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, 6

Edition

Brian Terlson

ECMA-262, Project Editor, 7

through 10

Editions

Jordan Harband

ECMA-262, Project Editor, 10

through 12

Editions

This Standard defines the ECMAScript 2021 general-purpose programming language.

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

12.6.2

of this specification.

A conforming implementation of ECMAScript must not implement any extension that is listed as a Forbidden
Extension in subclause

17.1

1 Scope

2 Conformance

A conforming implementation of ECMAScript must not redefine any facilities that are not

implementation-defined

implementation-approximated

, or

host-defined

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.

NORMATIVE OPTIONAL

Example clause contents.

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.

ISO/IEC 10646

Information Technology — Universal Multiple-Octet Coded Character Set (UCS) plus Amendment 1:2005,

Amendment 2:2006, Amendment 3:2008, and Amendment 4:2008

, plus additional amendments and corrigenda, or

successor

ECMA-402,

ECMAScript 2015 Internationalization API Specification

https://ecma-international.org/publications/standards/Ecma-402.htm

ECMA-404,

The JSON Data Interchange Format

https://ecma-international.org/publications/standards/Ecma-404.htm

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

2.1 Example Clause Heading

3 Normative References

4 Overview

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.

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.

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.

implementation

is an external source that further defines facilities enumerated in Annex

or those that are marked

implementation-defined

implementation-approximated

. In informal use, an implementation refers to a concrete

artefact, such as a particular web browser.

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.

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

4.1 Web Scripting

4.2 Hosts and Implementations

Math.exp

, are

implementation-approximated

host

is an external source that further defines facilities listed in Annex

but does not further define other

implementation-defined

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

. 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.

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

host-defined

facility is one that defers its definition to an external source without further qualification and is listed in

Annex

. Implementations that are not hosts may also provide definitions for

host-defined

facilities.

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.

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

4.3 ECMAScript Overview

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