets]] is true and rer.[[IgnoreCase]] is true, then
Return the CharSet containing all Unicode code points c that do not have a Simple Case Folding mapping (that is, scf(c)=c).
Return the CharSet containing all code point values.
Else,
Return the CharSet containing all code unit values.
22.2.2.9.5 MaybeSimpleCaseFolding ( rer, A )
The abstract operation MaybeSimpleCaseFolding takes arguments rer (a RegExp Record) and A (a CharSet) and returns a CharSet. If rer.[[UnicodeSets]] is false or rer.[[IgnoreCase]] is false, it returns A. Otherwise, it uses the Simple Case Folding (scf(cp)) definitions in the file CaseFolding.txt of the Unicode Character Database (each of which maps a single code point to another single code point) to map each CharSetElement of A character-by-character into a canonical form and returns the resulting CharSet. It performs the following steps when called:
If rer.[[UnicodeSets]] is false or rer.[[IgnoreCase]] is false, return A.
The abstract operation CharacterComplement takes arguments rer (a RegExp Record) and S (a CharSet) and returns a CharSet. It performs the following steps when called:
Return the CharSet containing the CharSetElements of A which are not also CharSetElements of S.
22.2.2.9.7 UnicodeMatchProperty ( rer, p )
The abstract operation UnicodeMatchProperty takes arguments rer (a RegExp Record) and p (ECMAScript source text) and returns a Unicode property name. It performs the following steps when called:
If rer.[[UnicodeSets]] is true and p is a Unicode property name listed in the “Property name” column of Table 67, then
Implementations must support the Unicode property names and aliases listed in Table 65, Table 66, and Table 67. To ensure interoperability, implementations must not support any other property names or aliases.
Note 1
For example, Script_Extensions (property name) and scx (property alias) are valid, but script_extensions or Scx aren't.
Note 2
The listed properties form a superset of what UTS18 RL1.2 requires.
Note 3
The spellings of entries in these tables (including casing) match the spellings used in the file PropertyAliases.txt in the Unicode Character Database. The precise spellings in that file are guaranteed to be stable.
Table 65: Non-binary Unicode property aliases and their canonical property names
The abstract operation UnicodeMatchPropertyValue takes arguments p (ECMAScript source text) and v (ECMAScript source text) and returns a Unicode property value. It performs the following steps when called:
Implementations must support the Unicode property values and property value aliases listed in PropertyValueAliases.txt for the properties listed in Table 65. To ensure interoperability, implementations must not support any other property values or property value aliases.
Note 1
For example, Xpeo and Old_Persian are valid Script_Extensions values, but xpeo and Old Persian aren't.
The syntax-directed operation CompileClassSetString takes argument rer (a RegExp Record) and returns a sequence of characters. It is defined piecewise over the following productions:
If F contains any code unit other than "d", "g", "i", "m", "s", "u", "v", or "y", or if F contains any code unit more than once, throw a SyntaxError exception.
If F contains "i", let i be true; else let i be false.
If F contains "m", let m be true; else let m be false.
If F contains "s", let s be true; else let s be false.
If F contains "u", let u be true; else let u be false.
If F contains "v", let v be true; else let v be false.
Let rer be the RegExp Record { [[IgnoreCase]]: i, [[Multiline]]: m, [[DotAll]]: s, [[Unicode]]: u, [[UnicodeSets]]: v, [[CapturingGroupsCount]]: capturingGroupsCount }.
Set obj.[[RegExpRecord]] to rer.
Set obj.[[RegExpMatcher]] to CompilePattern of parseResult with argument rer.
22.2.3.4 Static Semantics: ParsePattern ( patternText, u, v )
The abstract operation ParsePattern takes arguments patternText (a sequence of Unicode code points), u (a Boolean), and v (a Boolean) and returns a Parse Node or a non-empty List of SyntaxError objects.
is the initial value of the "RegExp" property of the global object.
creates and initializes a new RegExp object when called as a constructor.
when called as a function rather than as a constructor, returns either a new RegExp object, or the argument itself if the only argument is a RegExp object.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified RegExp behaviour must include a super call to the RegExp constructor to create and initialize subclass instances with the necessary internal slots.
22.2.4.1 RegExp ( pattern, flags )
This function performs the following steps when called:
If pattern is supplied using a StringLiteral, the usual escape sequence substitutions are performed before the String is processed by this function. If pattern must contain an escape sequence to be recognized by this function, any U+005C (REVERSE SOLIDUS) code points must be escaped within the StringLiteral to prevent them being removed when the contents of the StringLiteral are formed.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
22.2.5.2 get RegExp [ %Symbol.species% ]
RegExp[%Symbol.species%] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
Return the this value.
The value of the "name" property of this function is "get [Symbol.species]".
Note
RegExp prototype methods normally use their this value's constructor to create a derived object. However, a subclass constructor may over-ride that default behaviour by redefining its %Symbol.species% property.
The RegExp prototype object does not have a "valueOf" property of its own; however, it inherits the "valueOf" property from the Object prototype object.
22.2.6.1 RegExp.prototype.constructor
The initial value of RegExp.prototype.constructor is %RegExp%.
22.2.6.2 RegExp.prototype.exec ( string )
This method searches string for an occurrence of the regular expression pattern and returns an Array containing the results of the match, or null if string did not match.
RegExp.prototype.dotAll is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
Let R be the this value.
Let cu be the code unit 0x0073 (LATIN SMALL LETTER S).
RegExp.prototype.flags is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
RegExp.prototype.global is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
Let R be the this value.
Let cu be the code unit 0x0067 (LATIN SMALL LETTER G).
RegExp.prototype.hasIndices is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
Let R be the this value.
Let cu be the code unit 0x0064 (LATIN SMALL LETTER D).
RegExp.prototype.ignoreCase is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
Let R be the this value.
Let cu be the code unit 0x0069 (LATIN SMALL LETTER I).
The value of the "name" property of this method is "[Symbol.match]".
Note
The %Symbol.match% property is used by the IsRegExp abstract operation to identify objects that have the basic behaviour of regular expressions. The absence of a %Symbol.match% property or the existence of such a property whose value does not Boolean coerce to true indicates that the object is not intended to be used as a regular expression object.
The value of the "name" property of this method is "[Symbol.matchAll]".
22.2.6.10 get RegExp.prototype.multiline
RegExp.prototype.multiline is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
Let R be the this value.
Let cu be the code unit 0x006D (LATIN SMALL LETTER M).
NOTE: When n = 1, the preceding step puts the first element into captures (at index 0). More generally, the nth capture (the characters captured by the nth set of capturing parentheses) is at captures[n - 1].
Let replacementString be ? GetSubstitution(matched, S, position, captures, namedCaptures, replaceValue).
If position ≥ nextSourcePosition, then
NOTE: position should not normally move backwards. If it does, it is an indication of an ill-behaving RegExp subclass or use of an access triggered side-effect to change the global flag or other characteristics of rx. In such cases, the corresponding substitution is ignored.
Set accumulatedResult to the string-concatenation of accumulatedResult, the substring of S from nextSourcePosition to position, and replacementString.
Set nextSourcePosition to position + matchLength.
If nextSourcePosition ≥ lengthS, return accumulatedResult.
The value of the "name" property of this method is "[Symbol.search]".
Note
The "lastIndex" and "global" properties of this RegExp object are ignored when performing the search. The "lastIndex" property is left unchanged.
22.2.6.13 get RegExp.prototype.source
RegExp.prototype.source is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
The abstract operation EscapeRegExpPattern takes arguments P (a String) and F (a String) and returns a String. It performs the following steps when called:
If F contains "v", then
Let patternSymbol be Pattern[+UnicodeMode, +UnicodeSetsMode].
Else if F contains "u", then
Let patternSymbol be Pattern[+UnicodeMode, ~UnicodeSetsMode].
Else,
Let patternSymbol be Pattern[~UnicodeMode, ~UnicodeSetsMode].
Let S be a String in the form of a patternSymbol equivalent to P interpreted as UTF-16 encoded Unicode code points (6.1.4), in which certain code points are escaped as described below. S may or may not differ from P; however, the Abstract Closure that would result from evaluating S as a patternSymbol must behave identically to the Abstract Closure given by the constructed object's [[RegExpMatcher]] internal slot. Multiple calls to this abstract operation using the same values for P and F must produce identical results.
The code points / or any LineTerminator occurring in the pattern shall be escaped in S as necessary to ensure that the string-concatenation of "/", S, "/", and F can be parsed (in an appropriate lexical context) as a RegularExpressionLiteral that behaves identically to the constructed regular expression. For example, if P is "/", then S could be "\/" or "\u002F", among other possibilities, but not "/", because /// followed by F would be parsed as a SingleLineComment rather than a RegularExpressionLiteral. If P is the empty String, this specification can be met by letting S be "(?:)".
This method returns an Array into which substrings of the result of converting string to a String have been stored. The substrings are determined by searching from left to right for matches of the this value regular expression; these occurrences are not part of any String in the returned array, but serve to divide up the String value.
The this value may be an empty regular expression or a regular expression that can match an empty String. In this case, the regular expression does not match the empty substring at the beginning or end of the input String, nor does it match the empty substring at the end of the previous separator match. (For example, if the regular expression matches the empty String, the String is split up into individual code unit elements; the length of the result array equals the length of the String, and each substring contains one code unit.) Only the first match at a given index of the String is considered, even if backtracking could yield a non-empty substring match at that index. (For example, /a*?/[Symbol.split]("ab") evaluates to the array ["a", "b"], while /a*/[Symbol.split]("ab") evaluates to the array ["","b"].)
If string is (or converts to) the empty String, the result depends on whether the regular expression can match the empty String. If it can, the result array contains no elements. Otherwise, the result array contains one element, which is the empty String.
If the regular expression contains capturing parentheses, then each time separator is matched the results (including any undefined results) of the capturing parentheses are spliced into the output array. For example,
The value of the "name" property of this method is "[Symbol.split]".
Note 2
This method ignores the value of the "global" and "sticky" properties of this RegExp object.
22.2.6.15 get RegExp.prototype.sticky
RegExp.prototype.sticky is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
Let R be the this value.
Let cu be the code unit 0x0079 (LATIN SMALL LETTER Y).
The returned String has the form of a RegularExpressionLiteral that evaluates to another RegExp object with the same behaviour as this object.
22.2.6.18 get RegExp.prototype.unicode
RegExp.prototype.unicode is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
Let R be the this value.
Let cu be the code unit 0x0075 (LATIN SMALL LETTER U).
RegExp.prototype.unicodeSets is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
Let R be the this value.
Let cu be the code unit 0x0076 (LATIN SMALL LETTER V).
The abstract operation RegExpExec takes arguments R (an Object) and S (a String) and returns either a normal completion containing either an Object or null, or a throw completion. It performs the following steps when called:
If a callable "exec" property is not found this algorithm falls back to attempting to use the built-in RegExp matching algorithm. This provides compatible behaviour for code written for prior editions where most built-in algorithms that use regular expressions did not perform a dynamic property lookup of "exec".
22.2.7.2 RegExpBuiltinExec ( R, S )
The abstract operation RegExpBuiltinExec takes arguments R (an initialized RegExp instance) and S (a String) and returns either a normal completion containing either an Array exotic object or null, or a throw completion. It performs the following steps when called:
Let length be the length of S.
Let lastIndex be ℝ(? ToLength(? Get(R, "lastIndex"))).
Let flags be R.[[OriginalFlags]].
If flags contains "g", let global be true; else let global be false.
If flags contains "y", let sticky be true; else let sticky be false.
If flags contains "d", let hasIndices be true; else let hasIndices be false.
If global is false and sticky is false, set lastIndex to 0.
Let matcher be R.[[RegExpMatcher]].
If flags contains "u" or flags contains "v", let fullUnicode be true; else let fullUnicode be false.
Let matchSucceeded be false.
If fullUnicode is true, let input be StringToCodePoints(S). Otherwise, let input be a List whose elements are the code units that are the elements of S.
NOTE: Each element of input is considered to be a character.
The abstract operation AdvanceStringIndex takes arguments S (a String), index (a non-negative integer), and unicode (a Boolean) and returns an integer. It performs the following steps when called:
The abstract operation GetStringIndex takes arguments S (a String) and codePointIndex (a non-negative integer) and returns a non-negative integer. It interprets S as a sequence of UTF-16 encoded code points, as described in 6.1.4, and returns the code unit index corresponding to code point index codePointIndex when such an index exists. Otherwise, it returns the length of S. It performs the following steps when called:
If S is the empty String, return 0.
Let len be the length of S.
Let codeUnitCount be 0.
Let codePointCount be 0.
Repeat, while codeUnitCount < len,
If codePointCount = codePointIndex, return codeUnitCount.
The number of code units from the start of a string at which the match ends (exclusive).
22.2.7.6 GetMatchString ( S, match )
The abstract operation GetMatchString takes arguments S (a String) and match (a Match Record) and returns a String. It performs the following steps when called:
Assert: match.[[StartIndex]] ≤ match.[[EndIndex]] ≤ the length of S.
Return the substring of S from match.[[StartIndex]] to match.[[EndIndex]].
22.2.7.7 GetMatchIndexPair ( S, match )
The abstract operation GetMatchIndexPair takes arguments S (a String) and match (a Match Record) and returns an Array. It performs the following steps when called:
Assert: match.[[StartIndex]] ≤ match.[[EndIndex]] ≤ the length of S.
The abstract operation MakeMatchIndicesIndexPairArray takes arguments S (a String), indices (a List of either Match Records or undefined), groupNames (a List of either Strings or undefined), and hasGroups (a Boolean) and returns an Array. It performs the following steps when called:
RegExp instances are ordinary objects that inherit properties from the RegExp prototype object. RegExp instances have internal slots [[OriginalSource]], [[OriginalFlags]], [[RegExpRecord]], and [[RegExpMatcher]]. The value of the [[RegExpMatcher]] internal slot is an Abstract Closure representation of the Pattern of the RegExp object.
Note
Prior to ECMAScript 2015, RegExp instances were specified as having the own data properties"source", "global", "ignoreCase", and "multiline". Those properties are now specified as accessor properties of RegExp.prototype.
RegExp instances also have the following property:
22.2.8.1 lastIndex
The value of the "lastIndex" property specifies the String index at which to start the next match. It is coerced to an integral Number when used (see 22.2.7.2). This property shall have the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.
22.2.9 RegExp String Iterator Objects
A RegExp String Iterator is an object, that represents a specific iteration over some specific String instance object, matching against some specific RegExp instance object. There is not a named constructor for RegExp String Iterator objects. Instead, RegExp String Iterator objects are created by calling certain methods of RegExp instance objects.
The abstract operation CreateRegExpStringIterator takes arguments R (an Object), S (a String), global (a Boolean), and fullUnicode (a Boolean) and returns a Generator. It performs the following steps when called:
Let closure be a new Abstract Closure with no parameters that captures R, S, global, and fullUnicode and performs the following steps when called:
is the initial value of the "Array" property of the global object.
creates and initializes a new Array when called as a constructor.
also creates and initializes a new Array when called as a function rather than as a constructor. Thus the function call Array(…) is equivalent to the object creation expression new Array(…) with the same arguments.
is a function whose behaviour differs based upon the number and types of its arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the exotic Array behaviour must include a super call to the Array constructor to initialize subclass instances that are Array exotic objects. However, most of the Array.prototype methods are generic methods that are not dependent upon their this value being an Array exotic object.
23.1.1.1 Array ( ...values )
This function performs the following steps when called:
If NewTarget is undefined, let newTarget be the active function object; else let newTarget be NewTarget.
This method is an intentionally generic factory method; it does not require that its this value be the Array constructor. Therefore it can be transferred to or inherited by any other constructors that may be called with a single numeric argument.
23.1.2.2 Array.isArray ( arg )
This function performs the following steps when called:
This method is an intentionally generic factory method; it does not require that its this value be the Array constructor. Therefore it can be transferred to or inherited by other constructors that may be called with a single numeric argument.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
23.1.2.5 get Array [ %Symbol.species% ]
Array[%Symbol.species%] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
Return the this value.
The value of the "name" property of this function is "get [Symbol.species]".
Note
Array prototype methods normally use their this value's constructor to create a derived object. However, a subclass constructor may over-ride that default behaviour by redefining its %Symbol.species% property.
23.1.3 Properties of the Array Prototype Object
The Array prototype object:
is %Array.prototype%.
is an Array exotic object and has the internal methods specified for such objects.
has a "length" property whose initial value is +0𝔽 and whose attributes are { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.
The Array prototype object is specified to be an Array exotic object to ensure compatibility with ECMAScript code that was created prior to the ECMAScript 2015 specification.
The explicit setting of the "length" property in step 6 is intended to ensure the length is correct when the final non-empty element of items has trailing holes or when A is not a built-in Array.
Note 2
This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.
The initial value of Array.prototype.constructor is %Array%.
23.1.3.4 Array.prototype.copyWithin ( target, start [ , end ] )
Note 1
The end argument is optional. If it is not provided, the length of the this value is used.
Note 2
If target is negative, it is treated as length + target where length is the length of the array. If start is negative, it is treated as length + start. If end is negative, it is treated as length + end.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.5 Array.prototype.entries ( )
This method performs the following steps when called:
callback should be a function that accepts three arguments and returns a value that is coercible to a Boolean value. every calls callback once for each element present in the array, in ascending order, until it finds one where callback returns false. If such an element is found, every immediately returns false. Otherwise, every returns true. callback is called only for elements of the array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value for each invocation of callback. If it is not provided, undefined is used instead.
callback is called with three arguments: the value of the element, the index of the element, and the object being traversed.
every does not directly mutate the object on which it is called but the object may be mutated by the calls to callback.
The range of elements processed by every is set before the first call to callback. Elements which are appended to the array after the call to every begins will not be visited by callback. If existing elements of the array are changed, their value as passed to callback will be the value at the time every visits them; elements that are deleted after the call to every begins and before being visited are not visited. every acts like the "for all" quantifier in mathematics. In particular, for an empty array, it returns true.
This method performs the following steps when called:
Let testResult be ToBoolean(? Call(callback, thisArg, « kValue, 𝔽(k), O »)).
If testResult is false, return false.
Set k to k + 1.
Return true.
Note 2
This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.7 Array.prototype.fill ( value [ , start [ , end ] ] )
Note 1
The start argument is optional. If it is not provided, +0𝔽 is used.
The end argument is optional. If it is not provided, the length of the this value is used.
Note 2
If start is negative, it is treated as length + start where length is the length of the array. If end is negative, it is treated as length + end.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.
callback should be a function that accepts three arguments and returns a value that is coercible to a Boolean value. filter calls callback once for each element in the array, in ascending order, and constructs a new array of all the values for which callback returns true. callback is called only for elements of the array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value for each invocation of callback. If it is not provided, undefined is used instead.
callback is called with three arguments: the value of the element, the index of the element, and the object being traversed.
filter does not directly mutate the object on which it is called but the object may be mutated by the calls to callback.
The range of elements processed by filter is set before the first call to callback. Elements which are appended to the array after the call to filter begins will not be visited by callback. If existing elements of the array are changed their value as passed to callback will be the value at the time filter visits them; elements that are deleted after the call to filter begins and before being visited are not visited.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.
This method calls predicate once for each element of the array, in ascending index order, until it finds one where predicate returns a value that coerces to true. If such an element is found, find immediately returns that element value. Otherwise, find returns undefined.
Let findRec be ? FindViaPredicate(O, len, ascending, predicate, thisArg).
Return findRec.[[Value]].
Note 2
This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.
This method calls predicate once for each element of the array, in ascending index order, until it finds one where predicate returns a value that coerces to true. If such an element is found, findIndex immediately returns the index of that element value. Otherwise, findIndex returns -1.
Let findRec be ? FindViaPredicate(O, len, ascending, predicate, thisArg).
Return findRec.[[Index]].
Note 2
This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.
This method calls predicate once for each element of the array, in descending index order, until it finds one where predicate returns a value that coerces to true. If such an element is found, findLast immediately returns that element value. Otherwise, findLast returns undefined.
Let findRec be ? FindViaPredicate(O, len, descending, predicate, thisArg).
Return findRec.[[Value]].
Note 2
This method is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
This method calls predicate once for each element of the array, in descending index order, until it finds one where predicate returns a value that coerces to true. If such an element is found, findLastIndex immediately returns the index of that element value. Otherwise, findLastIndex returns -1.
Let findRec be ? FindViaPredicate(O, len, descending, predicate, thisArg).
Return findRec.[[Index]].
Note 2
This method is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.12.1 FindViaPredicate ( O, len, direction, predicate, thisArg )
O should be an array-like object or a TypedArray. This operation calls predicate once for each element of O, in either ascending index order or descending index order (as indicated by direction), until it finds one where predicate returns a value that coerces to true. At that point, this operation returns a Record that gives the index and value of the element found. If no such element is found, this operation returns a Record that specifies -1𝔽 for the index and undefined for the value.
predicate should be a function. When called for an element of the array, it is passed three arguments: the value of the element, the index of the element, and the object being traversed. Its return value will be coerced to a Boolean value.
thisArg will be used as the this value for each invocation of predicate.
This operation does not directly mutate the object on which it is called, but the object may be mutated by the calls to predicate.
The range of elements processed is set before the first call to predicate, just before the traversal begins. Elements that are appended to the array after this will not be visited by predicate. If existing elements of the array are changed, their value as passed to predicate will be the value at the time that this operation visits them. Elements that are deleted after traversal begins and before being visited are still visited and are either looked up from the prototype or are undefined.
It performs the following steps when called:
If IsCallable(predicate) is false, throw a TypeError exception.
If direction is ascending, then
Let indices be a List of the integers in the interval from 0 (inclusive) to len (exclusive), in ascending order.
Else,
Let indices be a List of the integers in the interval from 0 (inclusive) to len (exclusive), in descending order.
callback should be a function that accepts three arguments. forEach calls callback once for each element present in the array, in ascending order. callback is called only for elements of the array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value for each invocation of callback. If it is not provided, undefined is used instead.
callback is called with three arguments: the value of the element, the index of the element, and the object being traversed.
forEach does not directly mutate the object on which it is called but the object may be mutated by the calls to callback.
The range of elements processed by forEach is set before the first call to callback. Elements which are appended to the array after the call to forEach begins will not be visited by callback. If existing elements of the array are changed, their value as passed to callback will be the value at the time forEach visits them; elements that are deleted after the call to forEach begins and before being visited are not visited.
This method performs the following steps when called:
Perform ? Call(callback, thisArg, « kValue, 𝔽(k), O »).
Set k to k + 1.
Return undefined.
Note 2
This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.
This method compares searchElement to the elements of the array, in ascending order, using the SameValueZero algorithm, and if found at any position, returns true; otherwise, it returns false.
The optional second argument fromIndex defaults to +0𝔽 (i.e. the whole array is searched). If it is greater than or equal to the length of the array, false is returned, i.e. the array will not be searched. If it is less than -0𝔽, it is used as the offset from the end of the array to compute fromIndex. If the computed index is less than or equal to +0𝔽, the whole array will be searched.
This method performs the following steps when called:
If SameValueZero(searchElement, elementK) is true, return true.
Set k to k + 1.
Return false.
Note 2
This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.
Note 3
This method intentionally differs from the similar indexOf method in two ways. First, it uses the SameValueZero algorithm, instead of IsStrictlyEqual, allowing it to detect NaN array elements. Second, it does not skip missing array elements, instead treating them as undefined.
This method compares searchElement to the elements of the array, in ascending order, using the IsStrictlyEqual algorithm, and if found at one or more indices, returns the smallest such index; otherwise, it returns -1𝔽.
Note 1
The optional second argument fromIndex defaults to +0𝔽 (i.e. the whole array is searched). If it is greater than or equal to the length of the array, -1𝔽 is returned, i.e. the array will not be searched. If it is less than -0𝔽, it is used as the offset from the end of the array to compute fromIndex. If the computed index is less than or equal to +0𝔽, the whole array will be searched.
This method performs the following steps when called:
If IsStrictlyEqual(searchElement, elementK) is true, return 𝔽(k).
Set k to k + 1.
Return -1𝔽.
Note 2
This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.18 Array.prototype.join ( separator )
This method converts the elements of the array to Strings, and then concatenates these Strings, separated by occurrences of the separator. If no separator is provided, a single comma is used as the separator.
This method is intentionally generic; it does not require that its this value be an Array. Therefore, it can be transferred to other kinds of objects for use as a method.
23.1.3.19 Array.prototype.keys ( )
This method performs the following steps when called:
This method compares searchElement to the elements of the array in descending order using the IsStrictlyEqual algorithm, and if found at one or more indices, returns the largest such index; otherwise, it returns -1𝔽.
The optional second argument fromIndex defaults to the array's length minus one (i.e. the whole array is searched). If it is greater than or equal to the length of the array, the whole array will be searched. If it is less than -0𝔽, it is used as the offset from the end of the array to compute fromIndex. If the computed index is less than or equal to +0𝔽, -1𝔽 is returned.
This method performs the following steps when called:
If IsStrictlyEqual(searchElement, elementK) is true, return 𝔽(k).
Set k to k - 1.
Return -1𝔽.
Note 2
This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.
callback should be a function that accepts three arguments. map calls callback once for each element in the array, in ascending order, and constructs a new Array from the results. callback is called only for elements of the array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value for each invocation of callback. If it is not provided, undefined is used instead.
callback is called with three arguments: the value of the element, the index of the element, and the object being traversed.
map does not directly mutate the object on which it is called but the object may be mutated by the calls to callback.
The range of elements processed by map is set before the first call to callback. Elements which are appended to the array after the call to map begins will not be visited by callback. If existing elements of the array are changed, their value as passed to callback will be the value at the time map visits them; elements that are deleted after the call to map begins and before being visited are not visited.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.22 Array.prototype.pop ( )
Note 1
This method removes the last element of the array and returns it.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.23 Array.prototype.push ( ...items )
Note 1
This method appends the arguments to the end of the array, in the order in which they appear. It returns the new length of the array.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.
callback should be a function that takes four arguments. reduce calls the callback, as a function, once for each element after the first element present in the array, in ascending order.
callback is called with four arguments: the previousValue (value from the previous call to callback), the currentValue (value of the current element), the currentIndex, and the object being traversed. The first time that callback is called, the previousValue and currentValue can be one of two values. If an initialValue was supplied in the call to reduce, then previousValue will be initialValue and currentValue will be the first value in the array. If no initialValue was supplied, then previousValue will be the first value in the array and currentValue will be the second. It is a TypeError if the array contains no elements and initialValue is not provided.
reduce does not directly mutate the object on which it is called but the object may be mutated by the calls to callback.
The range of elements processed by reduce is set before the first call to callback. Elements that are appended to the array after the call to reduce begins will not be visited by callback. If existing elements of the array are changed, their value as passed to callback will be the value at the time reduce visits them; elements that are deleted after the call to reduce begins and before being visited are not visited.
This method performs the following steps when called:
Set accumulator to ? Call(callback, undefined, « accumulator, kValue, 𝔽(k), O »).
Set k to k + 1.
Return accumulator.
Note 2
This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.
callback should be a function that takes four arguments. reduceRight calls the callback, as a function, once for each element after the first element present in the array, in descending order.
callback is called with four arguments: the previousValue (value from the previous call to callback), the currentValue (value of the current element), the currentIndex, and the object being traversed. The first time the function is called, the previousValue and currentValue can be one of two values. If an initialValue was supplied in the call to reduceRight, then previousValue will be initialValue and currentValue will be the last value in the array. If no initialValue was supplied, then previousValue will be the last value in the array and currentValue will be the second-to-last value. It is a TypeError if the array contains no elements and initialValue is not provided.
reduceRight does not directly mutate the object on which it is called but the object may be mutated by the calls to callback.
The range of elements processed by reduceRight is set before the first call to callback. Elements that are appended to the array after the call to reduceRight begins will not be visited by callback. If existing elements of the array are changed by callback, their value as passed to callback will be the value at the time reduceRight visits them; elements that are deleted after the call to reduceRight begins and before being visited are not visited.
This method performs the following steps when called:
Set accumulator to ? Call(callback, undefined, « accumulator, kValue, 𝔽(k), O »).
Set k to k - 1.
Return accumulator.
Note 2
This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.26 Array.prototype.reverse ( )
Note 1
This method rearranges the elements of the array so as to reverse their order. It returns the reversed array.
This method performs the following steps when called:
Assert: lowerExists and upperExists are both false.
NOTE: No action is required.
Set lower to lower + 1.
Return O.
Note 2
This method is intentionally generic; it does not require that its this value be an Array. Therefore, it can be transferred to other kinds of objects for use as a method.
23.1.3.27 Array.prototype.shift ( )
This method removes the first element of the array and returns it.
This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.28 Array.prototype.slice ( start, end )
This method returns an array containing the elements of the array from element start up to, but not including, element end (or through the end of the array if end is undefined). If start is negative, it is treated as length + start where length is the length of the array. If end is negative, it is treated as length + end where length is the length of the array.
The explicit setting of the "length" property in step 15 is intended to ensure the length is correct even when A is not a built-in Array.
Note 2
This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.
callback should be a function that accepts three arguments and returns a value that is coercible to a Boolean value. some calls callback once for each element present in the array, in ascending order, until it finds one where callback returns true. If such an element is found, some immediately returns true. Otherwise, some returns false. callback is called only for elements of the array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value for each invocation of callback. If it is not provided, undefined is used instead.
callback is called with three arguments: the value of the element, the index of the element, and the object being traversed.
some does not directly mutate the object on which it is called but the object may be mutated by the calls to callback.
The range of elements processed by some is set before the first call to callback. Elements that are appended to the array after the call to some begins will not be visited by callback. If existing elements of the array are changed, their value as passed to callback will be the value at the time that some visits them; elements that are deleted after the call to some begins and before being visited are not visited. some acts like the "exists" quantifier in mathematics. In particular, for an empty array, it returns false.
This method performs the following steps when called:
Let testResult be ToBoolean(? Call(callback, thisArg, « kValue, 𝔽(k), O »)).
If testResult is true, return true.
Set k to k + 1.
Return false.
Note 2
This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.30 Array.prototype.sort ( comparator )
This method sorts the elements of this array. The sort must be stable (that is, elements that compare equal must remain in their original order). If comparator is not undefined, it should be a function that accepts two arguments x and y and returns a negative Number if x < y, a positive Number if x > y, or a zero otherwise.
It performs the following steps when called:
If comparator is not undefined and IsCallable(comparator) is false, throw a TypeError exception.
NOTE: The call to SortIndexedProperties in step 5 uses skip-holes. The remaining indices are deleted to preserve the number of holes that were detected and excluded from the sort.
Because non-existent property values always compare greater than undefined property values, and undefined always compares greater than any other value (see CompareArrayElements), undefined property values always sort to the end of the result, followed by non-existent property values.
This method is intentionally generic; it does not require that its this value be an Array. Therefore, it can be transferred to other kinds of objects for use as a method.
The sort order is the ordering of items after completion of step 4 of the algorithm above. The sort order is implementation-defined if SortCompare is not a consistent comparator for the elements of items. When SortIndexedProperties is invoked by Array.prototype.sort, the sort order is also implementation-defined if comparator is undefined, and all applications of ToString, to any specific value passed as an argument to SortCompare, do not produce the same result.
There must be some mathematical permutation π of the non-negative integers less than itemCount, such that for every non-negative integerj less than itemCount, the element old[j] is exactly the same as new[π(j)].
Then for all non-negative integersj and k, each less than itemCount, if ℝ(SortCompare(old[j], old[k])) < 0, then π(j) < π(k).
Here the notation old[j] is used to refer to items[j] before step 4 is executed, and the notation new[j] to refer to items[j] after step 4 has been executed.
An abstract closure or function comparator is a consistent comparator for a set of values S if all of the requirements below are met for all values a, b, and c (possibly the same value) in the set S: The notation a <Cb means ℝ(comparator(a, b)) < 0; a =Cb means ℝ(comparator(a, b)) = 0; and a >Cb means ℝ(comparator(a, b)) > 0.
Calling comparator(a, b) always returns the same value v when given a specific pair of values a and b as its two arguments. Furthermore, vis a Number, and v is not NaN. Note that this implies that exactly one of a <Cb, a =Cb, and a >Cb will be true for a given pair of a and b.
Calling comparator(a, b) does not modify obj or any object on obj's prototype chain.
a =Ca (reflexivity)
If a =Cb, then b =Ca (symmetry)
If a =Cb and b =Cc, then a =Cc (transitivity of =C)
If a <Cb and b <Cc, then a <Cc (transitivity of <C)
If a >Cb and b >Cc, then a >Cc (transitivity of >C)
Note
The above conditions are necessary and sufficient to ensure that comparator divides the set S into equivalence classes and that these equivalence classes are totally ordered.
23.1.3.30.2 CompareArrayElements ( x, y, comparator )
This method deletes the deleteCount elements of the array starting at integer indexstart and replaces them with the elements of items. It returns an Array containing the deleted elements (if any).
This method performs the following steps when called:
The explicit setting of the "length" property in steps 15 and 20 is intended to ensure the lengths are correct even when the objects are not built-in Arrays.
Note 3
This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of this method is used.
Note 1
The first edition of ECMA-402 did not include a replacement specification for this method.
The meanings of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.
This method performs the following steps when called:
This method converts the elements of the array to Strings using their toLocaleString methods, and then concatenates these Strings, separated by occurrences of an implementation-defined locale-sensitive separator String. This method is analogous to toString except that it is intended to yield a locale-sensitive result corresponding with conventions of the host environment's current locale.
Note 3
This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.33 Array.prototype.toReversed ( )
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.37 Array.prototype.unshift ( ...items )
This method prepends the arguments to the start of the array, such that their order within the array is the same as the order in which they appear in the argument list.
This method is intentionally generic; it does not require that its this value be an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.38 Array.prototype.values ( )
This method performs the following steps when called:
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
Note
The own property names of this object are property names that were not included as standard properties of Array.prototype prior to the ECMAScript 2015 specification. These names are ignored for with statement binding purposes in order to preserve the behaviour of existing code that might use one of these names as a binding in an outer scope that is shadowed by a with statement whose binding object is an Array.
The reason that "with" is not included in the unscopableList is because it is already a reserved word.
Array instances have a "length" property, and a set of enumerable properties with array index names.
23.1.4.1 length
The "length" property of an Array instance is a data property whose value is always numerically greater than the name of every configurable own property whose name is an array index.
The "length" property initially has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.
Note
Reducing the value of the "length" property has the side-effect of deleting own array elements whose array index is between the old and new length values. However, non-configurable properties can not be deleted. Attempting to set the "length" property of an Array to a value that is numerically less than or equal to the largest numeric own property name of an existing non-configurable array-indexed property of the array will result in the length being set to a numeric value that is one greater than that non-configurable numeric own property name. See 10.4.2.1.
23.1.5 Array Iterator Objects
An Array Iterator is an object, that represents a specific iteration over some specific Array instance object. There is not a named constructor for Array Iterator objects. Instead, Array iterator objects are created by calling certain methods of Array instance objects.
23.1.5.1 CreateArrayIterator ( array, kind )
The abstract operation CreateArrayIterator takes arguments array (an Object) and kind (key+value, key, or value) and returns a Generator. It is used to create iterator objects for Array methods that return such iterators. It performs the following steps when called:
Let closure be a new Abstract Closure with no parameters that captures kind and array and performs the following steps when called:
Let index be 0.
Repeat,
If array has a [[TypedArrayName]] internal slot, then
The initial value of the %Symbol.toStringTag% property is the String value "Array Iterator".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
23.2 TypedArray Objects
A TypedArray presents an array-like view of an underlying binary data buffer (25.1). A TypedArray element type is the underlying binary scalar data type that all elements of a TypedArray instance have. There is a distinct TypedArrayconstructor, listed in Table 69, for each of the supported element types. Each constructor in Table 69 has a corresponding distinct prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
23.2.2.4 get %TypedArray% [ %Symbol.species% ]
%TypedArray%[%Symbol.species%] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
Return the this value.
The value of the "name" property of this function is "get [Symbol.species]".
%TypedArray%.prototype.buffer is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
Assert: O has a [[ViewedArrayBuffer]] internal slot.
Let buffer be O.[[ViewedArrayBuffer]].
Return buffer.
23.2.3.3 get %TypedArray%.prototype.byteLength
%TypedArray%.prototype.byteLength is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
%TypedArray%.prototype.byteOffset is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
If IsStrictlyEqual(searchElement, elementK) is true, return 𝔽(k).
Set k to k - 1.
Return -1𝔽.
This method is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.
23.2.3.21 get %TypedArray%.prototype.length
%TypedArray%.prototype.length is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
This method sets multiple values in this TypedArray, reading the values from source. The details differ based upon the type of source. The optional offset value indicates the first element index in this TypedArray where values are written. If omitted, it is assumed to be 0.
The abstract operation SetTypedArrayFromTypedArray takes arguments target (a TypedArray), targetOffset (a non-negative integer or +∞), and source (a TypedArray) and returns either a normal completion containingunused or a throw completion. It sets multiple values in target, starting at index targetOffset, reading the values from source. It performs the following steps when called:
If targetOffset = +∞, throw a RangeError exception.
If srcLength + targetOffset > targetLength, throw a RangeError exception.
If target.[[ContentType]] is not source.[[ContentType]], throw a TypeError exception.
If IsSharedArrayBuffer(srcBuffer) is true, IsSharedArrayBuffer(targetBuffer) is true, and srcBuffer.[[ArrayBufferData]] is targetBuffer.[[ArrayBufferData]], let sameSharedArrayBuffer be true; otherwise, let sameSharedArrayBuffer be false.
If SameValue(srcBuffer, targetBuffer) is true or sameSharedArrayBuffer is true, then
The abstract operation SetTypedArrayFromArrayLike takes arguments target (a TypedArray), targetOffset (a non-negative integer or +∞), and source (an ECMAScript language value, but not a TypedArray) and returns either a normal completion containingunused or a throw completion. It sets multiple values in target, starting at index targetOffset, reading the values from source. It performs the following steps when called:
This is a distinct method that, except as described below, implements the same requirements as those of Array.prototype.sort as defined in 23.1.3.30. The implementation of this method may be optimized with the knowledge that the this value is an object that has a fixed length and whose integer-indexed properties are not sparse.
This method is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.
It performs the following steps when called:
If comparator is not undefined and IsCallable(comparator) is false, throw a TypeError exception.
Because NaN always compares greater than any other value (see CompareTypedArrayElements), NaN property values always sort to the end of the result when comparator is not provided.
23.2.3.30 %TypedArray%.prototype.subarray ( start, end )
This method returns a new TypedArray whose element type is the element type of this TypedArray and whose ArrayBuffer is the ArrayBuffer of this TypedArray, referencing the elements in the interval from start (inclusive) to end (exclusive). If either start or end is negative, it refers to an index from the end of the array, as opposed to from the beginning.
This is a distinct method that implements the same algorithm as Array.prototype.toLocaleString as defined in 23.1.3.32 except that TypedArrayLength is called in place of performing a [[Get]] of "length". The implementation of the algorithm may be optimized with the knowledge that the this value has a fixed length when the underlying buffer is not resizable and whose integer-indexed properties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm.
This method is not generic. ValidateTypedArray is called with the this value and seq-cst as arguments prior to evaluating the algorithm. If its result is an abrupt completion that exception is thrown instead of evaluating the algorithm.
Note
If the ECMAScript implementation includes the ECMA-402 Internationalization API this method is based upon the algorithm for Array.prototype.toLocaleString that is in the ECMA-402 specification.
23.2.3.32 %TypedArray%.prototype.toReversed ( )
This method performs the following steps when called:
The initial value of the %Symbol.iterator% property is %TypedArray.prototype.values%, defined in 23.2.3.35.
23.2.3.38 get %TypedArray%.prototype [ %Symbol.toStringTag% ]
%TypedArray%.prototype[%Symbol.toStringTag%] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
The abstract operation TypedArrayElementSize takes argument O (a TypedArray) and returns a non-negative integer. It performs the following steps when called:
Return the Element Size value specified in Table 69 for O.[[TypedArrayName]].
23.2.4.6 TypedArrayElementType ( O )
The abstract operation TypedArrayElementType takes argument O (a TypedArray) and returns a TypedArray element type. It performs the following steps when called:
Return the Element Type value specified in Table 69 for O.[[TypedArrayName]].
23.2.4.7 CompareTypedArrayElements ( x, y, comparator )
The abstract operation CompareTypedArrayElements takes arguments x (a Number or a BigInt), y (a Number or a BigInt), and comparator (a function object or undefined) and returns either a normal completion containing a Number or an abrupt completion. It performs the following steps when called:
is an intrinsic object that has the structure described below, differing only in the name used as the constructor name instead of TypedArray, in Table 69.
is a function whose behaviour differs based upon the number and types of its arguments. The actual behaviour of a call of TypedArray depends upon the number and kind of arguments that are passed to it.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified TypedArray behaviour must include a super call to the TypedArrayconstructor to create and initialize the subclass instance with the internal state necessary to support the %TypedArray%.prototype built-in methods.
23.2.5.1TypedArray ( ...args )
Each TypedArrayconstructor performs the following steps when called:
If NewTarget is undefined, throw a TypeError exception.
Let constructorName be the String value of the Constructor Name value specified in Table 69 for this TypedArrayconstructor.
Let proto be "%TypedArray.prototype%".
Let numberOfArgs be the number of elements in args.
The abstract operation AllocateTypedArray takes arguments constructorName (a String which is the name of a TypedArrayconstructor in Table 69), newTarget (a constructor), and defaultProto (a String) and optional argument length (a non-negative integer) and returns either a normal completion containing a TypedArray or a throw completion. It is used to validate and create an instance of a TypedArrayconstructor. If the length argument is passed, an ArrayBuffer of that length is also allocated and associated with the new TypedArray instance. AllocateTypedArray provides common semantics that is used by TypedArray. It performs the following steps when called:
23.2.5.1.5 InitializeTypedArrayFromArrayLike ( O, arrayLike )
The abstract operation InitializeTypedArrayFromArrayLike takes arguments O (a TypedArray) and arrayLike (an Object, but not a TypedArray or an ArrayBuffer) and returns either a normal completion containingunused or a throw completion. It performs the following steps when called:
The abstract operation AllocateTypedArrayBuffer takes arguments O (a TypedArray) and length (a non-negative integer) and returns either a normal completion containingunused or a throw completion. It allocates and associates an ArrayBuffer with O. It performs the following steps when called:
does not have a [[ViewedArrayBuffer]] or any other of the internal slots that are specific to TypedArray instance objects.
23.2.7.1TypedArray.prototype.BYTES_PER_ELEMENT
The value of TypedArray.prototype.BYTES_PER_ELEMENT is the Element Size value specified in Table 69 for TypedArray.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
23.2.7.2TypedArray.prototype.constructor
The initial value of the "constructor" property of the prototype for a given TypedArrayconstructor is the constructor itself.
23.2.8 Properties of TypedArray Instances
TypedArray instances are TypedArrays. Each TypedArray instance inherits properties from the corresponding TypedArray prototype object. Each TypedArray instance has the following internal slots: [[TypedArrayName]], [[ViewedArrayBuffer]], [[ByteLength]], [[ByteOffset]], and [[ArrayLength]].
24 Keyed Collections
24.1 Map Objects
Maps are collections of key/value pairs where both the keys and values may be arbitrary ECMAScript language values. A distinct key value may only occur in one key/value pair within the Map's collection. Distinct key values are discriminated using the SameValueZero comparison algorithm.
Maps must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of elements in the collection. The data structure used in this specification is only intended to describe the required observable semantics of Maps. It is not intended to be a viable implementation model.
is the initial value of the "Map" property of the global object.
creates and initializes a new Map when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified Map behaviour must include a super call to the Map constructor to create and initialize the subclass instance with the internal state necessary to support the Map.prototype built-in methods.
24.1.1.1 Map ( [ iterable ] )
This function performs the following steps when called:
If NewTarget is undefined, throw a TypeError exception.
If the parameter iterable is present, it is expected to be an object that implements an %Symbol.iterator% method that returns an iterator object that produces a two element array-like object whose first element is a value that will be used as a Map key and whose second element is the value to associate with that key.
The parameter iterable is expected to be an object that implements an %Symbol.iterator% method that returns an iterator object that produces a two element array-like object whose first element is a value that will be used as a Map key and whose second element is the value to associate with that key.
callback should be a function that accepts two arguments. groupBy calls callback once for each element in items, in ascending order, and constructs a new Map. Each value returned by callback is used as a key in the Map. For each such key, the result Map has an entry whose key is that key and whose value is an array containing all the elements for which callback returned that key.
callback is called with two arguments: the value of the element and the index of the element.
The return value of groupBy is a Map.
This function performs the following steps when called:
Let groups be ? GroupBy(items, callback, collection).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
24.1.2.3 get Map [ %Symbol.species% ]
Map[%Symbol.species%] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
Return the this value.
The value of the "name" property of this function is "get [Symbol.species]".
For each Record { [[Key]], [[Value]] } p of M.[[MapData]], do
Set p.[[Key]] to empty.
Set p.[[Value]] to empty.
Return undefined.
Note
The existing [[MapData]]List is preserved because there may be existing Map Iterator objects that are suspended midway through iterating over that List.
24.1.3.2 Map.prototype.constructor
The initial value of Map.prototype.constructor is %Map%.
24.1.3.3 Map.prototype.delete ( key )
This method performs the following steps when called:
For each Record { [[Key]], [[Value]] } p of M.[[MapData]], do
If p.[[Key]] is not empty and SameValue(p.[[Key]], key) is true, then
Set p.[[Key]] to empty.
Set p.[[Value]] to empty.
Return true.
Return false.
Note
The value empty is used as a specification device to indicate that an entry has been deleted. Actual implementations may take other actions such as physically removing the entry from internal data structures.
24.1.3.4 Map.prototype.entries ( )
This method performs the following steps when called:
If IsCallable(callback) is false, throw a TypeError exception.
Let entries be M.[[MapData]].
Let numEntries be the number of elements in entries.
Let index be 0.
Repeat, while index < numEntries,
Let e be entries[index].
Set index to index + 1.
If e.[[Key]] is not empty, then
Perform ? Call(callback, thisArg, « e.[[Value]], e.[[Key]], M »).
NOTE: The number of elements in entries may have increased during execution of callback.
Set numEntries to the number of elements in entries.
Return undefined.
Note
callback should be a function that accepts three arguments. forEach calls callback once for each key/value pair present in the Map, in key insertion order. callback is called only for keys of the Map which actually exist; it is not called for keys that have been deleted from the Map.
If a thisArg parameter is provided, it will be used as the this value for each invocation of callback. If it is not provided, undefined is used instead.
callback is called with three arguments: the value of the item, the key of the item, and the Map being traversed.
forEach does not directly mutate the object on which it is called but the object may be mutated by the calls to callback. Each entry of a map's [[MapData]] is only visited once. New keys added after the call to forEach begins are visited. A key will be revisited if it is deleted after it has been visited and then re-added before the forEach call completes. Keys that are deleted after the call to forEach begins and before being visited are not visited unless the key is added again before the forEach call completes.
24.1.3.6 Map.prototype.get ( key )
This method performs the following steps when called:
For each Record { [[Key]], [[Value]] } p of M.[[MapData]], do
If p.[[Key]] is not empty and SameValue(p.[[Key]], key) is true, then
Set p.[[Value]] to value.
Return M.
Let p be the Record { [[Key]]: key, [[Value]]: value }.
Append p to M.[[MapData]].
Return M.
24.1.3.10 get Map.prototype.size
Map.prototype.size is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
The initial value of the %Symbol.iterator% property is %Map.prototype.entries%, defined in 24.1.3.4.
24.1.3.13 Map.prototype [ %Symbol.toStringTag% ]
The initial value of the %Symbol.toStringTag% property is the String value "Map".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
24.1.4 Properties of Map Instances
Map instances are ordinary objects that inherit properties from the Map prototype. Map instances also have a [[MapData]] internal slot.
24.1.5 Map Iterator Objects
A Map Iterator is an object, that represents a specific iteration over some specific Map instance object. There is not a named constructor for Map Iterator objects. Instead, map iterator objects are created by calling certain methods of Map instance objects.
24.1.5.1 CreateMapIterator ( map, kind )
The abstract operation CreateMapIterator takes arguments map (an ECMAScript language value) and kind (key+value, key, or value) and returns either a normal completion containing a Generator or a throw completion. It is used to create iterator objects for Map methods that return such iterators. It performs the following steps when called:
The initial value of the %Symbol.toStringTag% property is the String value "Map Iterator".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
24.2 Set Objects
Set objects are collections of ECMAScript language values. A distinct value may only occur once as an element of a Set's collection. Distinct values are discriminated using the SameValueZero comparison algorithm.
Set objects must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of elements in the collection. The data structure used in this specification is only intended to describe the required observable semantics of Set objects. It is not intended to be a viable implementation model.
24.2.1 Abstract Operations For Set Objects
24.2.1.1 Set Records
An Set Record is a Record value used to encapsulate the interface of a Set or similar object.
If IsCallable(keys) is false, throw a TypeError exception.
Return a new Set Record { [[SetObject]]: obj, [[Size]]: intSize, [[Has]]: has, [[Keys]]: keys }.
24.2.1.3 SetDataHas ( setData, value )
The abstract operation SetDataHas takes arguments setData (a List of either ECMAScript language values or empty) and value (an ECMAScript language value) and returns a Boolean. It performs the following steps when called:
If SetDataIndex(setData, value) is not-found, return false.
Return true.
24.2.1.4 SetDataIndex ( setData, value )
The abstract operation SetDataIndex takes arguments setData (a List of either ECMAScript language values or empty) and value (an ECMAScript language value) and returns a non-negative integer or not-found. It performs the following steps when called:
The abstract operation SetDataSize takes argument setData (a List of either ECMAScript language values or empty) and returns a non-negative integer. It performs the following steps when called:
is the initial value of the "Set" property of the global object.
creates and initializes a new Set object when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified Set behaviour must include a super call to the Set constructor to create and initialize the subclass instance with the internal state necessary to support the Set.prototype built-in methods.
24.2.2.1 Set ( [ iterable ] )
This function performs the following steps when called:
If NewTarget is undefined, throw a TypeError exception.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
24.2.3.2 get Set [ %Symbol.species% ]
Set[%Symbol.species%] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
Return the this value.
The value of the "name" property of this function is "get [Symbol.species]".
Replace the element of S.[[SetData]] whose value is e with an element whose value is empty.
Return undefined.
Note
The existing [[SetData]]List is preserved because there may be existing Set Iterator objects that are suspended midway through iterating over that List.
24.2.4.3 Set.prototype.constructor
The initial value of Set.prototype.constructor is %Set%.
24.2.4.4 Set.prototype.delete ( value )
This method performs the following steps when called:
If e is not empty and SameValue(e, value) is true, then
Replace the element of S.[[SetData]] whose value is e with an element whose value is empty.
Return true.
Return false.
Note
The value empty is used as a specification device to indicate that an entry has been deleted. Actual implementations may take other actions such as physically removing the entry from internal data structures.
24.2.4.5 Set.prototype.difference ( other )
This method performs the following steps when called:
NOTE: The number of elements in entries may have increased during execution of callback.
Set numEntries to the number of elements in entries.
Return undefined.
Note
callback should be a function that accepts three arguments. forEach calls callback once for each value present in the Set object, in value insertion order. callback is called only for values of the Set which actually exist; it is not called for keys that have been deleted from the set.
If a thisArg parameter is provided, it will be used as the this value for each invocation of callback. If it is not provided, undefined is used instead.
callback is called with three arguments: the first two arguments are a value contained in the Set. The same value is passed for both arguments. The Set object being traversed is passed as the third argument.
The callback is called with three arguments to be consistent with the call back functions used by forEach methods for Map and Array. For Sets, each item value is considered to be both the key and the value.
forEach does not directly mutate the object on which it is called but the object may be mutated by the calls to callback.
Each value is normally visited only once. However, a value will be revisited if it is deleted after it has been visited and then re-added before the forEach call completes. Values that are deleted after the call to forEach begins and before being visited are not visited unless the value is added again before the forEach call completes. New values added after the call to forEach begins are visited.
24.2.4.8 Set.prototype.has ( value )
This method performs the following steps when called:
If SetDataSize(O.[[SetData]]) ≤ otherRec.[[Size]], then
Let thisSize be the number of elements in O.[[SetData]].
Let index be 0.
Repeat, while index < thisSize,
Let e be O.[[SetData]][index].
Set index to index + 1.
If e is not empty, then
Let inOther be ToBoolean(? Call(otherRec.[[Has]], otherRec.[[SetObject]], « e »)).
If inOther is true, then
NOTE: It is possible for earlier calls to otherRec.[[Has]] to remove and re-add an element of O.[[SetData]], which can cause the same element to be visited twice during this iteration.
The initial value of the "keys" property is %Set.prototype.values%, defined in 24.2.4.17.
Note
For iteration purposes, a Set appears similar to a Map where each entry has the same value for its key and value.
24.2.4.14 get Set.prototype.size
Set.prototype.size is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
The initial value of the %Symbol.toStringTag% property is the String value "Set".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
24.2.5 Properties of Set Instances
Set instances are ordinary objects that inherit properties from the Set prototype. Set instances also have a [[SetData]] internal slot.
24.2.6 Set Iterator Objects
A Set Iterator is an ordinary object, with the structure defined below, that represents a specific iteration over some specific Set instance object. There is not a named constructor for Set Iterator objects. Instead, set iterator objects are created by calling certain methods of Set instance objects.
24.2.6.1 CreateSetIterator ( set, kind )
The abstract operation CreateSetIterator takes arguments set (an ECMAScript language value) and kind (key+value or value) and returns either a normal completion containing a Generator or a throw completion. It is used to create iterator objects for Set methods that return such iterators. It performs the following steps when called:
The initial value of the %Symbol.toStringTag% property is the String value "Set Iterator".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
24.3 WeakMap Objects
WeakMaps are collections of key/value pairs where the keys are objects and/or symbols and values may be arbitrary ECMAScript language values. A WeakMap may be queried to see if it contains a key/value pair with a specific key, but no mechanism is provided for enumerating the values it holds as keys. In certain conditions, values which are not live are removed as WeakMap keys, as described in 9.9.3.
An implementation may impose an arbitrarily determined latency between the time a key/value pair of a WeakMap becomes inaccessible and the time when the key/value pair is removed from the WeakMap. If this latency was observable to ECMAScript program, it would be a source of indeterminacy that could impact program execution. For that reason, an ECMAScript implementation must not provide any means to observe a key of a WeakMap that does not require the observer to present the observed key.
WeakMaps must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of key/value pairs in the collection. The data structure used in this specification is only intended to describe the required observable semantics of WeakMaps. It is not intended to be a viable implementation model.
Note
WeakMap and WeakSet are intended to provide mechanisms for dynamically associating state with an object or symbol in a manner that does not “leak” memory resources if, in the absence of the WeakMap or WeakSet instance, the object or symbol otherwise became inaccessible and subject to resource reclamation by the implementation's garbage collection mechanisms. This characteristic can be achieved by using an inverted per-object/symbol mapping of WeakMap or WeakSet instances to keys. Alternatively, each WeakMap or WeakSet instance may internally store its key and value data, but this approach requires coordination between the WeakMap or WeakSet implementation and the garbage collector. The following references describe mechanism that may be useful to implementations of WeakMap and WeakSet:
Barry Hayes. 1997. Ephemerons: a new finalization mechanism. In Proceedings of the 12th ACM SIGPLAN conference on Object-oriented programming, systems, languages, and applications (OOPSLA '97), A. Michael Berman (Ed.). ACM, New York, NY, USA, 176-183, http://doi.acm.org/10.1145/263698.263733.
is the initial value of the "WeakMap" property of the global object.
creates and initializes a new WeakMap when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified WeakMap behaviour must include a super call to the WeakMap constructor to create and initialize the subclass instance with the internal state necessary to support the WeakMap.prototype built-in methods.
24.3.1.1 WeakMap ( [ iterable ] )
This function performs the following steps when called:
If NewTarget is undefined, throw a TypeError exception.
If the parameter iterable is present, it is expected to be an object that implements an %Symbol.iterator% method that returns an iterator object that produces a two element array-like object whose first element is a value that will be used as a WeakMap key and whose second element is the value to associate with that key.
For each Record { [[Key]], [[Value]] } p of M.[[WeakMapData]], do
If p.[[Key]] is not empty and SameValue(p.[[Key]], key) is true, then
Set p.[[Key]] to empty.
Set p.[[Value]] to empty.
Return true.
Return false.
Note
The value empty is used as a specification device to indicate that an entry has been deleted. Actual implementations may take other actions such as physically removing the entry from internal data structures.
24.3.3.3 WeakMap.prototype.get ( key )
This method performs the following steps when called:
The initial value of the %Symbol.toStringTag% property is the String value "WeakMap".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
24.3.4 Properties of WeakMap Instances
WeakMap instances are ordinary objects that inherit properties from the WeakMap prototype. WeakMap instances also have a [[WeakMapData]] internal slot.
24.4 WeakSet Objects
WeakSets are collections of objects and/or symbols. A distinct object or symbol may only occur once as an element of a WeakSet's collection. A WeakSet may be queried to see if it contains a specific value, but no mechanism is provided for enumerating the values it holds. In certain conditions, values which are not live are removed as WeakSet elements, as described in 9.9.3.
An implementation may impose an arbitrarily determined latency between the time a value contained in a WeakSet becomes inaccessible and the time when the value is removed from the WeakSet. If this latency was observable to ECMAScript program, it would be a source of indeterminacy that could impact program execution. For that reason, an ECMAScript implementation must not provide any means to determine if a WeakSet contains a particular value that does not require the observer to present the observed value.
WeakSets must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of elements in the collection. The data structure used in this specification is only intended to describe the required observable semantics of WeakSets. It is not intended to be a viable implementation model.
is the initial value of the "WeakSet" property of the global object.
creates and initializes a new WeakSet when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified WeakSet behaviour must include a super call to the WeakSet constructor to create and initialize the subclass instance with the internal state necessary to support the WeakSet.prototype built-in methods.
24.4.1.1 WeakSet ( [ iterable ] )
This function performs the following steps when called:
If NewTarget is undefined, throw a TypeError exception.
If e is not empty and SameValue(e, value) is true, then
Replace the element of S.[[WeakSetData]] whose value is e with an element whose value is empty.
Return true.
Return false.
Note
The value empty is used as a specification device to indicate that an entry has been deleted. Actual implementations may take other actions such as physically removing the entry from internal data structures.
24.4.3.4 WeakSet.prototype.has ( value )
This method performs the following steps when called:
The initial value of the %Symbol.toStringTag% property is the String value "WeakSet".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
24.4.4 Properties of WeakSet Instances
WeakSet instances are ordinary objects that inherit properties from the WeakSet prototype. WeakSet instances also have a [[WeakSetData]] internal slot.
The descriptions below in this section, 25.4, and 29 use the read-modify-write modification function internal data structure.
A read-modify-write modification function is a mathematical function that is represented as an abstract closure that takes two Lists of byte values as arguments and returns a List of byte values. These abstract closures satisfy all of the following properties:
They perform all their algorithm steps atomically.
Their individual algorithm steps are not observable.
Note
To aid verifying that a read-modify-write modification function's algorithm steps constitute a pure, mathematical function, the following editorial conventions are recommended:
They do not access, directly or transitively via invoked abstract operations and abstract closures, any language or specification values except their parameters and captured values.
The abstract operation AllocateArrayBuffer takes arguments constructor (a constructor) and byteLength (a non-negative integer) and optional argument maxByteLength (a non-negative integer or empty) and returns either a normal completion containing an ArrayBuffer or a throw completion. It is used to create an ArrayBuffer. It performs the following steps when called:
Let slots be « [[ArrayBufferData]], [[ArrayBufferByteLength]], [[ArrayBufferDetachKey]] ».
If maxByteLength is present and maxByteLength is not empty, let allocatingResizableBuffer be true; otherwise let allocatingResizableBuffer be false.
If allocatingResizableBuffer is true, then
If byteLength > maxByteLength, throw a RangeError exception.
If it is not possible to create a Data Blockblock consisting of maxByteLength bytes, throw a RangeError exception.
NOTE: Resizable ArrayBuffers are designed to be implementable with in-place growth. Implementations may throw if, for example, virtual memory cannot be reserved up front.
Set obj.[[ArrayBufferMaxByteLength]] to maxByteLength.
Return obj.
25.1.3.2 ArrayBufferByteLength ( arrayBuffer, order )
The abstract operation ArrayBufferByteLength takes arguments arrayBuffer (an ArrayBuffer or SharedArrayBuffer) and order (seq-cst or unordered) and returns a non-negative integer. It performs the following steps when called:
If IsSharedArrayBuffer(arrayBuffer) is true and arrayBuffer has an [[ArrayBufferByteLengthData]] internal slot, then
Let bufferByteLengthBlock be arrayBuffer.[[ArrayBufferByteLengthData]].
NOTE: Neither creation of the new Data Block nor copying from the old Data Block are observable. Implementations may implement this method as a zero-copy move or a realloc.
The abstract operation IsDetachedBuffer takes argument arrayBuffer (an ArrayBuffer or a SharedArrayBuffer) and returns a Boolean. It performs the following steps when called:
If arrayBuffer.[[ArrayBufferData]] is null, return true.
The abstract operation DetachArrayBuffer takes argument arrayBuffer (an ArrayBuffer) and optional argument key (anything) and returns either a normal completion containingunused or a throw completion. It performs the following steps when called:
If arrayBuffer.[[ArrayBufferDetachKey]] is not key, throw a TypeError exception.
Set arrayBuffer.[[ArrayBufferData]] to null.
Set arrayBuffer.[[ArrayBufferByteLength]] to 0.
Return unused.
Note
Detaching an ArrayBuffer instance disassociates the Data Block used as its backing store from the instance and sets the byte length of the buffer to 0.
The abstract operation CloneArrayBuffer takes arguments srcBuffer (an ArrayBuffer or a SharedArrayBuffer), srcByteOffset (a non-negative integer), and srcLength (a non-negative integer) and returns either a normal completion containing an ArrayBuffer or a throw completion. It creates a new ArrayBuffer whose data is a copy of srcBuffer's data over the range starting at srcByteOffset and continuing for srcLength bytes. It performs the following steps when called:
The host-defined abstract operation HostResizeArrayBuffer takes arguments buffer (an ArrayBuffer) and newByteLength (a non-negative integer) and returns either a normal completion containing either handled or unhandled, or a throw completion. It gives the host an opportunity to perform implementation-defined resizing of buffer. If the host chooses not to handle resizing of buffer, it may return unhandled for the default behaviour.
The implementation of HostResizeArrayBuffer must conform to the following requirements:
The abstract operation does not detach buffer.
If the abstract operation completes normally with handled, buffer.[[ArrayBufferByteLength]] is newByteLength.
The default implementation of HostResizeArrayBuffer is to return NormalCompletion(unhandled).
25.1.3.9 IsFixedLengthArrayBuffer ( arrayBuffer )
The abstract operation IsFixedLengthArrayBuffer takes argument arrayBuffer (an ArrayBuffer or a SharedArrayBuffer) and returns a Boolean. It performs the following steps when called:
If arrayBuffer has an [[ArrayBufferMaxByteLength]] internal slot, return false.
Return true.
25.1.3.10 IsUnsignedElementType ( type )
The abstract operation IsUnsignedElementType takes argument type (a TypedArray element type) and returns a Boolean. It verifies if the argument type is an unsigned TypedArray element type. It performs the following steps when called:
If type is one of uint8, uint8clamped, uint16, uint32, or biguint64, return true.
Return false.
25.1.3.11 IsUnclampedIntegerElementType ( type )
The abstract operation IsUnclampedIntegerElementType takes argument type (a TypedArray element type) and returns a Boolean. It verifies if the argument type is an IntegerTypedArray element type not including uint8clamped. It performs the following steps when called:
If type is one of int8, uint8, int16, uint16, int32, or uint32, return true.
If type is either biguint64 or bigint64, return true.
Return false.
25.1.3.13 IsNoTearConfiguration ( type, order )
The abstract operation IsNoTearConfiguration takes arguments type (a TypedArray element type) and order (seq-cst, unordered, or init) and returns a Boolean. It performs the following steps when called:
The abstract operation RawBytesToNumeric takes arguments type (a TypedArray element type), rawBytes (a List of byte values), and isLittleEndian (a Boolean) and returns a Number or a BigInt. It performs the following steps when called:
Let elementSize be the Element Size value specified in Table 69 for Element Type type.
If isLittleEndian is false, reverse the order of the elements of rawBytes.
If type is float32, then
Let value be the byte elements of rawBytes concatenated and interpreted as a little-endian bit string encoding of an IEEE 754-2019 binary32 value.
If value is an IEEE 754-2019 binary32 NaN value, return the NaN Number value.
Return the Number value that corresponds to value.
If type is float64, then
Let value be the byte elements of rawBytes concatenated and interpreted as a little-endian bit string encoding of an IEEE 754-2019 binary64 value.
If value is an IEEE 754-2019 binary64 NaN value, return the NaN Number value.
Return the Number value that corresponds to value.
Let intValue be the byte elements of rawBytes concatenated and interpreted as a bit string encoding of an unsigned little-endian binary number.
Else,
Let intValue be the byte elements of rawBytes concatenated and interpreted as a bit string encoding of a binary little-endian two's complement number of bit length elementSize × 8.
If IsBigIntElementType(type) is true, return the BigInt value that corresponds to intValue.
Otherwise, return the Number value that corresponds to intValue.
25.1.3.15 GetRawBytesFromSharedBlock ( block, byteIndex, type, isTypedArray, order )
The abstract operation GetRawBytesFromSharedBlock takes arguments block (a Shared Data Block), byteIndex (a non-negative integer), type (a TypedArray element type), isTypedArray (a Boolean), and order (seq-cst or unordered) and returns a List of byte values. It performs the following steps when called:
Let elementSize be the Element Size value specified in Table 69 for Element Type type.
If isTypedArray is true and IsNoTearConfiguration(type, order) is true, let noTear be true; otherwise let noTear be false.
Let rawValue be a List of length elementSize whose elements are nondeterministically chosen byte values.
NOTE: In implementations, rawValue is the result of a non-atomic or atomic read instruction on the underlying hardware. The nondeterminism is a semantic prescription of the memory model to describe observable behaviour of hardware with weak consistency.
Let readEvent be ReadSharedMemory { [[Order]]: order, [[NoTear]]: noTear, [[Block]]: block, [[ByteIndex]]: byteIndex, [[ElementSize]]: elementSize }.
Append readEvent to eventsRecord.[[EventList]].
Append Chosen Value Record { [[Event]]: readEvent, [[ChosenValue]]: rawValue } to execution.[[ChosenValues]].
The abstract operation GetValueFromBuffer takes arguments arrayBuffer (an ArrayBuffer or SharedArrayBuffer), byteIndex (a non-negative integer), type (a TypedArray element type), isTypedArray (a Boolean), and order (seq-cst or unordered) and optional argument isLittleEndian (a Boolean) and returns a Number or a BigInt. It performs the following steps when called:
Let rawValue be a List whose elements are bytes from block at indices in the interval from byteIndex (inclusive) to byteIndex + elementSize (exclusive).
Assert: The number of elements in rawValue is elementSize.
If isLittleEndian is not present, set isLittleEndian to the value of the [[LittleEndian]] field of the surrounding agent's Agent Record.
The abstract operation NumericToRawBytes takes arguments type (a TypedArray element type), value (a Number or a BigInt), and isLittleEndian (a Boolean) and returns a List of byte values. It performs the following steps when called:
If type is float32, then
Let rawBytes be a List whose elements are the 4 bytes that are the result of converting value to IEEE 754-2019 binary32 format using roundTiesToEven mode. The bytes are arranged in little endian order. If value is NaN, rawBytes may be set to any implementation chosen IEEE 754-2019 binary32 format Not-a-Number encoding. An implementation must always choose the same encoding for each implementation distinguishable NaN value.
Else if type is float64, then
Let rawBytes be a List whose elements are the 8 bytes that are the IEEE 754-2019 binary64 format encoding of value. The bytes are arranged in little endian order. If value is NaN, rawBytes may be set to any implementation chosen IEEE 754-2019 binary64 format Not-a-Number encoding. An implementation must always choose the same encoding for each implementation distinguishable NaN value.
Else,
Let n be the Element Size value specified in Table 69 for Element Type type.
Let conversionOperation be the abstract operation named in the Conversion Operation column in Table 69 for Element Type type.
The abstract operation SetValueInBuffer takes arguments arrayBuffer (an ArrayBuffer or SharedArrayBuffer), byteIndex (a non-negative integer), type (a TypedArray element type), value (a Number or a BigInt), isTypedArray (a Boolean), and order (seq-cst, unordered, or init) and optional argument isLittleEndian (a Boolean) and returns unused. It performs the following steps when called:
Store the individual bytes of rawBytes into block, starting at block[byteIndex].
Return unused.
25.1.3.19 GetModifySetValueInBuffer ( arrayBuffer, byteIndex, type, value, op )
The abstract operation GetModifySetValueInBuffer takes arguments arrayBuffer (an ArrayBuffer or a SharedArrayBuffer), byteIndex (a non-negative integer), type (a TypedArray element type), value (a Number or a BigInt), and op (a read-modify-write modification function) and returns a Number or a BigInt. It performs the following steps when called:
Let rawBytesRead be a List of length elementSize whose elements are nondeterministically chosen byte values.
NOTE: In implementations, rawBytesRead is the result of a load-link, of a load-exclusive, or of an operand of a read-modify-write instruction on the underlying hardware. The nondeterminism is a semantic prescription of the memory model to describe observable behaviour of hardware with weak consistency.
Let rmwEvent be ReadModifyWriteSharedMemory { [[Order]]: seq-cst, [[NoTear]]: true, [[Block]]: block, [[ByteIndex]]: byteIndex, [[ElementSize]]: elementSize, [[Payload]]: rawBytes, [[ModifyOp]]: op }.
Append rmwEvent to eventsRecord.[[EventList]].
Append Chosen Value Record { [[Event]]: rmwEvent, [[ChosenValue]]: rawBytesRead } to execution.[[ChosenValues]].
Else,
Let rawBytesRead be a List of length elementSize whose elements are the sequence of elementSize bytes starting with block[byteIndex].
Let rawBytesModified be op(rawBytesRead, rawBytes).
Store the individual bytes of rawBytesModified into block, starting at block[byteIndex].
is the initial value of the "ArrayBuffer" property of the global object.
creates and initializes a new ArrayBuffer when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified ArrayBuffer behaviour must include a super call to the ArrayBuffer constructor to create and initialize subclass instances with the internal state necessary to support the ArrayBuffer.prototype built-in methods.
25.1.4.1 ArrayBuffer ( length [ , options ] )
This function performs the following steps when called:
If NewTarget is undefined, throw a TypeError exception.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
25.1.5.3 get ArrayBuffer [ %Symbol.species% ]
ArrayBuffer[%Symbol.species%] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
Return the this value.
The value of the "name" property of this function is "get [Symbol.species]".
does not have an [[ArrayBufferData]] or [[ArrayBufferByteLength]] internal slot.
25.1.6.1 get ArrayBuffer.prototype.byteLength
ArrayBuffer.prototype.byteLength is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
The initial value of ArrayBuffer.prototype.constructor is %ArrayBuffer%.
25.1.6.3 get ArrayBuffer.prototype.detached
ArrayBuffer.prototype.detached is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
ArrayBuffer.prototype.maxByteLength is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
ArrayBuffer.prototype.resizable is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
NOTE: Neither creation of the new Data Block nor copying from the old Data Block are observable. Implementations may implement this method as in-place growth or shrinkage.
Set O.[[ArrayBufferData]] to newBlock.
Set O.[[ArrayBufferByteLength]] to newByteLength.
Return undefined.
25.1.6.7 ArrayBuffer.prototype.slice ( start, end )
This method performs the following steps when called:
The initial value of the %Symbol.toStringTag% property is the String value "ArrayBuffer".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
25.1.7 Properties of ArrayBuffer Instances
ArrayBuffer instances inherit properties from the ArrayBuffer prototype object. ArrayBuffer instances each have an [[ArrayBufferData]] internal slot, an [[ArrayBufferByteLength]] internal slot, and an [[ArrayBufferDetachKey]] internal slot. ArrayBuffer instances which are resizable each have an [[ArrayBufferMaxByteLength]] internal slot.
ArrayBuffer instances whose [[ArrayBufferData]] is null are considered to be detached and all operators to access or modify data contained in the ArrayBuffer instance will fail.
ArrayBuffer instances whose [[ArrayBufferDetachKey]] is set to a value other than undefined need to have all DetachArrayBuffer calls passing that same "detach key" as an argument, otherwise a TypeError will result. This internal slot is only ever set by certain embedding environments, not by algorithms in this specification.
25.1.8 Resizable ArrayBuffer Guidelines
Note 1
The following are guidelines for ECMAScript programmers working with resizable ArrayBuffer.
We recommend that programs be tested in their deployment environments where possible. The amount of available physical memory differs greatly between hardware devices. Similarly, virtual memory subsystems also differ greatly between hardware devices as well as operating systems. An application that runs without out-of-memory errors on a 64-bit desktop web browser could run out of memory on a 32-bit mobile web browser.
When choosing a value for the "maxByteLength" option for resizable ArrayBuffer, we recommend that the smallest possible size for the application be chosen. We recommend that "maxByteLength" does not exceed 1,073,741,824 (230 bytes or 1GiB).
Please note that successfully constructing a resizable ArrayBuffer for a particular maximum size does not guarantee that future resizes will succeed.
Note 2
The following are guidelines for ECMAScript implementers implementing resizable ArrayBuffer.
Resizable ArrayBuffer can be implemented as copying upon resize, as in-place growth via reserving virtual memory up front, or as a combination of both for different values of the constructor's "maxByteLength" option.
If a host is multi-tenanted (i.e. it runs many ECMAScript applications simultaneously), such as a web browser, and its implementations choose to implement in-place growth by reserving virtual memory, we recommend that both 32-bit and 64-bit implementations throw for values of "maxByteLength" ≥ 1GiB to 1.5GiB. This is to reduce the likelihood a single application can exhaust the virtual memory address space and to reduce interoperability risk.
If a host does not have virtual memory, such as those running on embedded devices without an MMU, or if a host only implements resizing by copying, it may accept any Number value for the "maxByteLength" option. However, we recommend a RangeError be thrown if a memory block of the requested size can never be allocated. For example, if the requested size is greater than the maximum amount of usable memory on the device.
25.2 SharedArrayBuffer Objects
25.2.1 Fixed-length and Growable SharedArrayBuffer Objects
A fixed-length SharedArrayBuffer is a SharedArrayBuffer whose byte length cannot change after creation.
The abstract operation AllocateSharedArrayBuffer takes arguments constructor (a constructor) and byteLength (a non-negative integer) and optional argument maxByteLength (a non-negative integer or empty) and returns either a normal completion containing a SharedArrayBuffer or a throw completion. It is used to create a SharedArrayBuffer. It performs the following steps when called:
Let slots be « [[ArrayBufferData]] ».
If maxByteLength is present and maxByteLength is not empty, let allocatingGrowableBuffer be true; otherwise let allocatingGrowableBuffer be false.
If allocatingGrowableBuffer is true, then
If byteLength > maxByteLength, throw a RangeError exception.
Append [[ArrayBufferByteLengthData]] and [[ArrayBufferMaxByteLength]] to slots.
Set obj.[[ArrayBufferByteLengthData]] to byteLengthBlock.
Set obj.[[ArrayBufferMaxByteLength]] to maxByteLength.
Else,
Set obj.[[ArrayBufferByteLength]] to byteLength.
Return obj.
25.2.2.2 IsSharedArrayBuffer ( obj )
The abstract operation IsSharedArrayBuffer takes argument obj (an ArrayBuffer or a SharedArrayBuffer) and returns a Boolean. It tests whether an object is an ArrayBuffer, a SharedArrayBuffer, or a subtype of either. It performs the following steps when called:
The host-defined abstract operation HostGrowSharedArrayBuffer takes arguments buffer (a SharedArrayBuffer) and newByteLength (a non-negative integer) and returns either a normal completion containing either handled or unhandled, or a throw completion. It gives the host an opportunity to perform implementation-defined growing of buffer. If the host chooses not to handle growing of buffer, it may return unhandled for the default behaviour.
The implementation of HostGrowSharedArrayBuffer must conform to the following requirements:
If the abstract operation does not complete normally with unhandled, and newByteLength < the current byte length of the buffer or newByteLength > buffer.[[ArrayBufferMaxByteLength]], throw a RangeError exception.
Let isLittleEndian be the value of the [[LittleEndian]] field of the surrounding agent's Agent Record. If the abstract operation completes normally with handled, a WriteSharedMemory or ReadModifyWriteSharedMemory event whose [[Order]] is seq-cst, [[Payload]] is NumericToRawBytes(biguint64, newByteLength, isLittleEndian), [[Block]] is buffer.[[ArrayBufferByteLengthData]], [[ByteIndex]] is 0, and [[ElementSize]] is 8 is added to the surrounding agent's candidate execution such that racing calls to SharedArrayBuffer.prototype.grow are not "lost", i.e. silently do nothing.
Note
The second requirement above is intentionally vague about how or when the current byte length of buffer is read. Because the byte length must be updated via an atomic read-modify-write operation on the underlying hardware, architectures that use load-link/store-conditional or load-exclusive/store-exclusive instruction pairs may wish to keep the paired instructions close in the instruction stream. As such, SharedArrayBuffer.prototype.grow itself does not perform bounds checking on newByteLength before calling HostGrowSharedArrayBuffer, nor is there a requirement on when the current byte length is read.
This is in contrast with HostResizeArrayBuffer, which is guaranteed that the value of newByteLength is ≥ 0 and ≤ buffer.[[ArrayBufferMaxByteLength]].
The default implementation of HostGrowSharedArrayBuffer is to return NormalCompletion(unhandled).
is the initial value of the "SharedArrayBuffer" property of the global object, if that property is present (see below).
creates and initializes a new SharedArrayBuffer when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified SharedArrayBuffer behaviour must include a super call to the SharedArrayBuffer constructor to create and initialize subclass instances with the internal state necessary to support the SharedArrayBuffer.prototype built-in methods.
Whenever a host does not provide concurrent access to SharedArrayBuffers it may omit the "SharedArrayBuffer" property of the global object.
Note
Unlike an ArrayBuffer, a SharedArrayBuffer cannot become detached, and its internal [[ArrayBufferData]] slot is never null.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
25.2.4.2 get SharedArrayBuffer [ %Symbol.species% ]
SharedArrayBuffer[%Symbol.species%] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
Return the this value.
The value of the "name" property of this function is "get [Symbol.species]".
25.2.5 Properties of the SharedArrayBuffer Prototype Object
does not have an [[ArrayBufferData]] or [[ArrayBufferByteLength]] internal slot.
25.2.5.1 get SharedArrayBuffer.prototype.byteLength
SharedArrayBuffer.prototype.byteLength is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
Let byteLengthBlock be O.[[ArrayBufferByteLengthData]].
Let currentByteLengthRawBytes be GetRawBytesFromSharedBlock(byteLengthBlock, 0, biguint64, true, seq-cst).
Let newByteLengthRawBytes be NumericToRawBytes(biguint64, ℤ(newByteLength), isLittleEndian).
Repeat,
NOTE: This is a compare-and-exchange loop to ensure that parallel, racing grows of the same buffer are totally ordered, are not lost, and do not silently do nothing. The loop exits if it was able to attempt to grow uncontended.
Let currentByteLength be ℝ(RawBytesToNumeric(biguint64, currentByteLengthRawBytes, isLittleEndian)).
If newByteLength = currentByteLength, return undefined.
If newByteLength < currentByteLength or newByteLength > O.[[ArrayBufferMaxByteLength]], throw a RangeError exception.
Let byteLengthDelta be newByteLength - currentByteLength.
If it is impossible to create a new Shared Data Block value consisting of byteLengthDelta bytes, throw a RangeError exception.
NOTE: No new Shared Data Block is constructed and used here. The observable behaviour of growable SharedArrayBuffers is specified by allocating a max-sized Shared Data Block at construction time, and this step captures the requirement that implementations that run out of memory must throw a RangeError.
Let readByteLengthRawBytes be AtomicCompareExchangeInSharedBlock(byteLengthBlock, 0, 8, currentByteLengthRawBytes, newByteLengthRawBytes).
If ByteListEqual(readByteLengthRawBytes, currentByteLengthRawBytes) is true, return undefined.
Set currentByteLengthRawBytes to readByteLengthRawBytes.
Note
Spurious failures of the compare-exchange to update the length are prohibited. If the bounds checking for the new length passes and the implementation is not out of memory, a ReadModifyWriteSharedMemory event (i.e. a successful compare-exchange) is always added into the candidate execution.
Parallel calls to SharedArrayBuffer.prototype.grow are totally ordered. For example, consider two racing calls: sab.grow(10) and sab.grow(20). One of the two calls is guaranteed to win the race. The call to sab.grow(10) will never shrink sab even if sab.grow(20) happened first; in that case it will instead throw a RangeError.
25.2.5.4 get SharedArrayBuffer.prototype.growable
SharedArrayBuffer.prototype.growable is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
25.2.5.5 get SharedArrayBuffer.prototype.maxByteLength
SharedArrayBuffer.prototype.maxByteLength is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
The initial value of the %Symbol.toStringTag% property is the String value "SharedArrayBuffer".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
25.2.6 Properties of SharedArrayBuffer Instances
SharedArrayBuffer instances inherit properties from the SharedArrayBuffer prototype object. SharedArrayBuffer instances each have an [[ArrayBufferData]] internal slot. SharedArrayBuffer instances which are not growable each have an [[ArrayBufferByteLength]] internal slot. SharedArrayBuffer instances which are growable each have an [[ArrayBufferByteLengthData]] internal slot and an [[ArrayBufferMaxByteLength]] internal slot.
Note
SharedArrayBuffer instances, unlike ArrayBuffer instances, are never detached.
We recommend that programs be tested in their deployment environments where possible. The amount of available physical memory differ greatly between hardware devices. Similarly, virtual memory subsystems also differ greatly between hardware devices as well as operating systems. An application that runs without out-of-memory errors on a 64-bit desktop web browser could run out of memory on a 32-bit mobile web browser.
When choosing a value for the "maxByteLength" option for growable SharedArrayBuffer, we recommend that the smallest possible size for the application be chosen. We recommend that "maxByteLength" does not exceed 1073741824, or 1GiB.
Please note that successfully constructing a growable SharedArrayBuffer for a particular maximum size does not guarantee that future grows will succeed.
Not all loads of a growable SharedArrayBuffer's length are synchronizing seq-cst loads. Loads of the length that are for bounds-checking of an integer-indexed property access, e.g. u8[idx], are not synchronizing. In general, in the absence of explicit synchronization, one property access being in-bound does not imply a subsequent property access in the same agent is also in-bound. In contrast, explicit loads of the length via the length and byteLength getters on SharedArrayBuffer, %TypedArray%.prototype, and DataView.prototype are synchronizing. Loads of the length that are performed by built-in methods to check if a TypedArray is entirely out-of-bounds are also synchronizing.
We recommend growable SharedArrayBuffer be implemented as in-place growth via reserving virtual memory up front.
Because grow operations can happen in parallel with memory accesses on a growable SharedArrayBuffer, the constraints of the memory model require that even unordered accesses do not "tear" (bits of their values will not be mixed). In practice, this means the underlying data block of a growable SharedArrayBuffer cannot be grown by being copied without stopping the world. We do not recommend stopping the world as an implementation strategy because it introduces a serialization point and is slow.
Grown memory must appear zeroed from the moment of its creation, including to any racy accesses in parallel. This can be accomplished via zero-filled-on-demand virtual memory pages, or careful synchronization if manually zeroing memory.
Integer-indexed property access on TypedArray views of growable SharedArrayBuffers is intended to be optimizable similarly to access on TypedArray views of non-growable SharedArrayBuffers, because integer-indexed property loads on are not synchronizing on the underlying buffer's length (see programmer guidelines above). For example, bounds checks for property accesses may still be hoisted out of loops.
In practice it is difficult to implement growable SharedArrayBuffer by copying on hosts that do not have virtual memory, such as those running on embedded devices without an MMU. Memory usage behaviour of growable SharedArrayBuffers on such hosts may significantly differ from that of hosts with virtual memory. Such hosts should clearly communicate memory usage expectations to users.
25.3 DataView Objects
25.3.1 Abstract Operations For DataView Objects
25.3.1.1 DataView With Buffer Witness Records
A DataView With Buffer Witness Record is a Record value used to encapsulate a DataView along with a cached byte length of the viewed buffer. It is used to help ensure there is a single shared memory read event of the byte length data block when the viewed buffer is a growable SharedArrayBuffers.
DataView With Buffer Witness Records have the fields listed in Table 71.
The byte length of the object's [[ViewedArrayBuffer]] when the Record was created.
25.3.1.2 MakeDataViewWithBufferWitnessRecord ( obj, order )
The abstract operation MakeDataViewWithBufferWitnessRecord takes arguments obj (a DataView) and order (seq-cst or unordered) and returns a DataView With Buffer Witness Record. It performs the following steps when called:
The abstract operation GetViewByteLength takes argument viewRecord (a DataView With Buffer Witness Record) and returns a non-negative integer. It performs the following steps when called:
The abstract operation IsViewOutOfBounds takes argument viewRecord (a DataView With Buffer Witness Record) and returns a Boolean. It performs the following steps when called:
Let view be viewRecord.[[Object]].
Let bufferByteLength be viewRecord.[[CachedBufferByteLength]].
Assert: IsDetachedBuffer(view.[[ViewedArrayBuffer]]) is true if and only if bufferByteLength is detached.
If bufferByteLength is detached, return true.
Let byteOffsetStart be view.[[ByteOffset]].
If view.[[ByteLength]] is auto, then
Let byteOffsetEnd be bufferByteLength.
Else,
Let byteOffsetEnd be byteOffsetStart + view.[[ByteLength]].
If byteOffsetStart > bufferByteLength or byteOffsetEnd > bufferByteLength, return true.
NOTE: 0-length DataViews are not considered out-of-bounds.
Return false.
25.3.1.5 GetViewValue ( view, requestIndex, isLittleEndian, type )
is the initial value of the "DataView" property of the global object.
creates and initializes a new DataView when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified DataView behaviour must include a super call to the DataView constructor to create and initialize subclass instances with the internal state necessary to support the DataView.prototype built-in methods.
does not have a [[DataView]], [[ViewedArrayBuffer]], [[ByteLength]], or [[ByteOffset]] internal slot.
25.3.4.1 get DataView.prototype.buffer
DataView.prototype.buffer is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
Assert: O has a [[ViewedArrayBuffer]] internal slot.
Let buffer be O.[[ViewedArrayBuffer]].
Return buffer.
25.3.4.2 get DataView.prototype.byteLength
DataView.prototype.byteLength is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
DataView.prototype.byteOffset is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
The initial value of the %Symbol.toStringTag% property is the String value "DataView".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
25.3.5 Properties of DataView Instances
DataView instances are ordinary objects that inherit properties from the DataView prototype object. DataView instances each have [[DataView]], [[ViewedArrayBuffer]], [[ByteLength]], and [[ByteOffset]] internal slots.
Note
The value of the [[DataView]] internal slot is not used within this specification. The simple presence of that internal slot is used within the specification to identify objects created using the DataView constructor.
25.4 The Atomics Object
The Atomics object:
is %Atomics%.
is the initial value of the "Atomics" property of the global object.
does not have a [[Construct]] internal method; it cannot be used as a constructor with the new operator.
does not have a [[Call]] internal method; it cannot be invoked as a function.
The Atomics object provides functions that operate indivisibly (atomically) on shared memory array cells as well as functions that let agents wait for and dispatch primitive events. When used with discipline, the Atomics functions allow multi-agent programs that communicate through shared memory to execute in a well-understood order even on parallel CPUs. The rules that govern shared-memory communication are provided by the memory model, defined below.
Note
For informative guidelines for programming and implementing shared memory in ECMAScript, please see the notes at the end of the memory model section.
25.4.1 Waiter Record
A Waiter Record is a Record value used to denote a particular call to Atomics.wait or Atomics.waitAsync.
The agent cluster has a store of WaiterList Records; the store is indexed by (block, i), where block is a Shared Data Block and i a byte offset into the memory of block. WaiterList Records are agent-independent: a lookup in the store of WaiterList Records by (block, i) will result in the same WaiterList Record in any agent in the agent cluster.
Each WaiterList Record has a critical section that controls exclusive access to that WaiterList Record during evaluation. Only a single agent may enter a WaiterList Record's critical section at one time. Entering and leaving a WaiterList Record's critical section is controlled by the abstract operationsEnterCriticalSection and LeaveCriticalSection. Operations on a WaiterList Record—adding and removing waiting agents, traversing the list of agents, suspending and notifying agents on the list, setting and retrieving the Synchronize event—may only be performed by agents that have entered the WaiterList Record's critical section.
The abstract operation RevalidateAtomicAccess takes arguments typedArray (a TypedArray) and byteIndexInBuffer (an integer) and returns either a normal completion containingunused or a throw completion. This operation revalidates the index within the backing buffer for atomic operations after all argument coercions are performed in Atomics methods, as argument coercions can have arbitrary side effects, which could cause the buffer to become out of bounds. This operation does not throw when typedArray's backing buffer is a SharedArrayBuffer. It performs the following steps when called:
If byteIndexInBuffer ≥ taRecord.[[CachedBufferByteLength]], throw a RangeError exception.
Return unused.
25.4.3.5 GetWaiterList ( block, i )
The abstract operation GetWaiterList takes arguments block (a Shared Data Block) and i (a non-negative integer that is evenly divisible by 4) and returns a WaiterList Record. It performs the following steps when called:
Assert: i and i + 3 are valid byte offsets within the memory of block.
Return the WaiterList Record that is referenced by the pair (block, i).
25.4.3.6 EnterCriticalSection ( WL )
The abstract operation EnterCriticalSection takes argument WL (a WaiterList Record) and returns unused. It performs the following steps when called:
Append (WL.[[MostRecentLeaveEvent]], enterEvent) to eventsRecord.[[AgentSynchronizesWith]].
Return unused.
EnterCriticalSection has contention when an agent attempting to enter the critical section must wait for another agent to leave it. When there is no contention, FIFO order of EnterCriticalSection calls is observable. When there is contention, an implementation may choose an arbitrary order but may not cause an agent to wait indefinitely.
25.4.3.7 LeaveCriticalSection ( WL )
The abstract operation LeaveCriticalSection takes argument WL (a WaiterList Record) and returns unused. It performs the following steps when called:
The abstract operation AddWaiter takes arguments WL (a WaiterList Record) and waiterRecord (a Waiter Record) and returns unused. It performs the following steps when called:
Assert: There is no Waiter Record in WL.[[Waiters]] whose [[PromiseCapability]] field is waiterRecord.[[PromiseCapability]] and whose [[AgentSignifier]] field is waiterRecord.[[AgentSignifier]].
Append waiterRecord to WL.[[Waiters]].
Return unused.
25.4.3.9 RemoveWaiter ( WL, waiterRecord )
The abstract operation RemoveWaiter takes arguments WL (a WaiterList Record) and waiterRecord (a Waiter Record) and returns unused. It performs the following steps when called:
The abstract operation RemoveWaiters takes arguments WL (a WaiterList Record) and c (a non-negative integer or +∞) and returns a List of Waiter Records. It performs the following steps when called:
Let L be a List whose elements are the first n elements of WL.[[Waiters]].
Remove the first n elements of WL.[[Waiters]].
Return L.
25.4.3.11 SuspendThisAgent ( WL, waiterRecord )
The abstract operation SuspendThisAgent takes arguments WL (a WaiterList Record) and waiterRecord (a Waiter Record) and returns unused. It performs the following steps when called:
Perform LeaveCriticalSection(WL) and suspend the surrounding agent until the time is waiterRecord.[[TimeoutTime]], performing the combined operation in such a way that a notification that arrives after the critical section is exited but before the suspension takes effect is not lost. The surrounding agent can only wake from suspension due to a timeout or due to another agent calling NotifyWaiter with arguments WL and thisAgent (i.e. via a call to Atomics.notify).
The abstract operation NotifyWaiter takes arguments WL (a WaiterList Record) and waiterRecord (a Waiter Record) and returns unused. It performs the following steps when called:
The abstract operation EnqueueResolveInAgentJob takes arguments agentSignifier (an agent signifier), promiseCapability (a PromiseCapability Record), and resolution ("ok" or "timed-out") and returns unused. It performs the following steps when called:
Let resolveJob be a new JobAbstract Closure with no parameters that captures agentSignifier, promiseCapability, and resolution and performs the following steps when called:
NOTE: There is no special handling of synchronous immediate timeouts. Asynchronous immediate timeouts have special handling in order to fail fast and avoid unnecessary Promise jobs.
Let timeoutTime be ℝ(now) + t + additionalTimeout.
NOTE: When t is +∞, timeoutTime is also +∞.
Let waiterRecord be a new Waiter Record { [[AgentSignifier]]: thisAgent, [[PromiseCapability]]: promiseCapability, [[TimeoutTime]]: timeoutTime, [[Result]]: "ok" }.
additionalTimeout allows implementations to pad timeouts as necessary, such as for reducing power consumption or coarsening timer resolution to mitigate timing attacks. This value may differ from call to call of DoWait.
The abstract operation EnqueueAtomicsWaitAsyncTimeoutJob takes arguments WL (a WaiterList Record) and waiterRecord (a Waiter Record) and returns unused. It performs the following steps when called:
Let timeoutJob be a new JobAbstract Closure with no parameters that captures WL and waiterRecord and performs the following steps when called:
The abstract operation AtomicCompareExchangeInSharedBlock takes arguments block (a Shared Data Block), byteIndexInBuffer (an integer), elementSize (a non-negative integer), expectedBytes (a List of byte values), and replacementBytes (a List of byte values) and returns a List of byte values. It performs the following steps when called:
Let rawBytesRead be a List of length elementSize whose elements are nondeterministically chosen byte values.
NOTE: In implementations, rawBytesRead is the result of a load-link, of a load-exclusive, or of an operand of a read-modify-write instruction on the underlying hardware. The nondeterminism is a semantic prescription of the memory model to describe observable behaviour of hardware with weak consistency.
NOTE: The comparison of the expected value and the read value is performed outside of the read-modify-write modification function to avoid needlessly strong synchronization when the expected value is not equal to the read value.
If ByteListEqual(rawBytesRead, expectedBytes) is true, then
Let second be a new read-modify-write modification function with parameters (oldBytes, newBytes) that captures nothing and performs the following steps atomically when called:
Return newBytes.
Let event be ReadModifyWriteSharedMemory { [[Order]]: seq-cst, [[NoTear]]: true, [[Block]]: block, [[ByteIndex]]: byteIndexInBuffer, [[ElementSize]]: elementSize, [[Payload]]: replacementBytes, [[ModifyOp]]: second }.
Else,
Let event be ReadSharedMemory { [[Order]]: seq-cst, [[NoTear]]: true, [[Block]]: block, [[ByteIndex]]: byteIndexInBuffer, [[ElementSize]]: elementSize }.
Append event to eventsRecord.[[EventList]].
Append Chosen Value Record { [[Event]]: event, [[ChosenValue]]: rawBytesRead } to execution.[[ChosenValues]].
Return rawBytesRead.
25.4.3.17 AtomicReadModifyWrite ( typedArray, index, value, op )
The abstract operation ByteListBitwiseOp takes arguments op (&, ^, or |), xBytes (a List of byte values), and yBytes (a List of byte values) and returns a List of byte values. The operation atomically performs a bitwise operation on all byte values of the arguments and returns a List of byte values. It performs the following steps when called:
Assert: xBytes and yBytes have the same number of elements.
Let resultByte be the result of applying the bitwise inclusive OR operation to xByte and yByte.
Set i to i + 1.
Append resultByte to result.
Return result.
25.4.3.19 ByteListEqual ( xBytes, yBytes )
The abstract operation ByteListEqual takes arguments xBytes (a List of byte values) and yBytes (a List of byte values) and returns a Boolean. It performs the following steps when called:
If xBytes and yBytes do not have the same number of elements, return false.
Let i be 0.
For each element xByte of xBytes, do
Let yByte be yBytes[i].
If xByte ≠ yByte, return false.
Set i to i + 1.
Return true.
25.4.4 Atomics.add ( typedArray, index, value )
This function performs the following steps when called:
Let add be a new read-modify-write modification function with parameters (xBytes, yBytes) that captures type and isLittleEndian and performs the following steps atomically when called:
This function performs the following steps when called:
Let and be a new read-modify-write modification function with parameters (xBytes, yBytes) that captures nothing and performs the following steps atomically when called:
25.4.7 Atomics.exchange ( typedArray, index, value )
This function performs the following steps when called:
Let second be a new read-modify-write modification function with parameters (oldBytes, newBytes) that captures nothing and performs the following steps atomically when called:
This function is an optimization primitive. The intuition is that if the atomic step of an atomic primitive (compareExchange, load, store, add, sub, and, or, xor, or exchange) on a datum of size n bytes will be performed without the surrounding agent acquiring a lock outside the n bytes comprising the datum, then Atomics.isLockFree(n) will return true. High-performance algorithms will use this function to determine whether to use locks or atomic operations in critical sections. If an atomic primitive is not lock-free then it is often more efficient for an algorithm to provide its own locking.
Atomics.isLockFree(4) always returns true as that can be supported on all known relevant hardware. Being able to assume this will generally simplify programs.
Regardless of the value returned by this function, all atomic operations are guaranteed to be atomic. For example, they will never have a visible operation take place in the middle of the operation (e.g., "tearing").
25.4.9 Atomics.load ( typedArray, index )
This function performs the following steps when called:
This function performs the following steps when called:
Let or be a new read-modify-write modification function with parameters (xBytes, yBytes) that captures nothing and performs the following steps atomically when called:
Let subtract be a new read-modify-write modification function with parameters (xBytes, yBytes) that captures type and isLittleEndian and performs the following steps atomically when called:
This function puts the surrounding agent in a wait queue and suspends it until notified or until the wait times out, returning a String differentiating those cases.
This function performs the following steps when called:
Let xor be a new read-modify-write modification function with parameters (xBytes, yBytes) that captures nothing and performs the following steps atomically when called:
does not have a [[Construct]] internal method; it cannot be used as a constructor with the new operator.
does not have a [[Call]] internal method; it cannot be invoked as a function.
The JSON Data Interchange Format is defined in ECMA-404. The JSON interchange format used in this specification is exactly that described by ECMA-404. Conforming implementations of JSON.parse and JSON.stringify must support the exact interchange format described in the ECMA-404 specification without any deletions or extensions to the format.
25.5.1 JSON.parse ( text [ , reviver ] )
This function parses a JSON text (a JSON-formatted String) and produces an ECMAScript language value. The JSON format represents literals, arrays, and objects with a syntax similar to the syntax for ECMAScript literals, Array Initializers, and Object Initializers. After parsing, JSON objects are realized as ECMAScript objects. JSON arrays are realized as ECMAScript Array instances. JSON strings, numbers, booleans, and null are realized as ECMAScript Strings, Numbers, Booleans, and null.
The optional reviver parameter is a function that takes two parameters, key and value. It can filter and transform the results. It is called with each of the key/value pairs produced by the parse, and its return value is used instead of the original value. If it returns what it received, the structure is not modified. If it returns undefined then the property is deleted from the result.
Parse StringToCodePoints(jsonString) as a JSON text as specified in ECMA-404. Throw a SyntaxError exception if it is not a valid JSON text as defined in that specification.
Valid JSON text is a subset of the ECMAScript PrimaryExpression syntax. Step 2 verifies that jsonString conforms to that subset, and step 10 asserts that that parsing and evaluation returns a value of an appropriate type.
However, because 13.2.5.5 behaves differently during JSON.parse, the same source text can produce different results when evaluated as a PrimaryExpression rather than as JSON. Furthermore, the Early Error for duplicate "__proto__" properties in object literals, which likewise does not apply during JSON.parse, means that not all texts accepted by JSON.parse are valid as a PrimaryExpression, despite matching the grammar.
It is not permitted for a conforming implementation of JSON.parse to extend the JSON grammars. If an implementation wishes to support a modified or extended JSON interchange format it must do so by defining a different parse function.
Note 2
In the case where there are duplicate name Strings within an object, lexically preceding values for the same key shall be overwritten.
25.5.2 JSON.stringify ( value [ , replacer [ , space ] ] )
This function returns a String in UTF-16 encoded JSON format representing an ECMAScript language value, or undefined. It can take three parameters. The value parameter is an ECMAScript language value, which is usually an object or array, although it can also be a String, Boolean, Number or null. The optional replacer parameter is either a function that alters the way objects and arrays are stringified, or an array of Strings and Numbers that acts as an inclusion list for selecting the object properties that will be stringified. The optional space parameter is a String or Number that allows the result to have white space injected into it to improve human readability.
Let state be the JSON Serialization Record { [[ReplacerFunction]]: ReplacerFunction, [[Stack]]: stack, [[Indent]]: indent, [[Gap]]: gap, [[PropertyList]]: PropertyList }.
JSON structures are allowed to be nested to any depth, but they must be acyclic. If value is or contains a cyclic structure, then this function must throw a TypeError exception. This is an example of a value that cannot be stringified:
a = [];
a[0] = a;
my_text = JSON.stringify(a); // This must throw a TypeError.
Note 2
Symbolic primitive values are rendered as follows:
The null value is rendered in JSON text as the String value "null".
The undefined value is not rendered.
The true value is rendered in JSON text as the String value "true".
The false value is rendered in JSON text as the String value "false".
Note 3
String values are wrapped in QUOTATION MARK (") code units. The code units " and \ are escaped with \ prefixes. Control characters code units are replaced with escape sequences \uHHHH, or with the shorter forms, \b (BACKSPACE), \f (FORM FEED), \n (LINE FEED), \r (CARRIAGE RETURN), \t (CHARACTER TABULATION).
Note 4
Finite numbers are stringified as if by calling ToString(number). NaN and Infinity regardless of sign are represented as the String value "null".
Note 5
Values that do not have a JSON representation (such as undefined and functions) do not produce a String. Instead they produce the undefined value. In arrays these values are represented as the String value "null". In objects an unrepresentable value causes the property to be excluded from stringification.
Note 6
An object is rendered as U+007B (LEFT CURLY BRACKET) followed by zero or more properties, separated with a U+002C (COMMA), closed with a U+007D (RIGHT CURLY BRACKET). A property is a quoted String representing the property name, a U+003A (COLON), and then the stringified property value. An array is rendered as an opening U+005B (LEFT SQUARE BRACKET) followed by zero or more values, separated with a U+002C (COMMA), closed with a U+005D (RIGHT SQUARE BRACKET).
25.5.2.1 JSON Serialization Record
A JSON Serialization Record is a Record value used to enable serialization to the JSON format.
JSON Serialization Records have the fields listed in Table 74.
The abstract operation SerializeJSONProperty takes arguments state (a JSON Serialization Record), key (a String), and holder (an Object) and returns either a normal completion containing either a String or undefined, or a throw completion. It performs the following steps when called:
The abstract operation QuoteJSONString takes argument value (a String) and returns a String. It wraps value in 0x0022 (QUOTATION MARK) code units and escapes certain other code units within it. This operation interprets value as a sequence of UTF-16 encoded code points, as described in 6.1.4. It performs the following steps when called:
Let product be the String value consisting solely of the code unit 0x0022 (QUOTATION MARK).
If C is listed in the “Code Point” column of Table 75, then
Set product to the string-concatenation of product and the escape sequence for C as specified in the “Escape Sequence” column of the corresponding row.
Set product to the string-concatenation of product and the code unit 0x0022 (QUOTATION MARK).
Return product.
Table 75: JSON Single Character Escape Sequences
Code Point
Unicode Character Name
Escape Sequence
U+0008
BACKSPACE
\b
U+0009
CHARACTER TABULATION
\t
U+000A
LINE FEED (LF)
\n
U+000C
FORM FEED (FF)
\f
U+000D
CARRIAGE RETURN (CR)
\r
U+0022
QUOTATION MARK
\"
U+005C
REVERSE SOLIDUS
\\
25.5.2.4 UnicodeEscape ( C )
The abstract operation UnicodeEscape takes argument C (a code unit) and returns a String. It represents C as a Unicode escape sequence. It performs the following steps when called:
Let properties be the String value formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with the code unit 0x002C (COMMA). A comma is not inserted either before the first String or after the last String.
Let separator be the string-concatenation of the code unit 0x002C (COMMA), the code unit 0x000A (LINE FEED), and state.[[Indent]].
Let properties be the String value formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with separator. The separator String is not inserted either before the first String or after the last String.
Let final be the string-concatenation of "{", the code unit 0x000A (LINE FEED), state.[[Indent]], properties, the code unit 0x000A (LINE FEED), stepBack, and "}".
Let properties be the String value formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with the code unit 0x002C (COMMA). A comma is not inserted either before the first String or after the last String.
Let separator be the string-concatenation of the code unit 0x002C (COMMA), the code unit 0x000A (LINE FEED), and state.[[Indent]].
Let properties be the String value formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with separator. The separator String is not inserted either before the first String or after the last String.
Let final be the string-concatenation of "[", the code unit 0x000A (LINE FEED), state.[[Indent]], properties, the code unit 0x000A (LINE FEED), stepBack, and "]".
Remove the last element of state.[[Stack]].
Set state.[[Indent]] to stepBack.
Return final.
Note
The representation of arrays includes only the elements in the interval from +0𝔽 (inclusive) to array.length (exclusive). Properties whose keys are not array indices are excluded from the stringification. An array is stringified as an opening LEFT SQUARE BRACKET, elements separated by COMMA, and a closing RIGHT SQUARE BRACKET.
25.5.3 JSON [ %Symbol.toStringTag% ]
The initial value of the %Symbol.toStringTag% property is the String value "JSON".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
26 Managing Memory
26.1 WeakRef Objects
A WeakRef is an object that is used to refer to a target object or symbol without preserving it from garbage collection. WeakRefs can be dereferenced to allow access to the target value, if the target hasn't been reclaimed by garbage collection.
is the initial value of the "WeakRef" property of the global object.
creates and initializes a new WeakRef when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified WeakRef behaviour must include a super call to the WeakRefconstructor to create and initialize the subclass instance with the internal state necessary to support the WeakRef.prototype built-in methods.
26.1.1.1 WeakRef ( target )
This function performs the following steps when called:
If NewTarget is undefined, throw a TypeError exception.
If CanBeHeldWeakly(target) is false, throw a TypeError exception.
If the WeakRef returns a target value that is not undefined, then this target value should not be garbage collected until the current execution of ECMAScript code has completed. The AddToKeptObjects operation makes sure read consistency is maintained.
let target = { foo() {} };
let weakRef = newWeakRef(target);
// ... later ...if (weakRef.deref()) {
weakRef.deref().foo();
}
In the above example, if the first deref does not evaluate to undefined then the second deref cannot either.
The initial value of the %Symbol.toStringTag% property is the String value "WeakRef".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
26.1.4 WeakRef Abstract Operations
26.1.4.1 WeakRefDeref ( weakRef )
The abstract operation WeakRefDeref takes argument weakRef (a WeakRef) and returns an ECMAScript language value. It performs the following steps when called:
A FinalizationRegistry is an object that manages registration and unregistration of cleanup operations that are performed when target objects and symbols are garbage collected.
is the initial value of the "FinalizationRegistry" property of the global object.
creates and initializes a new FinalizationRegistry when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified FinalizationRegistry behaviour must include a super call to the FinalizationRegistryconstructor to create and initialize the subclass instance with the internal state necessary to support the FinalizationRegistry.prototype built-in methods.
26.2.1.1 FinalizationRegistry ( cleanupCallback )
This function performs the following steps when called:
If NewTarget is undefined, throw a TypeError exception.
If IsCallable(cleanupCallback) is false, throw a TypeError exception.
Let finalizationRegistry be ? OrdinaryCreateFromConstructor(NewTarget, "%FinalizationRegistry.prototype%", « [[Realm]], [[CleanupCallback]], [[Cells]] »).
If unregisterToken is not undefined, throw a TypeError exception.
Set unregisterToken to empty.
Let cell be the Record { [[WeakRefTarget]]: target, [[HeldValue]]: heldValue, [[UnregisterToken]]: unregisterToken }.
Append cell to finalizationRegistry.[[Cells]].
Return undefined.
Note
Based on the algorithms and definitions in this specification, cell.[[HeldValue]] is live when finalizationRegistry.[[Cells]] contains cell; however, this does not necessarily mean that cell.[[UnregisterToken]] or cell.[[Target]] are live. For example, registering an object with itself as its unregister token would not keep the object alive forever.
An interface is a set of property keys whose associated values match a specific specification. Any object that provides all the properties as described by an interface's specification conforms to that interface. An interface is not represented by a distinct object. There may be many separately implemented objects that conform to any interface. An individual object may conform to multiple interfaces.
27.1.1.1 The Iterable Interface
The Iterable interface includes the property described in Table 76:
Table 76: Iterable Interface Required Properties
Property
Value
Requirements
%Symbol.iterator%
a function that returns an Iterator object
The returned object must conform to the Iterator interface.
27.1.1.2 The Iterator Interface
An object that implements the Iterator interface must include the property in Table 77. Such objects may also implement the properties in Table 78.
Table 77: Iterator Interface Required Properties
Property
Value
Requirements
"next"
a function that returns an IteratorResult object
The returned object must conform to the IteratorResult interface. If a previous call to the next method of an Iterator has returned an IteratorResult object whose "done" property is true, then all subsequent calls to the next method of that object should also return an IteratorResult object whose "done" property is true. However, this requirement is not enforced.
Note 1
Arguments may be passed to the next function but their interpretation and validity is dependent upon the target Iterator. The for-of statement and other common users of Iterators do not pass any arguments, so Iterator objects that expect to be used in such a manner must be prepared to deal with being called with no arguments.
Table 78: Iterator Interface Optional Properties
Property
Value
Requirements
"return"
a function that returns an IteratorResult object
The returned object must conform to the IteratorResult interface. Invoking this method notifies the Iterator object that the caller does not intend to make any more next method calls to the Iterator. The returned IteratorResult object will typically have a "done" property whose value is true, and a "value" property with the value passed as the argument of the return method. However, this requirement is not enforced.
"throw"
a function that returns an IteratorResult object
The returned object must conform to the IteratorResult interface. Invoking this method notifies the Iterator object that the caller has detected an error condition. The argument may be used to identify the error condition and typically will be an exception object. A typical response is to throw the value passed as the argument. If the method does not throw, the returned IteratorResult object will typically have a "done" property whose value is true.
Note 2
Typically callers of these methods should check for their existence before invoking them. Certain ECMAScript language features including for-of, yield*, and array destructuring call these methods after performing an existence check. Most ECMAScript library functions that accept Iterable objects as arguments also conditionally call them.
27.1.1.3 The AsyncIterable Interface
The AsyncIterable interface includes the properties described in Table 79:
The returned object must conform to the AsyncIterator interface.
27.1.1.4 The AsyncIterator Interface
An object that implements the AsyncIterator interface must include the properties in Table 80. Such objects may also implement the properties in Table 81.
a function that returns a promise for an IteratorResult object
The returned promise, when fulfilled, must fulfill with an object that conforms to the IteratorResult interface. If a previous call to the next method of an AsyncIterator has returned a promise for an IteratorResult object whose "done" property is true, then all subsequent calls to the next method of that object should also return a promise for an IteratorResult object whose "done" property is true. However, this requirement is not enforced.
Additionally, the IteratorResult object that serves as a fulfillment value should have a "value" property whose value is not a promise (or "thenable"). However, this requirement is also not enforced.
Note 1
Arguments may be passed to the next function but their interpretation and validity is dependent upon the target AsyncIterator. The for-await-of statement and other common users of AsyncIterators do not pass any arguments, so AsyncIterator objects that expect to be used in such a manner must be prepared to deal with being called with no arguments.
a function that returns a promise for an IteratorResult object
The returned promise, when fulfilled, must fulfill with an object that conforms to the IteratorResult interface. Invoking this method notifies the AsyncIterator object that the caller does not intend to make any more next method calls to the AsyncIterator. The returned promise will fulfill with an IteratorResult object which will typically have a "done" property whose value is true, and a "value" property with the value passed as the argument of the return method. However, this requirement is not enforced.
Additionally, the IteratorResult object that serves as a fulfillment value should have a "value" property whose value is not a promise (or "thenable"). If the argument value is used in the typical manner, then if it is a rejected promise, a promise rejected with the same reason should be returned; if it is a fulfilled promise, then its fulfillment value should be used as the "value" property of the returned promise's IteratorResult object fulfillment value. However, these requirements are also not enforced.
"throw"
a function that returns a promise for an IteratorResult object
The returned promise, when fulfilled, must fulfill with an object that conforms to the IteratorResult interface. Invoking this method notifies the AsyncIterator object that the caller has detected an error condition. The argument may be used to identify the error condition and typically will be an exception object. A typical response is to return a rejected promise which rejects with the value passed as the argument.
If the returned promise is fulfilled, the IteratorResult fulfillment value will typically have a "done" property whose value is true. Additionally, it should have a "value" property whose value is not a promise (or "thenable"), but this requirement is not enforced.
Note 2
Typically callers of these methods should check for their existence before invoking them. Certain ECMAScript language features including for-await-of and yield* call these methods after performing an existence check.
27.1.1.5 The IteratorResult Interface
The IteratorResult interface includes the properties listed in Table 82:
Table 82: IteratorResult Interface Properties
Property
Value
Requirements
"done"
a Boolean
This is the result status of an iteratornext method call. If the end of the iterator was reached "done" is true. If the end was not reached "done" is false and a value is available. If a "done" property (either own or inherited) does not exist, it is considered to have the value false.
If done is false, this is the current iteration element value. If done is true, this is the return value of the iterator, if it supplied one. If the iterator does not have a return value, "value" is undefined. In that case, the "value" property may be absent from the conforming object if it does not inherit an explicit "value" property.
All objects defined in this specification that implement the Iterator interface also inherit from %IteratorPrototype%. ECMAScript code may also define objects that inherit from %IteratorPrototype%. The %IteratorPrototype% object provides a place where additional methods that are applicable to all iterator objects may be added.
The following expression is one way that ECMAScript code can access the %IteratorPrototype% object:
All objects defined in this specification that implement the AsyncIterator interface also inherit from %AsyncIteratorPrototype%. ECMAScript code may also define objects that inherit from %AsyncIteratorPrototype%. The %AsyncIteratorPrototype% object provides a place where additional methods that are applicable to all async iterator objects may be added.
This function performs the following steps when called:
Return the this value.
The value of the "name" property of this function is "[Symbol.asyncIterator]".
27.1.4 Async-from-Sync Iterator Objects
An Async-from-Sync Iterator object is an async iterator that adapts a specific synchronous iterator. Async-from-Sync Iterator objects are never directly accessible to ECMAScript code. There is not a named constructor for Async-from-Sync Iterator objects. Instead, Async-from-Sync iterator objects are created by the CreateAsyncFromSyncIterator abstract operation as needed.
The abstract operation CreateAsyncFromSyncIterator takes argument syncIteratorRecord (an Iterator Record) and returns an Iterator Record. It is used to create an async Iterator Record from a synchronous Iterator Record. It performs the following steps when called:
27.1.4.3 Properties of Async-from-Sync Iterator Instances
Async-from-Sync Iterator instances are ordinary objects that inherit properties from the %AsyncFromSyncIteratorPrototype% intrinsic object. Async-from-Sync Iterator instances are initially created with the internal slots listed in Table 83.
Table 83: Internal Slots of Async-from-Sync Iterator Instances
The abstract operation AsyncFromSyncIteratorContinuation takes arguments result (an Object) and promiseCapability (a PromiseCapability Record for an intrinsic %Promise%) and returns a Promise. It performs the following steps when called:
NOTE: Because promiseCapability is derived from the intrinsic %Promise%, the calls to promiseCapability.[[Reject]] entailed by the use IfAbruptRejectPromise below are guaranteed not to throw.
NOTE: onFulfilled is used when processing the "value" property of an IteratorResult object in order to wait for its value if it is a promise and re-package the result in a new "unwrapped" IteratorResult object.
A Promise is an object that is used as a placeholder for the eventual results of a deferred (and possibly asynchronous) computation.
Any Promise is in one of three mutually exclusive states: fulfilled, rejected, and pending:
A promise p is fulfilled if p.then(f, r) will immediately enqueue a Job to call the function f.
A promise p is rejected if p.then(f, r) will immediately enqueue a Job to call the function r.
A promise is pending if it is neither fulfilled nor rejected.
A promise is said to be settled if it is not pending, i.e. if it is either fulfilled or rejected.
A promise is resolved if it is settled or if it has been “locked in” to match the state of another promise. Attempting to resolve or reject a resolved promise has no effect. A promise is unresolved if it is not resolved. An unresolved promise is always in the pending state. A resolved promise may be pending, fulfilled or rejected.
27.2.1 Promise Abstract Operations
27.2.1.1 PromiseCapability Records
A PromiseCapability Record is a Record value used to encapsulate a Promise or promise-like object along with the functions that are capable of resolving or rejecting that promise. PromiseCapability Records are produced by the NewPromiseCapability abstract operation.
PromiseCapability Records have the fields listed in Table 84.
A PromiseReaction Record is a Record value used to store information about how a promise should react when it becomes resolved or rejected with a given value. PromiseReaction Records are created by the PerformPromiseThen abstract operation, and are used by the Abstract Closure returned by NewPromiseReactionJob.
PromiseReaction Records have the fields listed in Table 85.
The function that should be applied to the incoming value, and whose return value will govern what happens to the derived promise. If [[Handler]] is empty, a function that depends on the value of [[Type]] will be used instead.
27.2.1.3 CreateResolvingFunctions ( promise )
The abstract operation CreateResolvingFunctions takes argument promise (a Promise) and returns a Record with fields [[Resolve]] (a function object) and [[Reject]] (a function object). It performs the following steps when called:
Let alreadyResolved be the Record { [[Value]]: false }.
The "length" property of a promise resolve function is 1𝔽.
27.2.1.4 FulfillPromise ( promise, value )
The abstract operation FulfillPromise takes arguments promise (a Promise) and value (an ECMAScript language value) and returns unused. It performs the following steps when called:
Assert: The value of promise.[[PromiseState]] is pending.
Let reactions be promise.[[PromiseFulfillReactions]].
Set promise.[[PromiseResult]] to value.
Set promise.[[PromiseFulfillReactions]] to undefined.
Set promise.[[PromiseRejectReactions]] to undefined.
If IsConstructor(C) is false, throw a TypeError exception.
NOTE: C is assumed to be a constructor function that supports the parameter conventions of the Promise constructor (see 27.2.3.1).
Let resolvingFunctions be the Record { [[Resolve]]: undefined, [[Reject]]: undefined }.
Let executorClosure be a new Abstract Closure with parameters (resolve, reject) that captures resolvingFunctions and performs the following steps when called:
If resolvingFunctions.[[Resolve]] is not undefined, throw a TypeError exception.
If resolvingFunctions.[[Reject]] is not undefined, throw a TypeError exception.
If IsCallable(resolvingFunctions.[[Resolve]]) is false, throw a TypeError exception.
If IsCallable(resolvingFunctions.[[Reject]]) is false, throw a TypeError exception.
Return the PromiseCapability Record { [[Promise]]: promise, [[Resolve]]: resolvingFunctions.[[Resolve]], [[Reject]]: resolvingFunctions.[[Reject]] }.
Note
This abstract operation supports Promise subclassing, as it is generic on any constructor that calls a passed executor function argument in the same way as the Promise constructor. It is used to generalize static methods of the Promise constructor to any subclass.
27.2.1.6 IsPromise ( x )
The abstract operation IsPromise takes argument x (an ECMAScript language value) and returns a Boolean. It checks for the promise brand on an object. It performs the following steps when called:
If x does not have a [[PromiseState]] internal slot, return false.
Return true.
27.2.1.7 RejectPromise ( promise, reason )
The abstract operation RejectPromise takes arguments promise (a Promise) and reason (an ECMAScript language value) and returns unused. It performs the following steps when called:
Assert: The value of promise.[[PromiseState]] is pending.
Let reactions be promise.[[PromiseRejectReactions]].
Set promise.[[PromiseResult]] to reason.
Set promise.[[PromiseFulfillReactions]] to undefined.
Set promise.[[PromiseRejectReactions]] to undefined.
The abstract operation TriggerPromiseReactions takes arguments reactions (a List of PromiseReaction Records) and argument (an ECMAScript language value) and returns unused. It enqueues a new Job for each record in reactions. Each such Job processes the [[Type]] and [[Handler]] of the PromiseReaction Record, and if the [[Handler]] is not empty, calls it passing the given argument. If the [[Handler]] is empty, the behaviour is determined by the [[Type]]. It performs the following steps when called:
The host-defined abstract operation HostPromiseRejectionTracker takes arguments promise (a Promise) and operation ("reject" or "handle") and returns unused. It allows host environments to track promise rejections.
The default implementation of HostPromiseRejectionTracker is to return unused.
Note 1
HostPromiseRejectionTracker is called in two scenarios:
When a promise is rejected without any handlers, it is called with its operation argument set to "reject".
When a handler is added to a rejected promise for the first time, it is called with its operation argument set to "handle".
A typical implementation of HostPromiseRejectionTracker might try to notify developers of unhandled rejections, while also being careful to notify them if such previous notifications are later invalidated by new handlers being attached.
Note 2
If operation is "handle", an implementation should not hold a reference to promise in a way that would interfere with garbage collection. An implementation may hold a reference to promise if operation is "reject", since it is expected that rejections will be rare and not on hot code paths.
The abstract operation NewPromiseReactionJob takes arguments reaction (a PromiseReaction Record) and argument (an ECMAScript language value) and returns a Record with fields [[Job]] (a JobAbstract Closure) and [[Realm]] (a Realm Record or null). It returns a new JobAbstract Closure that applies the appropriate handler to the incoming value, and uses the handler's return value to resolve or reject the derived promise associated with that handler. It performs the following steps when called:
Let job be a new JobAbstract Closure with no parameters that captures reaction and argument and performs the following steps when called:
NOTE: handlerRealm is never null unless the handler is undefined. When the handler is a revoked Proxy and no ECMAScript code runs, handlerRealm is used to create error objects.
Return the Record { [[Job]]: job, [[Realm]]: handlerRealm }.
27.2.2.2 NewPromiseResolveThenableJob ( promiseToResolve, thenable, then )
The abstract operation NewPromiseResolveThenableJob takes arguments promiseToResolve (a Promise), thenable (an Object), and then (a JobCallback Record) and returns a Record with fields [[Job]] (a JobAbstract Closure) and [[Realm]] (a Realm Record). It performs the following steps when called:
Let job be a new JobAbstract Closure with no parameters that captures promiseToResolve, thenable, and then and performs the following steps when called:
NOTE: thenRealm is never null. When then.[[Callback]] is a revoked Proxy and no code runs, thenRealm is used to create error objects.
Return the Record { [[Job]]: job, [[Realm]]: thenRealm }.
Note
This Job uses the supplied thenable and its then method to resolve the given promise. This process must take place as a Job to ensure that the evaluation of the then method occurs after evaluation of any surrounding code has completed.
is the initial value of the "Promise" property of the global object.
creates and initializes a new Promise when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified Promise behaviour must include a super call to the Promise constructor to create and initialize the subclass instance with the internal state necessary to support the Promise and Promise.prototype built-in methods.
27.2.3.1 Promise ( executor )
This function performs the following steps when called:
If NewTarget is undefined, throw a TypeError exception.
If IsCallable(executor) is false, throw a TypeError exception.
Let promise be ? OrdinaryCreateFromConstructor(NewTarget, "%Promise.prototype%", « [[PromiseState]], [[PromiseResult]], [[PromiseFulfillReactions]], [[PromiseRejectReactions]], [[PromiseIsHandled]] »).
Set promise.[[PromiseState]] to pending.
Set promise.[[PromiseFulfillReactions]] to a new empty List.
Set promise.[[PromiseRejectReactions]] to a new empty List.
The executor argument must be a function object. It is called for initiating and reporting completion of the possibly deferred action represented by this Promise. The executor is called with two arguments: resolve and reject. These are functions that may be used by the executor function to report eventual completion or failure of the deferred computation. Returning from the executor function does not mean that the deferred action has been completed but only that the request to eventually perform the deferred action has been accepted.
The resolve function that is passed to an executor function accepts a single argument. The executor code may eventually call the resolve function to indicate that it wishes to resolve the associated Promise. The argument passed to the resolve function represents the eventual value of the deferred action and can be either the actual fulfillment value or another promise which will provide the value if it is fulfilled.
The reject function that is passed to an executor function accepts a single argument. The executor code may eventually call the reject function to indicate that the associated Promise is rejected and will never be fulfilled. The argument passed to the reject function is used as the rejection value of the promise. Typically it will be an Error object.
The resolve and reject functions passed to an executor function by the Promise constructor have the capability to actually resolve and reject the associated promise. Subclasses may have different constructor behaviour that passes in customized values for resolve and reject.
This function returns a new promise which is fulfilled with an array of fulfillment values for the passed promises, or rejects with the reason of the first passed promise that rejects. It resolves all elements of the passed iterable to promises as it runs this algorithm.
A Promise.all resolve element function is an anonymous built-in function that is used to resolve a specific Promise.all element. Each Promise.all resolve element function has [[Index]], [[Values]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.
When a Promise.all resolve element function is called with argument x, the following steps are taken:
The "length" property of a Promise.all resolve element function is 1𝔽.
27.2.4.2 Promise.allSettled ( iterable )
This function returns a promise that is fulfilled with an array of promise state snapshots, but only after all the original promises have settled, i.e. become either fulfilled or rejected. It resolves all elements of the passed iterable to promises as it runs this algorithm.
27.2.4.2.2Promise.allSettled Resolve Element Functions
A Promise.allSettled resolve element function is an anonymous built-in function that is used to resolve a specific Promise.allSettled element. Each Promise.allSettled resolve element function has [[Index]], [[Values]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.
When a Promise.allSettled resolve element function is called with argument x, the following steps are taken:
The "length" property of a Promise.allSettled resolve element function is 1𝔽.
27.2.4.2.3Promise.allSettled Reject Element Functions
A Promise.allSettled reject element function is an anonymous built-in function that is used to reject a specific Promise.allSettled element. Each Promise.allSettled reject element function has [[Index]], [[Values]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.
When a Promise.allSettled reject element function is called with argument x, the following steps are taken:
The "length" property of a Promise.allSettled reject element function is 1𝔽.
27.2.4.3 Promise.any ( iterable )
This function returns a promise that is fulfilled by the first given promise to be fulfilled, or rejected with an AggregateError holding the rejection reasons if all of the given promises are rejected. It resolves all elements of the passed iterable to promises as it runs this algorithm.
A Promise.any reject element function is an anonymous built-in function that is used to reject a specific Promise.any element. Each Promise.any reject element function has [[Index]], [[Errors]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.
When a Promise.any reject element function is called with argument x, the following steps are taken:
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
27.2.4.5 Promise.race ( iterable )
This function returns a new promise which is settled in the same way as the first passed promise to settle. It resolves all elements of the passed iterable to promises as it runs this algorithm.
If the iterable argument yields no values or if none of the promises yielded by iterable ever settle, then the pending promise returned by this method will never be settled.
Note 2
This function expects its this value to be a constructor function that supports the parameter conventions of the Promise constructor. It also expects that its this value provides a resolve method.
Perform ? Call(promiseCapability.[[Reject]], undefined, « r »).
Return promiseCapability.[[Promise]].
Note
This function expects its this value to be a constructor function that supports the parameter conventions of the Promise constructor.
27.2.4.7 Promise.resolve ( x )
This function returns either a new promise resolved with the passed argument, or the argument itself if the argument is a promise produced by this constructor.
Promise[%Symbol.species%] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:
Return the this value.
The value of the "name" property of this function is "get [Symbol.species]".
Note
Promise prototype methods normally use their this value's constructor to create a derived object. However, a subclass constructor may over-ride that default behaviour by redefining its %Symbol.species% property.
The abstract operation PerformPromiseThen takes arguments promise (a Promise), onFulfilled (an ECMAScript language value), and onRejected (an ECMAScript language value) and optional argument resultCapability (a PromiseCapability Record) and returns an ECMAScript language value. It performs the “then” operation on promise using onFulfilled and onRejected as its settlement actions. If resultCapability is passed, the result is stored by updating resultCapability's promise. If it is not passed, then PerformPromiseThen is being called by a specification-internal operation where the result does not matter. It performs the following steps when called:
creates and initializes a new GeneratorFunction when called as a function rather than as a constructor. Thus the function call GeneratorFunction (…) is equivalent to the object creation expression new GeneratorFunction (…) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified GeneratorFunction behaviour must include a super call to the GeneratorFunction constructor to create and initialize subclass instances with the internal slots necessary for built-in GeneratorFunction behaviour. All ECMAScript syntactic forms for defining generator function objects create direct instances of GeneratorFunction. There is no syntactic means to create instances of GeneratorFunction subclasses.
The initial value of the %Symbol.toStringTag% property is the String value "GeneratorFunction".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
27.3.4 GeneratorFunction Instances
Every GeneratorFunction instance is an ECMAScript function object and has the internal slots listed in Table 30. The value of the [[IsClassConstructor]] internal slot for all such instances is false.
Each GeneratorFunction instance has the following own properties:
27.3.4.1 length
The specification for the "length" property of Function instances given in 20.2.4.1 also applies to GeneratorFunction instances.
27.3.4.2 name
The specification for the "name" property of Function instances given in 20.2.4.2 also applies to GeneratorFunction instances.
27.3.4.3 prototype
Whenever a GeneratorFunction instance is created another ordinary object is also created and is the initial value of the generator function's "prototype" property. The value of the prototype property is used to initialize the [[Prototype]] internal slot of a newly created Generator when the generator function object is invoked using [[Call]].
This property has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.
Note
Unlike Function instances, the object that is the value of a GeneratorFunction's "prototype" property does not have a "constructor" property whose value is the GeneratorFunction instance.
creates and initializes a new AsyncGeneratorFunction when called as a function rather than as a constructor. Thus the function call AsyncGeneratorFunction (...) is equivalent to the object creation expression new AsyncGeneratorFunction (...) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified AsyncGeneratorFunction behaviour must include a super call to the AsyncGeneratorFunction constructor to create and initialize subclass instances with the internal slots necessary for built-in AsyncGeneratorFunction behaviour. All ECMAScript syntactic forms for defining async generator function objects create direct instances of AsyncGeneratorFunction. There is no syntactic means to create instances of AsyncGeneratorFunction subclasses.
The initial value of the %Symbol.toStringTag% property is the String value "AsyncGeneratorFunction".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
27.4.4 AsyncGeneratorFunction Instances
Every AsyncGeneratorFunction instance is an ECMAScript function object and has the internal slots listed in Table 30. The value of the [[IsClassConstructor]] internal slot for all such instances is false.
Each AsyncGeneratorFunction instance has the following own properties:
27.4.4.1 length
The value of the "length" property is an integral Number that indicates the typical number of arguments expected by the AsyncGeneratorFunction. However, the language permits the function to be invoked with some other number of arguments. The behaviour of an AsyncGeneratorFunction when invoked on a number of arguments other than the number specified by its "length" property depends on the function.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
27.4.4.2 name
The specification for the "name" property of Function instances given in 20.2.4.2 also applies to AsyncGeneratorFunction instances.
27.4.4.3 prototype
Whenever an AsyncGeneratorFunction instance is created, another ordinary object is also created and is the initial value of the async generator function's "prototype" property. The value of the prototype property is used to initialize the [[Prototype]] internal slot of a newly created AsyncGenerator when the generator function object is invoked using [[Call]].
This property has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.
Note
Unlike function instances, the object that is the value of an AsyncGeneratorFunction's "prototype" property does not have a "constructor" property whose value is the AsyncGeneratorFunction instance.
27.5 Generator Objects
A Generator is created by calling a generator function and conforms to both the Iterator and Iterable interfaces.
Generator instances directly inherit properties from the initial value of the "prototype" property of the generator function that created the instance. Generator instances indirectly inherit properties from %GeneratorPrototype%.
The abstract operation GeneratorStart takes arguments generator (a Generator) and generatorBody (a FunctionBodyParse Node or an Abstract Closure with no parameters) and returns unused. It performs the following steps when called:
Assert: The value of generator.[[GeneratorState]] is undefined.
NOTE: Once a generator enters the completed state it never leaves it and its associated execution context is never resumed. Any execution state associated with acGenerator can be discarded at this point.
The abstract operation GeneratorValidate takes arguments generator (an ECMAScript language value) and generatorBrand (a String or empty) and returns either a normal completion containing one of suspended-start, suspended-yield, or completed, or a throw completion. It performs the following steps when called:
Resume the suspended evaluation of genContext using NormalCompletion(value) as the result of the operation that suspended it. Let result be the value returned by the resumed computation.
NOTE: Once a generator enters the completed state it never leaves it and its associated execution context is never resumed. Any execution state associated with generator can be discarded at this point.
Resume the suspended evaluation of genContext using abruptCompletion as the result of the operation that suspended it. Let result be the Completion Record returned by the resumed computation.
Resume callerContext passing NormalCompletion(iteratorResult). If genContext is ever resumed again, let resumptionValue be the Completion Record with which it is resumed.
The abstract operation CreateIteratorFromClosure takes arguments closure (an Abstract Closure with no parameters), generatorBrand (a String or empty), and generatorPrototype (an Object) and returns a Generator. It performs the following steps when called:
NOTE: closure can contain uses of the Yield operation to yield an IteratorResult object.
Let internalSlotsList be « [[GeneratorState]], [[GeneratorContext]], [[GeneratorBrand]] ».
An AsyncGenerator is created by calling an async generator function and conforms to both the AsyncIterator and AsyncIterable interfaces.
AsyncGenerator instances directly inherit properties from the initial value of the "prototype" property of the async generator function that created the instance. AsyncGenerator instances indirectly inherit properties from %AsyncGeneratorPrototype%.
Records which represent requests to resume the async generator. Except during state transitions, it is non-empty if and only if [[AsyncGeneratorState]] is either executing or draining-queue.
[[GeneratorBrand]]
a String or empty
A brand used to distinguish different kinds of async generators. The [[GeneratorBrand]] of async generators declared by ECMAScript source text is always empty.
27.6.3 AsyncGenerator Abstract Operations
27.6.3.1 AsyncGeneratorRequest Records
An AsyncGeneratorRequest is a Record value used to store information about how an async generator should be resumed and contains capabilities for fulfilling or rejecting the corresponding promise.
The abstract operation AsyncGeneratorStart takes arguments generator (an AsyncGenerator) and generatorBody (a FunctionBodyParse Node or an Abstract Closure with no parameters) and returns unused. It performs the following steps when called:
Assert: generator.[[AsyncGeneratorState]] is undefined.
The abstract operation AsyncGeneratorEnqueue takes arguments generator (an AsyncGenerator), completion (a Completion Record), and promiseCapability (a PromiseCapability Record) and returns unused. It performs the following steps when called:
Let request be AsyncGeneratorRequest { [[Completion]]: completion, [[Capability]]: promiseCapability }.
Append request to generator.[[AsyncGeneratorQueue]].
The abstract operation AsyncGeneratorCompleteStep takes arguments generator (an AsyncGenerator), completion (a Completion Record), and done (a Boolean) and optional argument realm (a Realm Record) and returns unused. It performs the following steps when called:
Assert: generator.[[AsyncGeneratorQueue]] is not empty.
Let next be the first element of generator.[[AsyncGeneratorQueue]].
Remove the first element from generator.[[AsyncGeneratorQueue]].
The abstract operation AsyncGeneratorResume takes arguments generator (an AsyncGenerator) and completion (a Completion Record) and returns unused. It performs the following steps when called:
Assert: generator.[[AsyncGeneratorState]] is either suspended-start or suspended-yield.
Let genContext be generator.[[AsyncGeneratorContext]].
Resume the suspended evaluation of genContext using completion as the result of the operation that suspended it. Let result be the Completion Record returned by the resumed computation.
The abstract operation AsyncGeneratorAwaitReturn takes argument generator (an AsyncGenerator) and returns unused. It performs the following steps when called:
Assert: generator.[[AsyncGeneratorState]] is draining-queue.
The abstract operation AsyncGeneratorDrainQueue takes argument generator (an AsyncGenerator) and returns unused. It drains the generator's AsyncGeneratorQueue until it encounters an AsyncGeneratorRequest which holds a return completion. It performs the following steps when called:
Assert: generator.[[AsyncGeneratorState]] is draining-queue.
Let queue be generator.[[AsyncGeneratorQueue]].
If queue is empty, then
Set generator.[[AsyncGeneratorState]] to completed.
Return unused.
Let done be false.
Repeat, while done is false,
Let next be the first element of queue.
Let completion be Completion(next.[[Completion]]).
The abstract operation CreateAsyncIteratorFromClosure takes arguments closure (an Abstract Closure with no parameters), generatorBrand (a String or empty), and generatorPrototype (an Object) and returns an AsyncGenerator. It performs the following steps when called:
NOTE: closure can contain uses of the Await operation and uses of the Yield operation to yield an IteratorResult object.
Let internalSlotsList be « [[AsyncGeneratorState]], [[AsyncGeneratorContext]], [[AsyncGeneratorQueue]], [[GeneratorBrand]] ».
creates and initializes a new AsyncFunction when called as a function rather than as a constructor. Thus the function call AsyncFunction(…) is equivalent to the object creation expression new AsyncFunction(…) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified AsyncFunction behaviour must include a super call to the AsyncFunction constructor to create and initialize a subclass instance with the internal slots necessary for built-in async function behaviour. All ECMAScript syntactic forms for defining async function objects create direct instances of AsyncFunction. There is no syntactic means to create instances of AsyncFunction subclasses.
The initial value of the %Symbol.toStringTag% property is the String value "AsyncFunction".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
27.7.4 AsyncFunction Instances
Every AsyncFunction instance is an ECMAScript function object and has the internal slots listed in Table 30. The value of the [[IsClassConstructor]] internal slot for all such instances is false. AsyncFunction instances are not constructors and do not have a [[Construct]] internal method. AsyncFunction instances do not have a prototype property as they are not constructable.
Each AsyncFunction instance has the following own properties:
27.7.4.1 length
The specification for the "length" property of Function instances given in 20.2.4.1 also applies to AsyncFunction instances.
27.7.4.2 name
The specification for the "name" property of Function instances given in 20.2.4.2 also applies to AsyncFunction instances.
The abstract operation AsyncBlockStart takes arguments promiseCapability (a PromiseCapability Record), asyncBody (a Parse Node), and asyncContext (an execution context) and returns unused. It performs the following steps when called:
Set the code evaluation state of asyncContext such that when evaluation is resumed for that execution context, closure will be called with no arguments.
Assert: result is a normal completion with a value of unused. The possible sources of this value are Await or, if the async function doesn't await anything, step 2.h above.
A Module Namespace Object is a module namespace exotic object that provides runtime property-based access to a module's exported bindings. There is no constructor function for Module Namespace Objects. Instead, such an object is created for each module that is imported by an ImportDeclaration that contains a NameSpaceImport.
In addition to the properties specified in 10.4.6 each Module Namespace Object has the following own property:
28.3.1 %Symbol.toStringTag%
The initial value of the %Symbol.toStringTag% property is the String value "Module".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
29 Memory Model
The memory consistency model, or memory model, specifies the possible orderings of Shared Data Block events, arising via accessing TypedArray instances backed by a SharedArrayBuffer and via methods on the Atomics object. When the program has no data races (defined below), the ordering of events appears as sequentially consistent, i.e., as an interleaving of actions from each agent. When the program has data races, shared memory operations may appear sequentially inconsistent. For example, programs may exhibit causality-violating behaviour and other astonishments. These astonishments arise from compiler transforms and the design of CPUs (e.g., out-of-order execution and speculation). The memory model defines both the precise conditions under which a program exhibits sequentially consistent behaviour as well as the possible values read from data races. To wit, there is no undefined behaviour.
The memory model is defined as relational constraints on events introduced by abstract operations on SharedArrayBuffer or by methods on the Atomics object during an evaluation.
Note
This section provides an axiomatic model on events introduced by the abstract operations on SharedArrayBuffers. It bears stressing that the model is not expressible algorithmically, unlike the rest of this specification. The nondeterministic introduction of events by abstract operations is the interface between the operational semantics of ECMAScript evaluation and the axiomatic semantics of the memory model. The semantics of these events is defined by considering graphs of all events in an evaluation. These are neither Static Semantics nor Runtime Semantics. There is no demonstrated algorithmic implementation, but instead a set of constraints that determine if a particular event graph is allowed or disallowed.
29.1 Memory Model Fundamentals
Shared memory accesses (reads and writes) are divided into two groups, atomic accesses and data accesses, defined below. Atomic accesses are sequentially consistent, i.e., there is a strict total ordering of events agreed upon by all agents in an agent cluster. Non-atomic accesses do not have a strict total ordering agreed upon by all agents, i.e., unordered.
Note 1
No orderings weaker than sequentially consistent and stronger than unordered, such as release-acquire, are supported.
A Shared Data Block event is either a ReadSharedMemory, WriteSharedMemory, or ReadModifyWriteSharedMemory Record.
These events are introduced by abstract operations or by methods on the Atomics object.
Some operations may also introduce Synchronize events. A Synchronize event has no fields, and exists purely to directly constrain the permitted orderings of other events.
In addition to Shared Data Block and Synchronize events, there are host-specific events.
Let the range of a ReadSharedMemory, WriteSharedMemory, or ReadModifyWriteSharedMemory event be the Set of contiguous integers from its [[ByteIndex]] to [[ByteIndex]] + [[ElementSize]] - 1. Two events' ranges are equal when the events have the same [[Block]], and the ranges are element-wise equal. Two events' ranges are overlapping when the events have the same [[Block]], the ranges are not equal and their intersection is non-empty. Two events' ranges are disjoint when the events do not have the same [[Block]] or their ranges are neither equal nor overlapping.
Note 2
Examples of host-specific synchronizing events that should be accounted for are: sending a SharedArrayBuffer from one agent to another (e.g., by postMessage in a browser), starting and stopping agents, and communicating within the agent cluster via channels other than shared memory. For a particular execution execution, those events are provided by the host via the host-synchronizes-withstrict partial order. Additionally, hosts can add host-specific synchronizing events to execution.[[EventList]] so as to participate in the is-agent-order-beforeRelation.
An empty candidate execution is a candidate execution Record whose fields are empty Lists.
29.5 Abstract Operations for the Memory Model
29.5.1 EventSet ( execution )
The abstract operation EventSet takes argument execution (a candidate execution) and returns a Set of events. It performs the following steps when called:
The abstract operation SharedDataBlockEventSet takes argument execution (a candidate execution) and returns a Set of events. It performs the following steps when called:
The abstract operation HostEventSet takes argument execution (a candidate execution) and returns a Set of events. It performs the following steps when called:
Let bytesModified be W.[[ModifyOp]](bytes, W.[[Payload]]).
Let byte be bytesModified[payloadIndex].
Append byte to bytesRead.
Set byteLocation to byteLocation + 1.
Return bytesRead.
Note 1
The read-modify-write modification [[ModifyOp]] is given by the function properties on the Atomics object that introduce ReadModifyWriteSharedMemory events.
For events E and D, E is-agent-order-before D in execution if there is some Agent Events Recordaer in execution.[[EventsRecords]] such that aer.[[EventList]] contains both E and D and E is before D in List order of aer.[[EventList]].
For events R and W, R reads-from W in execution if SharedDataBlockEventSet(execution) contains both R and W, and reads-bytes-from(R) in execution contains W.
For events R and W, W synchronizes-with R in execution if Rreads-fromW in execution, R.[[Order]] is seq-cst, W.[[Order]] is seq-cst, and R and W have equal ranges.
For each element eventsRecord of execution.[[EventsRecords]], the following is true.
For events S and Sw, S synchronizes-with Sw in execution if eventsRecord.[[AgentSynchronizesWith]] contains (S, Sw).
For events E and D, E synchronizes-with D in execution if execution.[[HostSynchronizesWith]] contains (E, D).
Note 1
Owing to convention in memory model literature, in a candidate executionexecution, write events synchronizes-with read events, instead of read events synchronizes-with write events.
In a candidate executionexecution, not all seq-cst events related by reads-from are related by synchronizes-with. Only events that also have equal ranges are related by synchronizes-with.
Assert: The remainder of dividing R.[[ByteIndex]] by R.[[ElementSize]] is 0.
For each event W such that Rreads-fromW in execution and W.[[NoTear]] is true, do
If R and W have equal ranges and there exists an event V such that V and W have equal ranges, V.[[NoTear]] is true, W and V are not the same Shared Data Block event, and Rreads-fromV in execution, then
Return false.
Return true.
Note
An event's [[NoTear]] field is true when that event was introduced via accessing an integerTypedArray, and false when introduced via accessing a floating point TypedArray or DataView.
Intuitively, this requirement says when a memory range is accessed in an aligned fashion via an integerTypedArray, a single write event on that range must "win" when in a data race with other write events with equal ranges. More precisely, this requirement says an aligned read event cannot read a value composed of bytes from multiple, different write events all with equal ranges. It is possible, however, for an aligned read event to read from multiple write events with overlapping ranges.
For events E and D, E is-memory-order-before D in execution if Ehappens-beforeD in execution.
For events R and W such that Rreads-fromW in execution, there is no WriteSharedMemory or ReadModifyWriteSharedMemory event V in SharedDataBlockEventSet(execution) such that V.[[Order]] is seq-cst, W is-memory-order-before V in execution, V is-memory-order-before R in execution, and any of the following conditions are true.
This clause together with the forward progress guarantee on agents ensure the liveness condition that seq-cst writes become visible to seq-cst reads with equal range in finite time.
While is-memory-order-before includes all events in EventSet(execution), those that are not constrained by happens-before or synchronizes-with in execution are allowed to occur anywhere in the order.
29.7.5 Valid Executions
A candidate executionexecution is a valid execution (or simply an execution) if all of the following are true.
For an execution execution and events E and D that are contained in SharedDataBlockEventSet(execution), E and D are in a race if the following algorithm returns true.
For an execution execution and events E and D that are contained in SharedDataBlockEventSet(execution), E and D are in a data race if the following algorithm returns true.
If E.[[Order]] is not seq-cst or D.[[Order]] is not seq-cst, then
Return true.
If E and D have overlapping ranges, then
Return true.
Return false.
29.10 Data Race Freedom
An execution execution is data race free if there are no two events in SharedDataBlockEventSet(execution) that are in a data race.
A program is data race free if all its executions are data race free.
The memory model guarantees sequential consistency of all events for data race free programs.
29.11 Shared Memory Guidelines
Note 1
The following are guidelines for ECMAScript programmers working with shared memory.
We recommend programs be kept data race free, i.e., make it so that it is impossible for there to be concurrent non-atomic operations on the same memory location. Data race free programs have interleaving semantics where each step in the evaluation semantics of each agent are interleaved with each other. For data race free programs, it is not necessary to understand the details of the memory model. The details are unlikely to build intuition that will help one to better write ECMAScript.
More generally, even if a program is not data race free it may have predictable behaviour, so long as atomic operations are not involved in any data races and the operations that race all have the same access size. The simplest way to arrange for atomics not to be involved in races is to ensure that different memory cells are used by atomic and non-atomic operations and that atomic accesses of different sizes are not used to access the same cells at the same time. Effectively, the program should treat shared memory as strongly typed as much as possible. One still cannot depend on the ordering and timing of non-atomic accesses that race, but if memory is treated as strongly typed the racing accesses will not "tear" (bits of their values will not be mixed).
Note 2
The following are guidelines for ECMAScript implementers writing compiler transformations for programs using shared memory.
It is desirable to allow most program transformations that are valid in a single-agent setting in a multi-agent setting, to ensure that the performance of each agent in a multi-agent program is as good as it would be in a single-agent setting. Frequently these transformations are hard to judge. We outline some rules about program transformations that are intended to be taken as normative (in that they are implied by the memory model or stronger than what the memory model implies) but which are likely not exhaustive. These rules are intended to apply to program transformations that precede the introductions of the events that make up the is-agent-order-beforeRelation.
Let possible read values of a read event be the set of all values of ValueOfReadEvent for that event across all valid executions.
Any transformation of an agent-order slice that is valid in the absence of shared memory is valid in the presence of shared memory, with the following exceptions.
Atomics are carved in stone: Program transformations must not cause the seq-cst events in an agent-order slice to be reordered with its unordered operations, nor its seq-cst operations to be reordered with each other, nor may a program transformation remove a seq-cst operation from the is-agent-order-beforeRelation.
(In practice, the prohibition on reorderings forces a compiler to assume that every seq-cst operation is a synchronization and included in the final is-memory-order-beforeRelation, which it would usually have to assume anyway in the absence of inter-agent program analysis. It also forces the compiler to assume that every call where the callee's effects on the memory-order are unknown may contain seq-cst operations.)
Reads must be stable: Any given shared memory read must only observe a single value in an execution.
(For example, if what is semantically a single read in the program is executed multiple times then the program is subsequently allowed to observe only one of the values read. A transformation known as rematerialization can violate this rule.)
Writes must be stable: All observable writes to shared memory must follow from program semantics in an execution.
(For example, a transformation may not introduce certain observable writes, such as by using read-modify-write operations on a larger location to write a smaller datum, writing a value to memory that the program could not have written, or writing a just-read value back to the location it was read from, if that location could have been overwritten by another agent after the read.)
Possible read values must be non-empty: Program transformations cannot cause the possible read values of a shared memory read to become empty.
(Counterintuitively, this rule in effect restricts transformations on writes, because writes have force in memory model insofar as to be read by read events. For example, writes may be moved and coalesced and sometimes reordered between two seq-cst operations, but the transformation may not remove every write that updates a location; some write must be preserved.)
Examples of transformations that remain valid are: merging multiple non-atomic reads from the same location, reordering non-atomic reads, introducing speculative non-atomic reads, merging multiple non-atomic writes to the same location, reordering non-atomic writes to different locations, and hoisting non-atomic reads out of loops even if that affects termination. Note in general that aliased TypedArrays make it hard to prove that locations are different.
Note 3
The following are guidelines for ECMAScript implementers generating machine code for shared memory accesses.
For architectures with memory models no weaker than those of ARM or Power, non-atomic stores and loads may be compiled to bare stores and loads on the target architecture. Atomic stores and loads may be compiled down to instructions that guarantee sequential consistency. If no such instructions exist, memory barriers are to be employed, such as placing barriers on both sides of a bare store or load. Read-modify-write operations may be compiled to read-modify-write instructions on the target architecture, such as LOCK-prefixed instructions on x86, load-exclusive/store-exclusive instructions on ARM, and load-link/store-conditional instructions on Power.
Specifically, the memory model is intended to allow code generation as follows.
Every atomic operation in the program is assumed to be necessary.
Atomic operations are never rearranged with each other or with non-atomic operations.
Functions are always assumed to perform atomic operations.
Atomic operations are never implemented as read-modify-write operations on larger data, but as non-lock-free atomics if the platform does not have atomic operations of the appropriate size. (We already assume that every platform has normal memory access operations of every interesting size.)
Naive code generation uses these patterns:
Regular loads and stores compile to single load and store instructions.
Lock-free atomic loads and stores compile to a full (sequentially consistent) fence, a regular load or store, and a full fence.
Lock-free atomic read-modify-write accesses compile to a full fence, an atomic read-modify-write instruction sequence, and a full fence.
Non-lock-free atomics compile to a spinlock acquire, a full fence, a series of non-atomic load and store instructions, a full fence, and a spinlock release.
That mapping is correct so long as an atomic operation on an address range does not race with a non-atomic write or with an atomic operation of different size. However, that is all we need: the memory model effectively demotes the atomic operations involved in a race to non-atomic status. On the other hand, the naive mapping is quite strong: it allows atomic operations to be used as sequentially consistent fences, which the memory model does not actually guarantee.
Local improvements to those basic patterns are also allowed, subject to the constraints of the memory model. For example:
There are obvious platform-dependent improvements that remove redundant fences. For example, on x86 the fences around lock-free atomic loads and stores can always be omitted except for the fence following a store, and no fence is needed for lock-free read-modify-write instructions, as these all use LOCK-prefixed instructions. On many platforms there are fences of several strengths, and weaker fences can be used in certain contexts without destroying sequential consistency.
Most modern platforms support lock-free atomics for all the data sizes required by ECMAScript atomics. Should non-lock-free atomics be needed, the fences surrounding the body of the atomic operation can usually be folded into the lock and unlock steps. The simplest solution for non-lock-free atomics is to have a single lock word per SharedArrayBuffer.
There are also more complicated platform-dependent local improvements, requiring some code analysis. For example, two back-to-back fences often have the same effect as a single fence, so if code is generated for two atomic operations in sequence, only a single fence need separate them. On x86, even a single fence separating atomic stores can be omitted, as the fence following a store is only needed to separate the store from a subsequent load.
The ECMAScript language syntax and semantics defined in this annex are required when the ECMAScript host is a web browser. The content of this annex is normative but optional if the ECMAScript host is not a web browser.
Note
This annex describes various legacy features and other characteristics of web browser ECMAScript hosts. All of the language features and behaviours specified in this annex have one or more undesirable characteristics and in the absence of legacy usage would be removed from this specification. However, the usage of these features by large numbers of existing web pages means that web browsers must continue to support them. The specifications in this annex define the requirements for interoperable implementations of these legacy features.
These features are not considered part of the core ECMAScript language. Programmers should not use or assume the existence of these features and behaviours when writing new ECMAScript code. ECMAScript implementations are discouraged from implementing these features unless the implementation is part of a web browser or is required to run the same legacy ECMAScript code that web browsers encounter.
B.1 Additional Syntax
B.1.1 HTML-like Comments
The syntax and semantics of 12.4 is extended as follows except that this extension is not allowed when parsing source text using the goal symbolModule:
The syntax of 22.2.1 is modified and extended as follows. These changes introduce ambiguities that are broken by the ordering of grammar productions and by contextual information. When parsing using the following grammar, each alternative is considered only if previous production alternatives do not match.
This alternative pattern grammar and semantics only changes the syntax and semantics of BMP patterns. The following grammar extensions include productions parameterized with the [UnicodeMode] parameter. However, none of these extensions change the syntax of Unicode patterns recognized when parsing with the [UnicodeMode] parameter present on the goal symbol.
Return the CharSet containing the single character \ U+005C (REVERSE SOLIDUS).
Note
This production can only be reached from the sequence \c within a character class where it is not followed by an acceptable control character.
B.1.2.8.1 CharacterRangeOrUnion ( rer, A, B )
The abstract operation CharacterRangeOrUnion takes arguments rer (a RegExp Record), A (a CharSet), and B (a CharSet) and returns a CharSet. It performs the following steps when called:
The abstract operation ParsePattern takes arguments patternText (a sequence of Unicode code points), u (a Boolean), and v (a Boolean). It performs the following steps when called:
If v is true and u is true, then
Let parseResult be a List containing one or more SyntaxError objects.
Else if v is true, then
Let parseResult be ParseText(patternText, Pattern[+UnicodeMode, +UnicodeSetsMode, +NamedCaptureGroups]).
Else if u is true, then
Let parseResult be ParseText(patternText, Pattern[+UnicodeMode, ~UnicodeSetsMode, +NamedCaptureGroups]).
Else,
Let parseResult be ParseText(patternText, Pattern[~UnicodeMode, ~UnicodeSetsMode, ~NamedCaptureGroups]).
This function is a property of the global object. It computes a new version of a String value in which certain code units have been replaced by a hexadecimal escape sequence.
When replacing a code unit of numeric value less than or equal to 0x00FF, a two-digit escape sequence of the form %xx is used. When replacing a code unit of numeric value strictly greater than 0x00FF, a four-digit escape sequence of the form %uxxxx is used.
The encoding is partly based on the encoding described in RFC 1738, but the entire encoding specified in this standard is described above without regard to the contents of RFC 1738. This encoding does not reflect changes to RFC 1738 made by RFC 3986.
B.2.1.2 unescape ( string )
This function is a property of the global object. It computes a new version of a String value in which each escape sequence of the sort that might be introduced by the escape function is replaced with the code unit that it represents.
B.2.2 Additional Properties of the String.prototype Object
B.2.2.1 String.prototype.substr ( start, length )
This method returns a substring of the result of converting the this value to a String, starting from index start and running for length code units (or through the end of the String if length is undefined). If start is negative, it is treated as sourceLength + start where sourceLength is the length of the String. The result is a String value, not a String object.
Return the substring of S from intStart to intEnd.
Note
This method is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
B.2.2.2 String.prototype.anchor ( name )
This method performs the following steps when called:
Let escapedV be the String value that is the same as V except that each occurrence of the code unit 0x0022 (QUOTATION MARK) in V has been replaced with the six code unit sequence """.
The property "trimStart" is preferred. The "trimLeft" property is provided principally for compatibility with old code. It is recommended that the "trimStart" property be used in new ECMAScript code.
The initial value of the "trimLeft" property is %String.prototype.trimStart%, defined in 22.1.3.34.
B.2.2.16 String.prototype.trimRight ( )
Note
The property "trimEnd" is preferred. The "trimRight" property is provided principally for compatibility with old code. It is recommended that the "trimEnd" property be used in new ECMAScript code.
The initial value of the "trimRight" property is %String.prototype.trimEnd%, defined in 22.1.3.33.
B.2.3 Additional Properties of the Date.prototype Object
B.2.3.1 Date.prototype.getYear ( )
Note
The getFullYear method is preferred for nearly all purposes, because it avoids the “year 2000 problem.”
This method performs the following steps when called:
This method completely reinitializes the this value RegExp with a new pattern and flags. An implementation may interpret use of this method as an assertion that the resulting RegExp object will be used multiple times and hence is a candidate for extra optimization.
B.3 Other Additional Features
B.3.1 Labelled Function Declarations
Prior to ECMAScript 2015, the specification of LabelledStatement did not allow for the association of a statement label with a FunctionDeclaration. However, a labelled FunctionDeclaration was an allowable extension for non-strict code and most browser-hosted ECMAScript implementations supported that extension. In ECMAScript 2015 and later, the grammar production for LabelledStatement permits use of FunctionDeclaration as a LabelledItem but 14.13.1 includes an Early Error rule that produces a Syntax Error if that occurs. That rule is modified with the addition of the highlighted text:
B.3.2 Block-Level Function Declarations Web Legacy Compatibility Semantics
Prior to ECMAScript 2015, the ECMAScript specification did not define the occurrence of a FunctionDeclaration as an element of a Block statement's StatementList. However, support for that form of FunctionDeclaration was an allowable extension and most browser-hosted ECMAScript implementations permitted them. Unfortunately, the semantics of such declarations differ among those implementations. Because of these semantic differences, existing web ECMAScript source text that uses Block level function declarations is only portable among browser implementations if the usage only depends upon the semantic intersection of all of the browser implementations for such declarations. The following are the use cases that fall within that intersection semantics:
A function is declared and only referenced within a single block.
One or more FunctionDeclarations whose BindingIdentifier is the name f occur within the function code of an enclosing function g and that declaration is nested within a Block.
No other declaration of f that is not a var declaration occurs within the function code of g.
A function is declared and possibly used within a single Block but also referenced by an inner function definition that is not contained within that same Block.
One or more FunctionDeclarations whose BindingIdentifier is the name f occur within the function code of an enclosing function g and that declaration is nested within a Block.
No other declaration of f that is not a var declaration occurs within the function code of g.
There is at least one occurrence of f as an IdentifierReference within another function h that is nested within g and no other declaration of f shadows the references to f from within h.
All invocations of h occur after the declaration of f has been evaluated.
A function is declared and possibly used within a single block but also referenced within subsequent blocks.
One or more FunctionDeclaration whose BindingIdentifier is the name f occur within the function code of an enclosing function g and that declaration is nested within a Block.
No other declaration of f that is not a var declaration occurs within the function code of g.
There is at least one occurrence of f as an IdentifierReference within the function code of g that lexically follows the Block containing the declaration of f.
The first use case is interoperable with the semantics of Block level function declarations provided by ECMAScript 2015. Any pre-existing ECMAScript source text that employs that use case will operate using the Block level function declarations semantics defined by clauses 10, 14, and 15.
ECMAScript 2015 interoperability for the second and third use cases requires the following extensions to the clause 10, clause 15, clause 19.2.1 and clause 16.1.7 semantics.
If an ECMAScript implementation has a mechanism for reporting diagnostic warning messages, a warning should be produced when code contains a FunctionDeclaration for which these compatibility semantics are applied and introduce observable differences from non-compatibility semantics. For example, if a var binding is not introduced because its introduction would create an early error, a warning message should not be produced.
B.3.2.1 Changes to FunctionDeclarationInstantiation
NOTE: A var binding for F is only instantiated here if it is neither a VarDeclaredName, the name of a formal parameter, or another FunctionDeclaration.
If instantiatedVarNames does not contain F and F is not "arguments", then
It is a Syntax Error if the LexicallyDeclaredNames of StatementList contains any duplicate entries, unless IsStrict(this production) is false and the duplicate entries are only bound by FunctionDeclarations.
It is a Syntax Error if the LexicallyDeclaredNames of CaseBlock contains any duplicate entries, unless IsStrict(this production) is false and the duplicate entries are only bound by FunctionDeclarations.
The Block of a Catch clause may contain var declarations that bind a name that is also bound by the CatchParameter. At runtime, such bindings are instantiated in the VariableDeclarationEnvironment. They do not shadow the same-named bindings introduced by the CatchParameter and hence the Initializer for such var declarations will assign to the corresponding catch parameter rather than the var binding.
This modified behaviour also applies to var and function declarations introduced by direct eval calls contained within the Block of a Catch clause. This change is accomplished by modifying the algorithm of 19.2.1.3 as follows:
Objects with an [[IsHTMLDDA]] internal slot are never created by this specification. However, the document.all object in web browsers is a host-definedexotic object with this slot that exists for web compatibility purposes. There are no other known examples of this type of object and implementations should not create any with the exception of document.all.
Assignment to an undeclared identifier or otherwise unresolvable reference does not create a property in the global object. When a simple assignment occurs within strict mode code, its LeftHandSideExpression must not evaluate to an unresolvable Reference. If it does a ReferenceError exception is thrown (6.2.5.6). The LeftHandSideExpression also may not be a reference to a data property with the attribute value { [[Writable]]: false }, to an accessor property with the attribute value { [[Set]]: undefined }, nor to a non-existent property of an object whose [[Extensible]] internal slot is false. In these cases a TypeError exception is thrown (13.15).
Arguments objects for strict functions do not dynamically share their array-indexed property values with the corresponding formal parameter bindings of their functions. (10.4.4).
For strict functions, if an arguments object is created the binding of the local identifier arguments to the arguments object is immutable and hence may not be the target of an assignment expression. (10.2.11).
Strict mode eval code cannot instantiate variables or functions in the variable environment of the caller to eval. Instead, a new variable environment is created and that environment is used for declaration binding instantiation for the eval code (19.2.1).
If this is evaluated within strict mode code, then the this value is not coerced to an object. A this value of either undefined or null is not converted to the global object and primitive values are not converted to wrapper objects. The this value passed via a function call (including calls made using Function.prototype.apply and Function.prototype.call) do not coerce the passed this value to an object (10.2.1.2, 20.2.3.1, 20.2.3.3).
When a delete operator occurs within strict mode code, a SyntaxError is thrown if its UnaryExpression is a direct reference to a variable, function argument, or function name (13.5.1.1).
When a delete operator occurs within strict mode code, a TypeError is thrown if the property to be deleted has the attribute { [[Configurable]]: false } or otherwise cannot be deleted (13.5.1.2).
An implementation may not extend, beyond that defined in this specification, the meanings within strict functions of properties named "caller" or "arguments" of function instances.
Preparation steps before, and cleanup steps after, invocation of JobAbstract Closures. See 9.5.
D.5 Internal Methods of Exotic Objects
Any of the essential internal methods in Table 4 for any exotic object not specified within this specification.
D.6 Built-in Objects and Methods
Any built-in objects and methods not defined within this specification, except as restricted in 17.1.
E Corrections and Clarifications in ECMAScript 2015 with Possible Compatibility Impact
9.1.1.4.15-9.1.1.4.18 Edition 5 and 5.1 used a property existence test to determine whether a global object property corresponding to a new global declaration already existed. ECMAScript 2015 uses an own property existence test. This corresponds to what has been most commonly implemented by web browsers.
10.4.2.1: The 5th Edition moved the capture of the current array length prior to the integer conversion of the array index or new length value. However, the captured length value could become invalid if the conversion process has the side-effect of changing the array length. ECMAScript 2015 specifies that the current array length must be captured after the possible occurrence of such side-effects.
21.4.1.31: Previous editions permitted the TimeClip abstract operation to return either +0𝔽 or -0𝔽 as the representation of a 0 time value. ECMAScript 2015 specifies that +0𝔽 always returned. This means that for ECMAScript 2015 the time value of a Date is never observably -0𝔽 and methods that return time values never return -0𝔽.
21.4.1.32: If a UTC offset representation is not present, the local time zone is used. Edition 5.1 incorrectly stated that a missing time zone should be interpreted as "z".
21.4.4.36: If the year cannot be represented using the Date Time String Format specified in 21.4.1.32 a RangeError exception is thrown. Previous editions did not specify the behaviour for that case.
21.4.4.41: Previous editions did not specify the value returned by Date.prototype.toString when the time value is NaN. ECMAScript 2015 specifies the result to be the String value "Invalid Date".
22.2.4.1, 22.2.6.13.1: Any LineTerminator code points in the value of the "source" property of a RegExp instance must be expressed using an escape sequence. Edition 5.1 only required the escaping of /.
22.2.6.8, 22.2.6.11: In previous editions, the specifications for String.prototype.match and String.prototype.replace was incorrect for cases where the pattern argument was a RegExp value whose global flag is set. The previous specifications stated that for each attempt to match the pattern, if lastIndex did not change, it should be incremented by 1. The correct behaviour is that lastIndex should be incremented by 1 only if the pattern matched the empty String.
23.1.3.30: Previous editions did not specify how a NaN value returned by a comparator was interpreted by Array.prototype.sort. ECMAScript 2015 specifies that such as value is treated as if +0𝔽 was returned from the comparator. ECMAScript 2015 also specifies that ToNumber is applied to the result returned by a comparator. In previous editions, the effect of a comparator result that is not a Number value was implementation-defined. In practice, implementations call ToNumber.
F Additions and Changes That Introduce Incompatibilities with Prior Editions
6.2.5: In ECMAScript 2015, Function calls are not allowed to return a Reference Record.
9.3: In ECMAScript 2018, Template objects are canonicalized based on Parse Node (source location), instead of across all occurrences of that template literal or tagged template in a Realm in previous editions.
12.2: In ECMAScript 2016, Unicode 8.0.0 or higher is mandated, as opposed to ECMAScript 2015 which mandated Unicode 5.1. In particular, this caused U+180E MONGOLIAN VOWEL SEPARATOR, which was in the Space_Separator (Zs) category and thus treated as whitespace in ECMAScript 2015, to be moved to the Format (Cf) category (as of Unicode 6.3.0). This causes whitespace-sensitive methods to behave differently. For example, "\u180E".trim().length was 0 in previous editions, but 1 in ECMAScript 2016 and later. Additionally, ECMAScript 2017 mandated always using the latest version of the Unicode Standard.
12.7: In ECMAScript 2015, the valid code points for an IdentifierName are specified in terms of the Unicode properties “ID_Start” and “ID_Continue”. In previous editions, the valid IdentifierName or Identifier code points were specified by enumerating various Unicode code point categories.
12.10.1: In ECMAScript 2015, Automatic Semicolon Insertion adds a semicolon at the end of a do-while statement if the semicolon is missing. This change aligns the specification with the actual behaviour of most existing implementations.
13.2.5.1: In ECMAScript 2015, it is no longer an early error to have duplicate property names in Object Initializers.
13.15.1: In ECMAScript 2015, strict mode code containing an assignment to an immutable binding such as the function name of a FunctionExpression does not produce an early error. Instead it produces a runtime error.
14.2: In ECMAScript 2015, a StatementList beginning with the token let followed by the input elements LineTerminator then Identifier is the start of a LexicalDeclaration. In previous editions, automatic semicolon insertion would always insert a semicolon before the Identifier input element.
14.7: In ECMAScript 2015, if the ( token of a for statement is immediately followed by the token sequence let [ then the let is treated as the start of a LexicalDeclaration. In previous editions such a token sequence would be the start of an Expression.
14.7: In ECMAScript 2015, if the ( token of a for-in statement is immediately followed by the token sequence let [ then the let is treated as the start of a ForDeclaration. In previous editions such a token sequence would be the start of an LeftHandSideExpression.
14.7: Prior to ECMAScript 2015, an initialization expression could appear as part of the VariableDeclaration that precedes the inkeyword. In ECMAScript 2015, the ForBinding in that same position does not allow the occurrence of such an initializer. In ECMAScript 2017, such an initializer is permitted only in non-strict code.
14.15: In ECMAScript 2015, it is an early error for a Catch clause to contain a var declaration for the same Identifier that appears as the Catch clause parameter. In previous editions, such a variable declaration would be instantiated in the enclosing variable environment but the declaration's Initializer value would be assigned to the Catch parameter.
14.15, 19.2.1.3: In ECMAScript 2015, a runtime SyntaxError is thrown if a Catch clause evaluates a non-strict direct eval whose eval code includes a var or FunctionDeclaration declaration that binds the same Identifier that appears as the Catch clause parameter.
15.4.5 In ECMAScript 2015, the function objects that are created as the values of the [[Get]] or [[Set]] attribute of accessor properties in an ObjectLiteral are not constructor functions and they do not have a "prototype" own property. In the previous edition, they were constructors and had a "prototype" property.
20.1.2.6: In ECMAScript 2015, if the argument to Object.freeze is not an object it is treated as if it was a non-extensible ordinary object with no own properties. In the previous edition, a non-object argument always causes a TypeError to be thrown.
20.1.2.8: In ECMAScript 2015, if the argument to Object.getOwnPropertyDescriptor is not an object an attempt is made to coerce the argument using ToObject. If the coercion is successful the result is used in place of the original argument value. In the previous edition, a non-object argument always causes a TypeError to be thrown.
20.1.2.10: In ECMAScript 2015, if the argument to Object.getOwnPropertyNames is not an object an attempt is made to coerce the argument using ToObject. If the coercion is successful the result is used in place of the original argument value. In the previous edition, a non-object argument always causes a TypeError to be thrown.
20.1.2.12: In ECMAScript 2015, if the argument to Object.getPrototypeOf is not an object an attempt is made to coerce the argument using ToObject. If the coercion is successful the result is used in place of the original argument value. In the previous edition, a non-object argument always causes a TypeError to be thrown.
20.1.2.16: In ECMAScript 2015, if the argument to Object.isExtensible is not an object it is treated as if it was a non-extensible ordinary object with no own properties. In the previous edition, a non-object argument always causes a TypeError to be thrown.
20.1.2.17: In ECMAScript 2015, if the argument to Object.isFrozen is not an object it is treated as if it was a non-extensible ordinary object with no own properties. In the previous edition, a non-object argument always causes a TypeError to be thrown.
20.1.2.18: In ECMAScript 2015, if the argument to Object.isSealed is not an object it is treated as if it was a non-extensible ordinary object with no own properties. In the previous edition, a non-object argument always causes a TypeError to be thrown.
20.1.2.19: In ECMAScript 2015, if the argument to Object.keys is not an object an attempt is made to coerce the argument using ToObject. If the coercion is successful the result is used in place of the original argument value. In the previous edition, a non-object argument always causes a TypeError to be thrown.
20.1.2.20: In ECMAScript 2015, if the argument to Object.preventExtensions is not an object it is treated as if it was a non-extensible ordinary object with no own properties. In the previous edition, a non-object argument always causes a TypeError to be thrown.
20.1.2.22: In ECMAScript 2015, if the argument to Object.seal is not an object it is treated as if it was a non-extensible ordinary object with no own properties. In the previous edition, a non-object argument always causes a TypeError to be thrown.
20.2.3.2: In ECMAScript 2015, the [[Prototype]] internal slot of a bound function is set to the [[GetPrototypeOf]] value of its target function. In the previous edition, [[Prototype]] was always set to %Function.prototype%.
20.2.4.1: In ECMAScript 2015, the "length" property of function instances is configurable. In previous editions it was non-configurable.
21.4.4 In ECMAScript 2015, the Date prototype object is not a Date instance. In previous editions it was a Date instance whose TimeValue was NaN.
22.1.3.12 In ECMAScript 2015, the String.prototype.localeCompare function must treat Strings that are canonically equivalent according to the Unicode Standard as being identical. In previous editions implementations were permitted to ignore canonical equivalence and could instead use a bit-wise comparison.
22.1.3.28 and 22.1.3.30 In ECMAScript 2015, lowercase/upper conversion processing operates on code points. In previous editions such the conversion processing was only applied to individual code units. The only affected code points are those in the Deseret block of Unicode.
22.1.3.32 In ECMAScript 2015, the String.prototype.trim method is defined to recognize white space code points that may exist outside of the Unicode BMP. However, as of Unicode 7 no such code points are defined. In previous editions such code points would not have been recognized as white space.
22.2.4.1 In ECMAScript 2015, If the pattern argument is a RegExp instance and the flags argument is not undefined, a new RegExp instance is created just like pattern except that pattern's flags are replaced by the argument flags. In previous editions a TypeError exception was thrown when pattern was a RegExp instance and flags was not undefined.
22.2.6 In ECMAScript 2015, the RegExp prototype object is not a RegExp instance. In previous editions it was a RegExp instance whose pattern is the empty String.
25.4.15: In ECMAScript 2019, Atomics.wake has been renamed to Atomics.notify to prevent confusion with Atomics.wait.
27.1.4.4, 27.6.3.6: In ECMAScript 2019, the number of Jobs enqueued by await was reduced, which could create an observable difference in resolution order between a then() call and an await expression.
G Colophon
This specification is authored on GitHub in a plaintext source format called Ecmarkup. Ecmarkup is an HTML and Markdown dialect that provides a framework and toolset for authoring ECMAScript specifications in plaintext and processing the specification into a full-featured HTML rendering that follows the editorial conventions for this document. Ecmarkup builds on and integrates a number of other formats and technologies including Grammarkdown for defining syntax and Ecmarkdown for authoring algorithm steps. PDF renderings of this specification are produced by printing the HTML rendering to a PDF.
Prior editions of this specification were authored using Word—the Ecmarkup source text that formed the basis of this edition was produced by converting the ECMAScript 2015 Word document to Ecmarkup using an automated conversion tool.
H Bibliography
IEEE 754-2019: IEEE Standard for Floating-Point Arithmetic. Institute of Electrical and Electronic Engineers, New York (2019)
Note
There are no normative changes between IEEE 754-2008 and IEEE 754-2019 that affect the ECMA-262 specification.
This draft document may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published, and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this section are included on all such copies and derivative works. However, this document itself may not be modified in any way, including by removing the copyright notice or references to Ecma International, except as needed for the purpose of developing any document or deliverable produced by Ecma International.
This disclaimer is valid only prior to final version of this document. After approval all rights on the standard are reserved by Ecma International.
The limited permissions are granted through the standardization phase and will not be revoked by Ecma International or its successors or assigns during this time.
This document and the information contained herein is provided on an "AS IS" basis and ECMA INTERNATIONAL DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY OWNERSHIP RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Software License
All Software contained in this document ("Software") is protected by copyright and is being made available under the "BSD License", included below. This Software may be subject to third party rights (rights from parties other than Ecma International), including patent rights, and no licenses under such third party rights are granted under this license even if the third party concerned is a member of Ecma International. SEE THE ECMA CODE OF CONDUCT IN PATENT MATTERS AVAILABLE AT https://ecma-international.org/memento/codeofconduct.htm FOR INFORMATION REGARDING THE LICENSING OF PATENT CLAIMS THAT ARE REQUIRED TO IMPLEMENT ECMA INTERNATIONAL STANDARDS.
Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:
Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
Neither the name of the authors nor Ecma International may be used to endorse or promote products derived from this software without specific prior written permission.
THIS SOFTWARE IS PROVIDED BY THE ECMA INTERNATIONAL "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL ECMA INTERNATIONAL BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.