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ECMA-262 (12th Edition) ECMAScript 2021 Language Specification - page 8

Batak

Batk

Bengali

Beng

Bhaiksuki

Bhks

Bopomofo

Bopo

Brahmi

Brah

Braille

Brai

Buginese

Bugi

Buhid

Buhd

Canadian_Aboriginal

Cans

Carian

Cari

Caucasian_Albanian

Aghb

Chakma

Cakm

Cham

Chorasmian

Chrs

Cherokee

Cher

Common

Zyyy

Coptic

Copt

Qaac

Cuneiform

Xsux

604

Cypriot

Cprt

Cyrillic

Cyrl

Deseret

Dsrt

Devanagari

Deva

Dives_Akuru

Diak

Dogra

Dogr

Duployan

Dupl

Egyptian_Hieroglyphs

Egyp

Elbasan

Elba

Elymaic

Elym

Ethiopic

Ethi

Georgian

Geor

Glagolitic

Glag

Gothic

Goth

Grantha

Gran

Greek

Grek

Gujarati

Gujr

Gunjala_Gondi

Gong

605

Gurmukhi

Guru

Han

Hani

Hangul

Hang

Hanifi_Rohingya

Rohg

Hanunoo

Hano

Hatran

Hatr

Hebrew

Hebr

Hiragana

Hira

Imperial_Aramaic

Armi

Inherited

Zinh

Qaai

Inscriptional_Pahlavi

Phli

Inscriptional_Parthian

Prti

Javanese

Java

Kaithi

Kthi

Kannada

Knda

Katakana

Kana

Kayah_Li

Kali

Kharoshthi

606

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.

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

"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.

4.3.1 Objects

Figure 1: Object/Prototype Relationships

implicit prototype link

explicit prototype property

prototype
P1
P2

CFP1

q1
q2

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:

is a

constructor

(and also an object). Five objects have been created by using

new

expressions:

, and

. Each of these objects contains properties named

"q1"

and

"q2"

. The dashed lines represent the implicit prototype

relationship; so, for example,

's prototype is

. The

constructor

, has two properties itself, named

"P1"

and

"P2"

, which are not visible to

, or

. The property named

"CFP1"

is shared by

, and

(but not by

), as are any properties found in

's implicit prototype chain that are not named

"q1"

"q2"

, or

"CFP1"

. Notice that there is no implicit prototype link between

and

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

, and

by assigning a new value to the

property in

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.

For the purposes of this document, the following terms and definitions apply.

implementation-approximated

facility is defined in whole or in part by an external source but has a recommended,

ideal behaviour in this specification

implementation-defined

facility is defined in whole or in part by an external source to this specification

same as

implementation-defined

NOTE

set of data values as defined in clause

Editorially, see clause

4.2

4.4 Terms and Definitions

4.4.1 implementation-approximated

4.4.2 implementation-defined

4.4.3 host-defined

4.4.4 type

4.4.5 primitive value

member of one of the types Undefined, Null, Boolean, Number, BigInt, Symbol, or String as defined in clause

NOTE

member of the type Object

NOTE

function object

that creates and initializes objects

NOTE

object that provides shared properties for other objects

NOTE

object that has the default behaviour for the essential internal methods that must be supported by all objects

object that does not have the default behaviour for one or more of the essential internal methods

NOTE

object whose semantics are defined by this specification

A primitive value is a datum that is represented directly at the lowest level of the language
implementation.

An object is a collection of properties and has a single prototype object. The prototype may be the
null value.

The value of a

constructor

"prototype"

property is a prototype object that is used to implement

inheritance and shared properties.

When a

constructor

creates an object, that object implicitly references the

constructor

"prototype"

property for the purpose of resolving property references. The

constructor

"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.

Any object that is not an

ordinary object

is an

exotic object

4.4.6 object

4.4.7 constructor

4.4.8 prototype

4.4.9 ordinary object

4.4.10 exotic object

4.4.11 standard object

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