Annotated ECMAScript 5.1

‟Ex igne vita”

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This is not the normative ECMAScript Language specification. The normative spec (ECMA 262) is a PDF file maintained by ECMA TC39 and is available from http://www.ecmascript.org/. An auto-generated HTML version is available, too: http://ecma-international.org/ecma-262/5.1/

This is an annotated, hyperlinked, HTML version of Edition 5.1 of the ECMAScript Specification, the source for which is maintained at https://github.com/es5/es5.github.io. No copyright is asserted on its front matter (everything up through the Table of Contents), but any reuse of its body text (everything following the Table of Contents) must include the normative spec’s copyright notice and license statement.

To view annotations, follow the Ⓐ, Ⓑ, Ⓓ, Ⓡ, Ⓖ, Ⓔ, and ① hyperlinks in the headings. A key to the markers explains the different types of annotations. Also included are a variety of hyperlinked cross-references, following the example of Jason Orendorff’s version at http://people.mozilla.org/~jorendorff/es5.html

© Ecma International 2010

Introduction # Ⓣ Ⓔ ① Ⓐ

This Ecma Standard 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 this Standard 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 forthcoming internationalisation facilities and 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.

Since publication of the third edition, ECMAScript has 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. Although that work was not completed and not published [1] as the fourth edition of ECMAScript, it informs continuing evolution of the language. The present fifth edition of ECMAScript (published as ECMA-262 5^th edition) codifies de facto interpretations of the language specification that have become common among browser implementations and adds support for new features that have 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.

ECMAScript is a vibrant language and the evolution of the language is not complete. Significant technical enhancement will continue with future editions of this specification.

1 Scope # Ⓣ Ⓔ ① Ⓐ

This Standard defines the ECMAScript scripting language.

2 Conformance # Ⓣ Ⓔ ① Ⓐ

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 this International standard shall interpret characters in conformance with the Unicode Standard, Version 3.0 or later and ISO/IEC 10646-1 with either UCS-2 or UTF-16 as the adopted encoding form, implementation level 3. If the adopted ISO/IEC 10646-1 subset is not otherwise specified, it is presumed to be the BMP subset, collection 300. If the adopted encoding form is not otherwise specified, it is presumed to be the UTF-16 encoding form.

A conforming implementation of ECMAScript is permitted to provide additional types, values, objects, properties, and functions beyond those described in this specification. In particular, a conforming implementation of ECMAScript is permitted to 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 is permitted to support program and regular expression syntax not described in this specification. In particular, a conforming implementation of ECMAScript is permitted to support program syntax that makes use of the “future reserved words” listed in 7.6.1.2 of this specification.

3 Normative references # Ⓣ Ⓔ ① Ⓐ

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 9899:1996, Programming Languages – C, including amendment 1 and technical corrigenda 1 and 2

ISO/IEC 10646-1:1993, Information Technology – Universal Multiple-Octet Coded Character Set (UCS) plus its amendments and corrigenda

4 Overview # Ⓣ Ⓔ ① Ⓐ

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 host 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.

A scripting language is a programming language that is used to manipulate, customise, 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 can provide core scripting capabilities for a variety of host environments, and therefore the core scripting language is specified in this document apart from any particular host environment.

Some of the facilities of ECMAScript are similar to those used in other programming languages; in particular Java™, Self, and Scheme as described in:

Gosling, James, Bill Joy and Guy Steele. The Java^™ Language Specification. Addison Wesley Publishing Co., 1996.

Ungar, David, and Smith, Randall B. Self: The Power of Simplicity. OOPSLA '87 Conference Proceedings, pp. 227–241, Orlando, FL, October 1987.

IEEE Standard for the Scheme Programming Language. IEEE Std 1178-1990.

4.1 Web Scripting # Ⓣ Ⓔ ① Ⓐ

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 customised 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 Language Overview # Ⓣ Ⓔ ① Ⓐ

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. An ECMAScript object is a collection of properties each with zero or more 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 change the value of 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, and String; an object is a member of the remaining built-in type Object; and a function is a callable object. A function that is associated with an object via a property is 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, the Object object, the Function object, the Array object, the String object, the Boolean object, the Number object, the Math object, the Date object, the RegExp object, the JSON object, and the Error objects Error, EvalError, RangeError, ReferenceError, SyntaxError, TypeError and URIError.

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.

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.

4.2.1 Objects # Ⓣ Ⓔ ① Ⓐ

ECMAScript does not use classes 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 initialises 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.

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, and 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 CF_p. The constructor, CF, has two properties itself, named P1 and P2, which are not visible to CF_p, cf₁, cf₂, cf₃, cf₄, or cf₅. The property named CFP1 in CF_p is shared by cf₁, cf₂, cf₃, cf₄, and cf₅ (but not by CF), as are any properties found in CF_p’s implicit prototype chain that are not named q1, q2, or CFP1. Notice that there is no implicit prototype link between CF and CF_p.

Unlike 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 CF_p.

4.2.2 The Strict Variant of ECMAScript # Ⓣ Ⓔ ① Ⓐ

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 code units. Because strict mode is selected at the level of a syntactic code unit, strict mode only imposes restrictions that have local effect within such a code unit. Strict mode does not restrict or modify any aspect of the ECMAScript semantics that must operate consistently across multiple code units. A complete ECMAScript program may be composed for both strict mode and non-strict mode ECMAScript code units. In this case, strict mode only applies when actually executing code that is defined within a strict mode code unit.

In order to conform to this specification, an ECMAScript implementation must implement both the full unrestricted ECMAScript language and the strict mode variant of the ECMAScript language as defined by this specification. In addition, an implementation must support the combination of unrestricted and strict mode code units into a single composite program.

4.3 Definitions # Ⓣ Ⓔ ① Ⓐ

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

4.3.1
type # Ⓣ Ⓔ ① Ⓐ

set of data values as defined in Clause 8 of this specification.

4.3.2
primitive value # Ⓣ Ⓔ ① Ⓐ

member of one of the types Undefined, Null, Boolean, Number, or String as defined in Clause 8.

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

4.3.3
object # Ⓣ Ⓔ ① Ⓐ

member of the type Object.

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

4.3.4
constructor # Ⓣ Ⓔ ① Ⓐ

Function object that creates and initialises objects.

NOTE The value of a constructor’s “prototype” property is a prototype object that is used to implement inheritance and shared properties.

4.3.5
prototype # Ⓣ Ⓔ ① Ⓐ

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.

4.3.6
native object # Ⓣ Ⓔ ① Ⓐ

object in an ECMAScript implementation whose semantics are fully defined by this specification rather than by the host environment.

NOTE Standard native objects are defined in this specification. Some native objects are built-in; others may be constructed during the course of execution of an ECMAScript program.

4.3.7
built-in object # Ⓣ Ⓔ ① Ⓐ

object supplied by an ECMAScript implementation, independent of the host environment, that is present at the start of the execution of an ECMAScript program.

NOTE Standard built-in objects are defined in this specification, and an ECMAScript implementation may specify and define others. Every built-in object is a native object. A built-in constructor is a built-in object that is also a constructor.

4.3.8
host object # Ⓣ Ⓔ ① Ⓐ

object supplied by the host environment to complete the execution environment of ECMAScript.

NOTE Any object that is not native is a host object.

4.3.9
undefined value # Ⓣ Ⓔ ① Ⓐ

primitive value used when a variable has not been assigned a value.

4.3.10
Undefined type # Ⓣ Ⓔ ① Ⓐ

type whose sole value is the undefined value.

4.3.11
null value # Ⓣ Ⓔ ① Ⓐ

primitive value that represents the intentional absence of any object value.

4.3.12
Null type # Ⓣ Ⓔ ① Ⓐ

type whose sole value is the null value.

4.3.13
Boolean value # Ⓣ Ⓔ ① Ⓐ

member of the Boolean type.

NOTE There are only two Boolean values, true and false.

4.3.14
Boolean type # Ⓣ Ⓔ ① Ⓐ

type consisting of the primitive values true and false.

4.3.15
Boolean object # Ⓣ Ⓔ ① Ⓐ

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 property whose value is the Boolean value. A Boolean object can be coerced to a Boolean value.

4.3.16
String value # Ⓣ Ⓔ ① Ⓐ

primitive value that is a finite ordered sequence of zero or more 16-bit unsigned integer.

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.

4.3.17
String type # Ⓣ Ⓔ ① Ⓐ

set of all possible String values.

4.3.18
String object # Ⓣ Ⓔ ① Ⓐ

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 property whose value is the String value. A String object can be coerced to a String value by calling the String constructor as a function (15.5.1).

4.3.19
Number value # Ⓣ Ⓔ ① Ⓐ

primitive value corresponding to a double-precision 64-bit binary format IEEE 754 value.

NOTE A Number value is a member of the Number type and is a direct representation of a number.

4.3.20
Number type # Ⓣ Ⓔ ① Ⓐ

set of all possible Number values including the special “Not-a-Number” (NaN) values, positive infinity, and negative infinity.

4.3.21
Number object # Ⓣ Ⓔ ① Ⓐ

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 property whose value is the Number value. A Number object can be coerced to a Number value by calling the Number constructor as a function (15.7.1).

4.3.22
Infinity # Ⓣ Ⓔ ① Ⓐ

Number value that is the positive infinite Number value.

4.3.23
NaN # Ⓣ Ⓔ ① Ⓐ

Number value that is a IEEE 754 “Not-a-Number” value.

4.3.24
function # Ⓣ Ⓔ ① Ⓐ

member of the Object type that is an instance of the standard built-in Function constructor and that may be invoked as a subroutine.

NOTE In addition to its named 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.

4.3.25
built-in function # Ⓣ Ⓔ ① Ⓐ

built-in object that is a function.

NOTE Examples of built-in functions include parseInt and Math.exp. An implementation may provide implementation-dependent built-in functions that are not described in this specification.

4.3.26
property # Ⓣ Ⓔ ① Ⓐ

association between a name and a value that is a part of an object.

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.

4.3.27
method # Ⓣ Ⓔ ① Ⓐ

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.

4.3.28
built-in method # Ⓣ Ⓔ ① Ⓐ

method that is a built-in function.

NOTE Standard built-in methods are defined in this specification, and an ECMAScript implementation may specify and provide other additional built-in methods.

4.3.29
attribute # Ⓣ Ⓔ ① Ⓐ

internal value that defines some characteristic of a property.

4.3.30
own property # Ⓣ Ⓔ ① Ⓐ

property that is directly contained by its object.

4.3.31
inherited property # Ⓣ Ⓔ ① Ⓐ

property of an object that is not an own property but is a property (either own or inherited) of the object’s prototype.

5 Notational Conventions # Ⓣ Ⓔ ① Ⓐ

5.1 Syntactic and Lexical Grammars # Ⓣ Ⓔ ① Ⓐ

5.1.1 Context-Free Grammars # Ⓣ Ⓔ ① Ⓐ

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.

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.

5.1.2 The Lexical and RegExp Grammars # Ⓣ Ⓔ ① Ⓐ

A lexical grammar for ECMAScript is given in clause 7. This grammar has as its terminal symbols characters (Unicode code units) that conform to the rules for SourceCharacter defined in Clause 6. It defines a set of productions, starting from the goal symbol InputElementDiv or InputElementRegExp, that describe how sequences of such characters are translated into a sequence of input elements.

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 (7.9). 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 15.10. This grammar also has as its terminal symbols the characters as defined by SourceCharacter. It defines a set of productions, starting from the goal symbol Pattern, that describe how sequences of characters 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.

5.1.3 The Numeric String Grammar # Ⓣ Ⓔ ① Ⓐ

Another grammar is used for translating Strings into numeric values. This grammar is similar to the part of the lexical grammar having to do with numeric literals and has as its terminal symbols SourceCharacter. This grammar appears in 9.3.1.

Productions of the numeric string grammar are distinguished by having three colons “:::” as punctuation.

5.1.4 The Syntactic Grammar # Ⓣ Ⓔ ① Ⓐ

The syntactic grammar for ECMAScript is given in clauses 11, 12, 13 and 14. 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 the goal symbol Program, that describe how sequences of tokens can form syntactically correct ECMAScript programs.

When a stream of characters is to be parsed as an ECMAScript program, 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 program is syntactically in error if the tokens in the stream of input elements cannot be parsed as a single instance of the goal nonterminal Program, with no tokens left over.

Productions of the syntactic grammar are distinguished by having just one colon “:” as punctuation.

The syntactic grammar as presented in clauses 11, 12, 13 and 14 is actually not a complete account of which token sequences are accepted as correct ECMAScript programs. 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 terminator character appears in certain “awkward” places.

5.1.5 The JSON Grammar # Ⓣ Ⓔ ① Ⓐ

The JSON grammar is used to translate a String describing a set of ECMAScript objects into actual objects. The JSON grammar is given in 15.12.1.

The JSON grammar consists of the JSON lexical grammar and the JSON syntactic grammar. The JSON lexical grammar is used to translate character sequences into tokens and is similar to parts of the ECMAScript lexical grammar. The JSON syntactic grammar describes how sequences of tokens from the JSON lexical grammar can form syntactically correct JSON object descriptions.

Productions of the JSON lexical grammar are distinguished by having two colons “::” as separating punctuation. The JSON lexical grammar uses some productions from the ECMAScript lexical grammar. The JSON syntactic grammar is similar to parts of the ECMAScript syntactic grammar. Productions of the JSON syntactic grammar are distinguished by using one colon “:” as separating punctuation.

5.1.6 Grammar Notation # Ⓣ Ⓔ ① Ⓐ

Terminal symbols of the lexical and string grammars, and some of the terminal symbols of the syntactic grammar, are shown in fixed width font, both in the productions of the grammars and throughout this specification whenever the text directly refers to such a terminal symbol. These are to appear in a program exactly as written. All terminal symbol characters specified in this way are to be understood as the appropriate Unicode character from the ASCII range, as opposed to any similar-looking characters from other Unicode ranges.

Nonterminal symbols are shown in italic type. The definition of a nonterminal 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:

WhileStatement :

while ( Expression ) Statement

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:

ArgumentList :

AssignmentExpression
ArgumentList , AssignmentExpression

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.

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:

VariableDeclaration :

Identifier Initialiser_opt

is a convenient abbreviation for:

VariableDeclaration :

Identifier
Identifier Initialiser

and that:

IterationStatement :

for ( ExpressionNoIn_opt ; Expression_opt ; Expression_opt ) Statement

is a convenient abbreviation for:

IterationStatement :

for ( ; Expression_opt ; Expression_opt ) Statement
for ( ExpressionNoIn ; Expression_opt ; Expression_opt ) Statement

which in turn is an abbreviation for:

IterationStatement :

for ( ; ; Expression_opt ) Statement
for ( ; Expression ; Expression_opt ) Statement
for ( ExpressionNoIn ; ; Expression_opt ) Statement
for ( ExpressionNoIn ; Expression ; Expression_opt ) Statement

which in turn is an abbreviation for:

IterationStatement :

for ( ; ; ) Statement
for ( ; ; Expression ) Statement
for ( ; Expression ; ) Statement
for ( ; Expression ; Expression ) Statement
for ( ExpressionNoIn ; ; ) Statement
for ( ExpressionNoIn ; ; Expression ) Statement
for ( ExpressionNoIn ; Expression ; ) Statement
for ( ExpressionNoIn ; Expression ; Expression ) Statement

so the nonterminal IterationStatement actually has eight alternative right-hand sides.

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.

If the phrase “[lookahead ∉ set]” appears in the right-hand side of a production, it indicates that the production may not be used if the immediately following input token is a member of the given set. The set can be written as a list of terminals enclosed in curly braces. For convenience, the set can also be written as a nonterminal, in which case it represents the set of all terminals to which that nonterminal could expand. For example, given the definitions

DecimalDigit :: one of

0 1 2 3 4 5 6 7 8 9

DecimalDigits ::

DecimalDigit
DecimalDigits DecimalDigit

the definition

LookaheadExample ::

n [lookahead ∉ {1 , 3 , 5 , 7 , 9}]DecimalDigits
DecimalDigit [lookahead ∉ DecimalDigit ]

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.

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:

ReturnStatement :

return [no LineTerminator here] Expression_opt ;

indicates that the production may not be used if a LineTerminator occurs in the program between the return 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 program.

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:

NonZeroDigit :: one of

1 2 3 4 5 6 7 8 9

which is merely a convenient abbreviation for:

NonZeroDigit ::

1
2
3
4
5
6
7
8
9

When an alternative in a production of the lexical grammar or the numeric string grammar appears to be a multi-character token, it represents the sequence of characters that would make up such a token.

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:

Identifier ::

IdentifierName but not ReservedWord

means that the nonterminal Identifier may be replaced by any sequence of characters that could replace IdentifierName provided that the same sequence of characters could not replace ReservedWord.

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:

SourceCharacter ::

any Unicode code unit

5.2 Algorithm Conventions # Ⓣ Ⓔ ① Ⓐ

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.

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.

When an algorithm is to produce a value as a result, the directive “return x” is used to indicate that the result of the algorithm is the value of x and that the algorithm should terminate. The notation Result(n) is used as shorthand for “the result of step n”.

For clarity of expression, 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 lower case alphabetic characters and the second level of substeps labelled with lower case roman numerals. If more than three levels are required these rules repeat with the fourth level using numeric labels. For example:

1. Top-level step

   1. Substep.

   2. Substep

      1. Subsubstep.

      2. Subsubstep.

         1. Subsubsubstep

            1. Subsubsubsubstep

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.

Mathematical operations such as addition, subtraction, negation, multiplication, division, and the mathematical functions defined later in this clause should always be understood as computing exact mathematical results on mathematical real numbers, which do not include infinities and do not include a negative zero that is distinguished from positive zero. Algorithms in this standard that model floating-point arithmetic include explicit steps, where necessary, to handle infinities and signed zero and to perform rounding. If a mathematical operation or function is applied to a floating-point number, it should be understood as being applied to the exact mathematical value represented by that floating-point number; such a floating-point number must be finite, and if it is +0 or −0 then the corresponding mathematical value is simply 0.

The mathematical function abs(x) yields the absolute value of x, which is −x if x is negative (less than zero) and otherwise is x itself.

The mathematical function sign(x) yields 1 if x is positive and −1 if x is negative. The sign function is not used in this standard for cases when x is zero.

The notation “x modulo y” (y must be finite and nonzero) 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 integer q.

The mathematical function floor(x) yields the largest integer (closest to positive infinity) that is not larger than x.

NOTE floor(x) = x−(x modulo 1).

If an algorithm is defined to “throw an exception”, execution of the algorithm is terminated and no result is returned. The calling algorithms are also terminated, until an algorithm step is reached that explicitly deals with the exception, using terminology such as “If an exception was thrown…”. Once such an algorithm step has been encountered the exception is no longer considered to have occurred.

6 Source Text # Ⓣ Ⓔ ① Ⓐ

ECMAScript source text is represented as a sequence of characters in the Unicode character encoding, version 3.0 or later. The text is expected to have been normalised to Unicode Normalised Form C (canonical composition), as described in Unicode Technical Report #15. Conforming ECMAScript implementations are not required to perform any normalisation of text, or behave as though they were performing normalisation of text, themselves. ECMAScript source text is assumed to be a sequence of 16-bit code units for the purposes of this specification. Such a source text may include sequences of 16-bit code units that are not valid UTF-16 character encodings. If an actual source text is encoded in a form other than 16-bit code units it must be processed as if it was first convert to UTF-16.

SourceCharacter ::

any Unicode code unit

Throughout the rest of this document, the phrase “code unit” and the word “character” will be used to refer to a 16-bit unsigned value used to represent a single 16-bit unit of text. The phrase “Unicode character” will be used to refer to the abstract linguistic or typographical unit represented by a single Unicode scalar value (which may be longer than 16 bits and thus may be represented by more than one code unit). The phrase “code point” refers to such a Unicode scalar value. “Unicode character” only refers to entities represented by single Unicode scalar values: the components of a combining character sequence are still individual “Unicode characters,” even though a user might think of the whole sequence as a single character.

In string literals, regular expression literals, and identifiers, any character (code unit) may also be expressed as a Unicode escape sequence consisting of six characters, namely \u plus four hexadecimal digits. Within a comment, such an escape sequence is effectively ignored as part of the comment. Within a string literal or regular expression literal, the Unicode escape sequence contributes one character to the value of the literal. Within an identifier, the escape sequence contributes one character to the identifier.

NOTE Although this document sometimes refers to a “transformation” between a “character” within a “string” and the 16-bit unsigned integer that is the code unit of that character, there is actually no transformation because a “character” within a “string” is actually represented using that 16-bit unsigned value.

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 character 000A is line feed) and therefore the next character 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 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 a character to the String value of the literal and is never interpreted as a line terminator or as a quote mark that might terminate the string literal.

7 Lexical Conventions # Ⓣ Ⓔ ① Ⓐ

The source text of an ECMAScript program 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 characters as the next input element.

There are two goal symbols for the lexical grammar. The InputElementDiv symbol is used in those syntactic grammar contexts where a leading division (/) or division-assignment (/=) operator is permitted. The InputElementRegExp symbol is used in other syntactic grammar contexts.

NOTE 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 7.9); in examples such as the following:

a = b
/hi/g.exec(c).map(d);

where the first non-whitespace, non-comment character after a LineTerminator is slash (/) 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:

a = b / hi / g.exec(c).map(d);

Syntax

InputElementDiv ::

WhiteSpace
LineTerminator
Comment
Token
DivPunctuator

InputElementRegExp ::

WhiteSpace
LineTerminator
Comment
Token
RegularExpressionLiteral

7.1 Unicode Format-Control Characters # Ⓣ Ⓔ ① Ⓐ

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 and regular expression literals.

<ZWNJ> and <ZWJ> are format-control characters that are used to make necessary distinctions when forming words or phrases in certain languages. In ECMAScript source text, <ZWNJ> and <ZWJ> may also be used in an identifier after the first character.

<BOM> 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. <BOM> characters intended for this purpose can sometimes also appear after the start of a text, for example as a result of concatenating files. <BOM> characters are treated as white space characters (see 7.2).

The special treatment of certain format-control characters outside of comments, string literals, and regular expression literals is summarized in Table 1.

7.2 White Space # Ⓣ Ⓔ ① Ⓐ

White space characters are used to improve source text readability and to separate tokens (indivisible lexical units) from each other, but are otherwise insignificant. White space characters may occur between any two tokens and at the start or end of input. White space characters may also occur within a StringLiteral or a RegularExpressionLiteral (where they are considered significant characters forming part of the literal value) or within a Comment, but cannot appear within any other kind of token.

The ECMAScript white space characters are listed in Table 2.

ECMAScript implementations must recognize all of the white space characters defined in Unicode 3.0. Later editions of the Unicode Standard may define other white space characters. ECMAScript implementations may recognize white space characters from later editions of the Unicode Standard.

Syntax

WhiteSpace ::

<TAB>
<VT>
<FF>
<SP>
<#x0a>
<BOM>
<USP>

7.3 Line Terminators # Ⓣ Ⓔ ① Ⓐ

Like white space characters, line terminator characters are used to improve source text readability and to separate tokens (indivisible lexical units) from each other. However, unlike white space characters, 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 (7.9). A line terminator cannot occur within any token except a StringLiteral. Line terminators may only occur within a StringLiteral token as part of a LineContinuation.

A line terminator can occur within a MultiLineComment (7.4) but cannot occur within a SingleLineComment.

Line terminators are included in the set of white space characters that are matched by the \s class in regular expressions.

The ECMAScript line terminator characters are listed in Table 3.

Only the characters in Table 3 are treated as line terminators. Other new line or line breaking characters are treated as white space but not as line terminators. The character sequence <CR><LF> is commonly used as a line terminator. It should be considered a single character for the purpose of reporting line numbers.

Syntax

LineTerminator ::

<LF>
<CR>
<LS>
<PS>

LineTerminatorSequence ::

<LF>
<CR> [lookahead ∉ <LF> ]
<LS>
<PS>
<CR> <LF>

7.4 Comments # Ⓣ Ⓔ ① Ⓐ

Comments can be either single or multi-line. Multi-line comments cannot nest.

Because a single-line comment can contain any character except a LineTerminator character, and because of the general rule that a token is always as long as possible, a single-line comment always consists of all characters 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 recognised 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 7.9).

Comments behave like white space and are discarded except that, if a MultiLineComment contains a line terminator character, then the entire comment is considered to be a LineTerminator for purposes of parsing by the syntactic grammar.

Syntax

Comment ::

MultiLineComment
SingleLineComment

MultiLineComment ::

/* MultiLineCommentChars_opt */

MultiLineCommentChars ::

MultiLineNotAsteriskChar MultiLineCommentChars_opt
* PostAsteriskCommentChars_opt

PostAsteriskCommentChars ::

MultiLineNotForwardSlashOrAsteriskChar MultiLineCommentChars_opt
* PostAsteriskCommentChars_opt

MultiLineNotAsteriskChar ::

SourceCharacter but not asterisk *

MultiLineNotForwardSlashOrAsteriskChar ::

SourceCharacter but not forward-slash / orasterisk *

SingleLineComment ::

// SingleLineCommentChars_opt

SingleLineCommentChars ::

SingleLineCommentChar SingleLineCommentChars_opt

SingleLineCommentChar ::

SourceCharacter but not LineTerminator

7.5 Tokens # Ⓣ Ⓔ ① Ⓐ

Syntax

Token ::

IdentifierName
Punctuator
NumericLiteral
StringLiteral

NOTE The DivPunctuator and RegularExpressionLiteral productions define tokens, but are not included in the Token production.

7.6 Identifier Names and Identifiers # Ⓣ Ⓔ ① Ⓐ

Identifier Names are tokens that are interpreted according to the grammar given in the “Identifiers” section of chapter 5 of the Unicode standard, with some small modifications. An Identifier is an IdentifierName that is not a ReservedWord (see 7.6.1). The Unicode identifier grammar is based on both normative and informative character categories specified by the Unicode Standard. The characters in the specified categories in version 3.0 of the Unicode standard must be treated as in those categories by all conforming ECMAScript implementations.

This standard specifies specific character additions: The dollar sign ($) and the underscore (_) are permitted anywhere in an IdentifierName.

Unicode escape sequences are also permitted in an IdentifierName, where they contribute a single character to the IdentifierName, as computed by the CV of the UnicodeEscapeSequence (see 7.8.4). The \ preceding the UnicodeEscapeSequence does not contribute a character to the IdentifierName. A UnicodeEscapeSequence cannot be used to put a character into an IdentifierName that would otherwise be illegal. In other words, if a \ UnicodeEscapeSequence sequence were replaced by its UnicodeEscapeSequence's CV, the result must still be a valid IdentifierName that has the exact same sequence of characters as the original IdentifierName. All interpretations of identifiers within this specification are based upon their actual characters regardless of whether or not an escape sequence was used to contribute any particular characters.

Two IdentifierName that are canonically equivalent according to the Unicode standard are not equal unless they are represented by the exact same sequence of code units (in other words, conforming ECMAScript implementations are only required to do bitwise comparison on IdentifierName values). The intent is that the incoming source text has been converted to normalised form C before it reaches the compiler.

ECMAScript implementations may recognize identifier characters defined in later editions of the Unicode Standard. If portability is a concern, programmers should only employ identifier characters defined in Unicode 3.0.

Syntax

Identifier ::

IdentifierName but not ReservedWord

IdentifierName ::

IdentifierStart
IdentifierName IdentifierPart

IdentifierStart ::

UnicodeLetter
$
_
\ UnicodeEscapeSequence

IdentifierPart ::

IdentifierStart
UnicodeCombiningMark
UnicodeDigit
UnicodeConnectorPunctuation
<ZWNJ>
<ZWJ>

UnicodeLetter

any character in the Unicode categories “Uppercase letter (Lu)”, “Lowercase letter (Ll)”, “Titlecase letter (Lt)”, “Modifier letter (Lm)”, “Other letter (Lo)”, or “Letter number (Nl)”.

UnicodeCombiningMark

any character in the Unicode categories “Non-spacing mark (Mn)” or “Combining spacing mark (Mc)”

UnicodeDigit

any character in the Unicode category “Decimal number (Nd)”

UnicodeConnectorPunctuation

any character in the Unicode category “Connector punctuation (Pc)”

UnicodeEscapeSequence

see 7.8.4.

7.6.1 Reserved Words # Ⓣ Ⓔ ① Ⓐ

A reserved word is an IdentifierName that cannot be used as an Identifier.

Syntax

ReservedWord ::

Keyword
FutureReservedWord
NullLiteral
BooleanLiteral

7.6.1.1 Keywords # Ⓣ Ⓔ ① Ⓐ

The following tokens are ECMAScript keywords and may not be used as Identifiers in ECMAScript programs.

Syntax

Keyword :: one of

break	do	instanceof	typeof
case	else	new	var
catch	finally	return	void
continue	for	switch	while
debugger	function	this	with
default	if	throw
delete	in	try

7.6.1.2 Future Reserved Words # Ⓣ Ⓔ ① Ⓐ

The following words are used as keywords in proposed extensions and are therefore reserved to allow for the possibility of future adoption of those extensions.

Syntax

FutureReservedWord :: one of

class	enum	extends	super
const	export	import

The following tokens are also considered to be FutureReservedWords when they occur within strict mode code (see 10.1.1). The occurrence of any of these tokens within strict mode code in any context where the occurrence of a FutureReservedWord would produce an error must also produce an equivalent error:

7.7 Punctuators # Ⓣ Ⓔ ① Ⓐ

Syntax

Punctuator :: one of

{	}	(	)	[	]
.	;	,	<	>	<=
>=	==	!=	===	!==
+	-	*	%	++	--
<<	>>	>>>	&	|	^
!	~	&&	||	?	:
=	+=	-=	*=	%=	<<=
>>=	>>>=	&=	|=	^=

DivPunctuator :: one of

/

/=

7.8 Literals # Ⓣ Ⓔ ① Ⓐ

Syntax

Literal ::

NullLiteral
BooleanLiteral
NumericLiteral
StringLiteral
RegularExpressionLiteral

7.8.1 Null Literals # Ⓣ Ⓔ ① Ⓐ

Syntax

NullLiteral ::

null

Semantics

The value of the null literal null is the sole value of the Null type, namely null.

7.8.2 Boolean Literals # Ⓣ Ⓔ ① Ⓐ

Syntax

BooleanLiteral ::

true
false

Semantics

The value of the Boolean literal true is a value of the Boolean type, namely true.

The value of the Boolean literal false is a value of the Boolean type, namely false.

7.8.3 Numeric Literals # Ⓣ Ⓔ ① Ⓐ

Syntax

NumericLiteral ::

DecimalLiteral
HexIntegerLiteral

DecimalLiteral ::

DecimalIntegerLiteral . DecimalDigits_opt ExponentPart_opt
. DecimalDigits ExponentPart_opt
DecimalIntegerLiteral ExponentPart_opt

DecimalIntegerLiteral ::

0
NonZeroDigit DecimalDigits_opt

DecimalDigits ::

DecimalDigit
DecimalDigits DecimalDigit

DecimalDigit :: one of

0 1 2 3 4 5 6 7 8 9

NonZeroDigit :: one of

1 2 3 4 5 6 7 8 9

ExponentPart ::

ExponentIndicator SignedInteger

ExponentIndicator :: one of

e E

SignedInteger ::

DecimalDigits
+ DecimalDigits
- DecimalDigits

HexIntegerLiteral ::

0x HexDigit
0X HexDigit
HexIntegerLiteral HexDigit

HexDigit :: one of

0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F

The source character immediately following a NumericLiteral must not be an IdentifierStart or DecimalDigit.

NOTE For example:

3in

is an error and not the two input elements 3 and in.

Semantics

A numeric literal stands for a value of the Number type. This value is determined in two steps: first, a mathematical value (MV) is derived from the literal; second, this mathematical value is rounded as described below.

• The MV of NumericLiteral :: DecimalLiteral is the MV of DecimalLiteral.

• The MV of NumericLiteral :: HexIntegerLiteral is the MV of HexIntegerLiteral.

• The MV of DecimalLiteral :: DecimalIntegerLiteral . is the MV of DecimalIntegerLiteral.

• The MV of DecimalLiteral :: DecimalIntegerLiteral . DecimalDigits is the MV of DecimalIntegerLiteral plus (the MV of DecimalDigits times 10^–n), where n is the number of characters in DecimalDigits.

• The MV of DecimalLiteral :: DecimalIntegerLiteral . ExponentPart is the MV of DecimalIntegerLiteral times 10^e, where e is the MV of ExponentPart.

• The MV of DecimalLiteral :: DecimalIntegerLiteral . DecimalDigits ExponentPart is (the MV of DecimalIntegerLiteral plus (the MV of DecimalDigits times 10^–n)) times 10^e, where n is the number of characters in DecimalDigits and e is the MV of ExponentPart.

• The MV of DecimalLiteral ::. DecimalDigits is the MV of DecimalDigits times 10^–n, where n is the number of characters in DecimalDigits.

• The MV of DecimalLiteral ::. DecimalDigits ExponentPart is the MV of DecimalDigits times 10^e–n, where n is the number of characters in DecimalDigits and e is the MV of ExponentPart.

• The MV of DecimalLiteral :: DecimalIntegerLiteral is the MV of DecimalIntegerLiteral.

• The MV of DecimalLiteral :: DecimalIntegerLiteral ExponentPart is the MV of DecimalIntegerLiteral times 10^e, where e is the MV of ExponentPart.

• The MV of DecimalIntegerLiteral :: 0 is 0.

• The MV of DecimalIntegerLiteral :: NonZeroDigit DecimalDigits is (the MV of NonZeroDigit times 10ⁿ) plus the MV of DecimalDigits, where n is the number of characters in DecimalDigits.

• The MV of DecimalDigits :: DecimalDigit is the MV of DecimalDigit.

• The MV of DecimalDigits :: DecimalDigits DecimalDigit is (the MV of DecimalDigits times 10) plus the MV of DecimalDigit.

• The MV of ExponentPart :: ExponentIndicator SignedInteger is the MV of SignedInteger.

• The MV of SignedInteger :: DecimalDigits is the MV of DecimalDigits.

• The MV of SignedInteger :: + DecimalDigits is the MV of DecimalDigits.

• The MV of SignedInteger :: - DecimalDigits is the negative of the MV of DecimalDigits.

• The MV of DecimalDigit :: 0 or of HexDigit :: 0 is 0.

• The MV of DecimalDigit :: 1 or of NonZeroDigit :: 1 or of HexDigit :: 1 is 1.

• The MV of DecimalDigit :: 2 or of NonZeroDigit :: 2 or of HexDigit :: 2 is 2.

• The MV of DecimalDigit :: 3 or of NonZeroDigit :: 3 or of HexDigit :: 3 is 3.

• The MV of DecimalDigit :: 4 or of NonZeroDigit :: 4 or of HexDigit :: 4 is 4.

• The MV of DecimalDigit :: 5 or of NonZeroDigit :: 5 or of HexDigit :: 5 is 5.

• The MV of DecimalDigit :: 6 or of NonZeroDigit :: 6 or of HexDigit :: 6 is 6.

• The MV of DecimalDigit :: 7 or of NonZeroDigit :: 7 or of HexDigit :: 7 is 7.

• The MV of DecimalDigit :: 8 or of NonZeroDigit :: 8 or of HexDigit :: 8 is 8.

• The MV of DecimalDigit :: 9 or of NonZeroDigit :: 9 or of HexDigit :: 9 is 9.

• The MV of HexDigit :: a or of HexDigit :: A is 10.

• The MV of HexDigit :: b or of HexDigit :: B is 11.

• The MV of HexDigit :: c or of HexDigit :: C is 12.

• The MV of HexDigit :: d or of HexDigit :: D is 13.

• The MV of HexDigit :: e or of HexDigit :: E is 14.

• The MV of HexDigit :: f or of HexDigit :: F is 15.

• The MV of HexIntegerLiteral :: 0x HexDigit is the MV of HexDigit.

• The MV of HexIntegerLiteral :: 0X HexDigit is the MV of HexDigit.

• The MV of HexIntegerLiteral :: HexIntegerLiteral HexDigit is (the MV of HexIntegerLiteral times 16) plus the MV of HexDigit.

Once the exact MV for a numeric literal has been determined, it is then rounded to a value of the Number type. If the MV is 0, then the rounded value is +0; otherwise, the rounded value must be the Number value for the MV (as specified in 8.5), unless the literal is a DecimalLiteral and the literal has more than 20 significant digits, in which case the Number value may be either the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit or the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit and then incrementing the literal at the 20th significant digit position. A digit is significant if it is not part of an ExponentPart and

• it is not 0; or

• there is a nonzero digit to its left and there is a nonzero digit, not in the ExponentPart, to its right.

A conforming implementation, when processing strict mode code (see 10.1.1), must not extend the syntax of NumericLiteral to include OctalIntegerLiteral as described in B.1.1.

7.8.4 String Literals # Ⓣ Ⓔ ① Ⓐ

A string literal is zero or more characters enclosed in single or double quotes. Each character may be represented by an escape sequence. All characters may appear literally in a string literal except for the closing quote character, backslash, carriage return, line separator, paragraph separator, and line feed. Any character may appear in the form of an escape sequence.

Syntax

StringLiteral ::

" DoubleStringCharacters_opt "
' SingleStringCharacters_opt '

DoubleStringCharacters ::

DoubleStringCharacter DoubleStringCharacters_opt

SingleStringCharacters ::

SingleStringCharacter SingleStringCharacters_opt

DoubleStringCharacter ::

SourceCharacter but not double-quote " or backslash \ or LineTerminator
\ EscapeSequence
LineContinuation

SingleStringCharacter ::

SourceCharacter but not single-quote ' orbackslash \ or LineTerminator
\ EscapeSequence
LineContinuation

LineContinuation ::

\ LineTerminatorSequence

EscapeSequence ::

CharacterEscapeSequence
0 [lookahead ∉ DecimalDigit]
HexEscapeSequence
UnicodeEscapeSequence

CharacterEscapeSequence ::

SingleEscapeCharacter
NonEscapeCharacter

SingleEscapeCharacter :: one of

' " \ b f n r t v

NonEscapeCharacter ::

SourceCharacter but not EscapeCharacter or LineTerminator

EscapeCharacter ::

SingleEscapeCharacter
DecimalDigit
x
u

HexEscapeSequence ::

x HexDigit HexDigit

UnicodeEscapeSequence ::

u HexDigit HexDigit HexDigit HexDigit

The definitions of the nonterminal HexDigit is given in 7.6. SourceCharacter is defined in clause 6.

Semantics

A string literal stands for a value of the String type. The String value (SV) of the literal is described in terms of character values (CV) contributed by the various parts of the string literal. As part of this process, some characters within the string literal are interpreted as having a mathematical value (MV), as described below or in 7.8.3.

• The SV of StringLiteral :: "" is the empty character sequence.

• The SV of StringLiteral :: '' is the empty character sequence.

• The SV of StringLiteral :: " DoubleStringCharacters " is the SV of DoubleStringCharacters.

• The SV of StringLiteral :: ' SingleStringCharacters ' is the SV of SingleStringCharacters.

• The SV of DoubleStringCharacters :: DoubleStringCharacter is a sequence of one character, the CV of DoubleStringCharacter.

• The SV of DoubleStringCharacters :: DoubleStringCharacter DoubleStringCharacters is a sequence of the CV of DoubleStringCharacter followed by all the characters in the SV of DoubleStringCharacters in order.

• The SV of SingleStringCharacters :: SingleStringCharacter is a sequence of one character, the CV of SingleStringCharacter.

• The SV of SingleStringCharacters :: SingleStringCharacter SingleStringCharacters is a sequence of the CV of SingleStringCharacter followed by all the characters in the SV of SingleStringCharacters in order.

• The SV of LineContinuation :: \ LineTerminatorSequence is the empty character sequence.

• The CV of DoubleStringCharacter :: SourceCharacter but not double-quote " or backslash \ or LineTerminator is theSourceCharacter character itself.

• The CV of DoubleStringCharacter :: \ EscapeSequence is the CV of the EscapeSequence.

• The CV of DoubleStringCharacter :: LineContinuation is the empty character sequence.

• The CV of SingleStringCharacter :: SourceCharacter but not single-quote ' or backslash \ or LineTerminator is theSourceCharacter character itself.

• The CV of SingleStringCharacter :: \ EscapeSequence is the CV of the EscapeSequence.

• The CV of SingleStringCharacter :: LineContinuation is the empty character sequence.

• The CV of EscapeSequence :: CharacterEscapeSequence is the CV of the CharacterEscapeSequence.

• The CV of EscapeSequence :: 0 [lookahead ∉ DecimalDigit] is a <NUL> character (Unicode value 0000).

• The CV of EscapeSequence :: HexEscapeSequence is the CV of the HexEscapeSequence.

• The CV of EscapeSequence :: UnicodeEscapeSequence is the CV of the UnicodeEscapeSequence.

• The CV of CharacterEscapeSequence ::SingleEscapeCharacter is the character whose code unit value is determined by theSingleEscapeCharacter according to Table 4:

  
  

  Table 4 — String Single Character Escape Sequences

  Escape Sequence	Code Unit Value	Name	Symbol
  \b	\u0008	backspace	<BS>
  \t	\u0009	horizontal tab	<HT>
  \n	\u000A	line feed (new line)	<LF>
  \v	\u000B	vertical tab	<VT>
  \f	\u000C	form feed	<FF>
  \r	\u000D	carriage return	<CR>
  \"	\u0022	double quote	"
  \'	\u0027	single quote	'
  \\	\u005C	backslash	\

  
  

• The CV of CharacterEscapeSequence :: NonEscapeCharacter is the CV of the NonEscapeCharacter.

• The CV of NonEscapeCharacter :: SourceCharacter but not EscapeCharacter or LineTerminator is the SourceCharacter character itself.

• The CV of HexEscapeSequence :: x HexDigit HexDigit is the character whose code unit value is (16 times the MV of the first HexDigit) plus the MV of the second HexDigit.

• The CV of UnicodeEscapeSequence :: u HexDigit HexDigit HexDigit HexDigit is the character whose code unit value is (4096 times the MV of the first HexDigit) plus (256 times the MV of the second HexDigit) plus (16 times the MV of the third HexDigit) plus the MV of the fourth HexDigit.

A conforming implementation, when processing strict mode code (see 10.1.1), may not extend the syntax of EscapeSequence to include OctalEscapeSequence as described in B.1.2.

NOTE A line terminator character cannot appear in a string literal, except as part of a LineContinuation to produce the empty character sequence. The correct way to cause a line terminator character to be part of the String value of a string literal is to use an escape sequence such as \n or \u000A.

7.8.5 Regular Expression Literals # Ⓣ Ⓔ ① Ⓐ

A regular expression literal is an input element that is converted to a RegExp object (see 15.10) 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 (see 15.10.4) or calling the RegExp constructor as a function (15.10.3).

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 Strings of characters comprising the RegularExpressionBody and the RegularExpressionFlags are passed uninterpreted to the regular expression constructor, which interprets them according to its own, more stringent grammar. An implementation may extend the regular expression constructor's grammar, but it must not extend the RegularExpressionBody and RegularExpressionFlags productions or the productions used by these productions.

Syntax

RegularExpressionLiteral ::

/ RegularExpressionBody / RegularExpressionFlags

RegularExpressionBody ::

RegularExpressionFirstChar RegularExpressionChars

RegularExpressionChars ::

[empty]
RegularExpressionChars RegularExpressionChar

RegularExpressionFirstChar ::

RegularExpressionNonTerminator but not * or \ or / or [
RegularExpressionBackslashSequence
RegularExpressionClass

RegularExpressionChar ::

RegularExpressionNonTerminator but not \ or / or [
RegularExpressionBackslashSequence
RegularExpressionClass

RegularExpressionBackslashSequence ::

\ RegularExpressionNonTerminator

RegularExpressionNonTerminator ::

SourceCharacter but not LineTerminator

RegularExpressionClass ::

[ RegularExpressionClassChars ]

RegularExpressionClassChars ::

[empty]
RegularExpressionClassChars RegularExpressionClassChar

RegularExpressionClassChar ::

RegularExpressionNonTerminator but not ] or \
RegularExpressionBackslashSequence

RegularExpressionFlags ::

[empty]
RegularExpressionFlags IdentifierPart

NOTE Regular expression literals may not be empty; instead of representing an empty regular expression literal, the characters // start a single-line comment. To specify an empty regular expression, use: /(?:)/.

Semantics

A regular expression literal evaluates to a value of the Object type that is an instance of the standard built-in constructor RegExp. This value is determined in two steps: first, the characters comprising the regular expression's RegularExpressionBody and RegularExpressionFlags production expansions are collected uninterpreted into two Strings Pattern and Flags, respectively. Then each time the literal is evaluated, a new object is created as if by the expression new RegExp(Pattern, Flags) where RegExp is the standard built-in constructor with that name. The newly constructed object becomes the value of the RegularExpressionLiteral. If the call to new RegExp would generate an error as specified in 15.10.4.1, the error must be treated as an early error (Clause 16).

7.9 Automatic Semicolon Insertion # Ⓣ Ⓔ ① Ⓐ

Certain ECMAScript statements (empty statement, variable statement, expression statement, do-while statement, continue statement, break statement, return statement, and throw statement) must be terminated with semicolons. 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.

7.9.1 Rules of Automatic Semicolon Insertion # Ⓣ Ⓔ ① Ⓐ

There are three basic rules of semicolon insertion:

1. When, as the program 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 }.

2. When, as the program 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 complete ECMAScript Program, then a semicolon is automatically inserted at the end of the input stream.

3. When, as the program 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 12.6.3).

NOTE The following are the only restricted productions in the grammar:

PostfixExpression :

LeftHandSideExpression [no LineTerminator here] ++
LeftHandSideExpression [no LineTerminator here] --

ContinueStatement :

continue [no LineTerminator here] Identifier;

BreakStatement :

break [no LineTerminator here] Identifier;

ReturnStatement :

return [no LineTerminator here] Expression;

ThrowStatement :

throw [no LineTerminator here] Expression;

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, or throw token is encountered and a LineTerminator is encountered before the next token, a semicolon is automatically inserted after the continue, break, return, or throw token.

The resulting practical advice to ECMAScript programmers is:

A postfix ++ or -- operator should appear on the same line as its operand.

An Expression in a return or throw statement should start on the same line as the return or throw token.

A Identifier in a break or continue statement should be on the same line as the break or continue token.

7.9.2 Examples of Automatic Semicolon Insertion # Ⓣ Ⓔ ① Ⓐ

The source

{ 1 2 } 3

is not a valid sentence in the ECMAScript grammar, even with the automatic semicolon insertion rules. In contrast, the source

{ 1
2 } 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 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 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 parenthesised 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.

8 Types # Ⓣ Ⓔ ① Ⓐ

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 subclassified into ECMAScript language types and specification 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, Number, and Object.

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 are Reference, List, Completion, Property Descriptor, Property Identifier, Lexical Environment, and Environment Record. 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.

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.

8.1 The Undefined Type # Ⓣ Ⓔ ① Ⓐ

The Undefined type has exactly one value, called undefined. Any variable that has not been assigned a value has the value undefined.

8.2 The Null Type # Ⓣ Ⓔ ① Ⓐ

The Null type has exactly one value, called null.

8.3 The Boolean Type # Ⓣ Ⓔ ① Ⓐ

The Boolean type represents a logical entity having two values, called true and false.

8.4 The String Type # Ⓣ Ⓔ ① Ⓐ

The String type is the set of all finite ordered sequences of zero or more 16-bit unsigned integer values (“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 code unit value (see Clause 6). Each element is regarded as occupying a position within the sequence. These positions are indexed with nonnegative integers. The first element (if any) is at position 0, the next element (if any) at position 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.

When a String contains actual textual data, each element is considered to be a single UTF-16 code unit. Whether or not this is the actual storage format of a String, the characters within a String are numbered by their initial code unit element position as though they were represented using UTF-16. All operations on Strings (except as otherwise stated) treat them as sequences of undifferentiated 16-bit unsigned integers; they do not ensure the resulting String is in normalised form, nor do they ensure language-sensitive results.

NOTE The rationale behind this design was to keep the implementation of Strings as simple and high-performing as possible. The intent is that textual data coming into the execution environment from outside (e.g., user input, text read from a file or received over the network, etc.) be converted to Unicode Normalised Form C before the running program sees it. Usually this would occur at the same time incoming text is converted from its original character encoding to Unicode (and would impose no additional overhead). Since it is recommended that ECMAScript source code be in Normalised Form C, string literals are guaranteed to be normalised (if source text is guaranteed to be normalised), as long as they do not contain any Unicode escape sequences.

8.5 The Number Type # Ⓣ Ⓔ ① Ⓐ

The Number type has exactly 18437736874454810627 (that is, 2⁶⁴−2⁵³+3) values, representing the double-precision 64-bit format IEEE 754 values as specified in the IEEE Standard for Binary Floating-Point Arithmetic, except that the 9007199254740990 (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-dependent; to ECMAScript code, all NaN values are indistinguishable from each other.

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 +∞ and −∞, respectively. (Note that these two infinite Number values are produced by the program expressions +Infinity (or simply Infinity) and -Infinity.)

The other 18437736874454810624 (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 +0 and −0, respectively. (Note that these two different zero Number values are produced by the program expressions +0 (or simply 0) and -0.)

The 18437736874454810622 (that is, 2⁶⁴−2⁵³−2) finite nonzero values are of two kinds:

18428729675200069632 (that is, 2⁶⁴−2⁵⁴) of them are normalised, having the form

s × m × 2^e

where s is +1 or −1, m is a positive integer less than 2⁵³ but not less than 2⁵², and e is an integer ranging from −1074 to 971, inclusive.

The remaining 9007199254740990 (that is, 2⁵³−2) values are denormalised, having the form

s × m × 2^e

where s is +1 or −1, m is a positive integer less than 2⁵², 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 (indeed, the integer 0 has two representations, +0 and -0).

A finite number has an odd significand if it is nonzero and the integer m 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 nonzero 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 −0 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 +∞; if −2¹⁰²⁴ was chosen, replace it with −∞; if +0 was chosen, replace it with −0 if and only if x is less than zero; any other chosen value is used unchanged. The result is the Number value for x. (This procedure corresponds exactly to the behaviour of the IEEE 754 “round to nearest” mode.)

Some ECMAScript operators deal only with integers in the range −2³¹ through 2³¹−1, inclusive, or in the range 0 through 2³²−1, inclusive. These operators accept any value of the Number type but first convert each such value to one of 2³² integer values. See the descriptions of the ToInt32 and ToUint32 operators in 9.5 and 9.6, respectively.

8.6 The Object Type # Ⓣ Ⓔ ① Ⓐ

An Object is a collection of properties. Each property is either a named data property, a named accessor property, or an internal property:

• A named data property associates a name with an ECMAScript language value and a set of Boolean attributes.

• A named accessor property associates a name 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.

• An internal property has no name and is not directly accessible via ECMAScript language operators. Internal properties exist purely for specification purposes.

There are two kinds of access for named (non-internal) properties: get and put, corresponding to retrieval and assignment, respectively.

8.6.1 Property Attributes # Ⓣ Ⓔ ① Ⓐ

Attributes are used in this specification to define and explain the state of named properties. A named data property associates a name with the attributes listed in Table 5

Table 5 — Attributes of a Named Data Property

Attribute Name	Value Domain	Description
[[Value]]	Any ECMAScript language type	The value retrieved by reading the property.
[[Writable]]	Boolean	If false, attempts by ECMAScript code to change the property’s [[Value]] attribute using [[Put]] will not succeed.
[[Enumerable]]	Boolean	If true, the property will be enumerated by a for-in enumeration (see 12.6.4). Otherwise, the property is said to be non-enumerable.
[[Configurable]]	Boolean	If false, attempts to delete the property, change the property to be an accessor property, or change its attributes (other than [[Value]]) will fail.

A named accessor property associates a name with the attributes listed in Table 6.

Table 6 — Attributes of a Named Accessor Property

Attribute Name	Value Domain	Description
[[Get]]	Object or Undefined	If the value is an Object it must be a function Object. The function’s [[Call]] internal method (8.6.2) is called with an empty arguments list to return the property value each time a get access of the property is performed.
[[Set]]	Object or Undefined	If the value is an Object it must be a function Object. The function’s [[Call]] internal method (8.6.2) 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.
[[Enumerable]]	Boolean	If true, the property is to be enumerated by a for-in enumeration (see 12.6.4). Otherwise, the property is said to be non-enumerable.
[[Configurable]]	Boolean	If false, attempts to delete the property, change the property to be a data property, or change its attributes will fail.

If the value of an attribute is not explicitly specified by this specification for a named property, the default value defined in Table 7 is used.

Table 7 — Default Attribute Values

Attribute Name	Default Value
[[Value]]	undefined
[[Get]]	undefined
[[Set]]	undefined
[[Writable]]	false
[[Enumerable]]	false
[[Configurable]]	false

8.6.2 Object Internal Properties and Methods # Ⓣ Ⓔ ① Ⓐ

This specification uses various internal properties to define the semantics of object values. These internal properties are not part of the ECMAScript language. They are defined by this specification purely for expository purposes. An implementation of ECMAScript must behave as if it produced and operated upon internal properties in the manner described here. The names of internal properties are enclosed in double square brackets [[ ]]. When an algorithm uses an internal property of an object and the object does not implement the indicated internal property, a TypeError exception is thrown.

The Table 8 summarises the internal properties used by this specification that are applicable to all ECMAScript objects. The Table 9 summarises the internal properties used by this specification that are only applicable to some ECMAScript objects. The descriptions in these tables indicates their behaviour for native ECMAScript objects, unless stated otherwise in this document for particular kinds of native ECMAScript objects. Host objects may support these internal properties with any implementation-dependent behaviour as long as it is consistent with the specific host object restrictions stated in this document.

The “Value Type Domain” columns of the following tables define the types of values associated with internal properties. The type names refer to the types defined in Clause 8 augmented by the following additional names. “any” means the value may be any ECMAScript language type. “primitive” means Undefined, Null, Boolean, String, or Number. “SpecOp” means the internal property is an internal method, an implementation provided procedure defined by an abstract operation specification. “SpecOp” is followed by a list of descriptive parameter names. If a parameter name is the same as a type name then the name describes the type of the parameter. If a “SpecOp” returns a value, its parameter list is followed by the symbol “→” and the type of the returned value.

Every object (including host objects) must implement all of the internal properties listed in Table 8. However, the [[DefaultValue]] internal method may, for some objects, simply throw a TypeError exception.

All objects have an internal property called [[Prototype]]. The value of this property is either null or an object and is used for implementing inheritance. Whether or not a native object can have a host object as its [[Prototype]] depends on the implementation. Every [[Prototype]] chain must have finite length (that is, starting from any object, recursively accessing the [[Prototype]] internal property must eventually lead to a null value). Named data properties of the [[Prototype]] object are inherited (are visible as properties of the child object) for the purposes of get access, but not for put access. Named accessor properties are inherited for both get access and put access.

Every ECMAScript object has a Boolean-valued [[Extensible]] internal property that controls whether or not named properties may be added to the object. If the value of the [[Extensible]] internal property is false then additional named properties may not be added to the object. In addition, if [[Extensible]] is false the value of the [[Class]] and [[Prototype]] internal properties of the object may not be modified. Once the value of an [[Extensible]] internal property has been set to false it may not be subsequently changed to true.

NOTE This specification defines no ECMAScript language operators or built-in functions that permit a program to modify an object’s [[Class]] or [[Prototype]] internal properties or to change the value of [[Extensible]] from false to true. Implementation specific extensions that modify [[Class]], [[Prototype]] or [[Extensible]] must not violate the invariants defined in the preceding paragraph.

The value of the [[Class]] internal property is defined by this specification for every kind of built-in object. The value of the [[Class]] internal property of a host object may be any String value except one of "Arguments", "Array", "Boolean", "Date", "Error", "Function", "JSON", "Math", "Number", "Object", "RegExp", and "String". The value of a [[Class]] internal property is used internally to distinguish different kinds of objects. Note that this specification does not provide any means for a program to access that value except through Object.prototype.toString (see 15.2.4.2).

Unless otherwise specified, the common internal methods of native ECMAScript objects behave as described in 8.12. Array objects have a slightly different implementation of the [[DefineOwnProperty]] internal method (see 15.4.5.1) and String objects have a slightly different implementation of the [[GetOwnProperty]] internal method (see 15.5.5.2). Arguments objects (10.6) have different implementations of [[Get]], [[GetOwnProperty]], [[DefineOwnProperty]], and [[Delete]]. Function objects (15.3) have a different implementation of [[Get]].

Host objects may implement these internal methods in any manner unless specified otherwise; for example, one possibility is that [[Get]] and [[Put]] for a particular host object indeed fetch and store property values but [[HasProperty]] always generates false. However, if any specified manipulation of a host object's internal properties is not supported by an implementation, that manipulation must throw a TypeError exception when attempted.

The [[GetOwnProperty]] internal method of a host object must conform to the following invariants for each property of the host object:

• 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 internal methods.

• If a property is described as a data property and its [[Writable]] and [[Configurable]] are both false, then the SameValue (according to 9.12) must be returned for the [[Value]] attribute of the property on all calls to [[GetOwnProperty]].

• If the attributes other than [[Writable]] may change over time or if the property might disappear, then the [[Configurable]] attribute must be true.

• If the [[Writable]] attribute may change from false to true, then the [[Configurable]] attribute must be true.

• If the value of the host object’s [[Extensible]] internal property is has been observed by ECMAScript code to be false, then if a call to [[GetOwnProperty]] describes a property as non-existent all subsequent calls must also describe that property as non-existent.

The [[DefineOwnProperty]] internal method of a host object must not permit the addition of a new property to a host object if the [[Extensible]] internal property of that host object has been observed by ECMAScript code to be false.

If the [[Extensible]] internal property of that host object has been observed by ECMAScript code to be false then it must not subsequently become true.

8.7 The Reference Specification Type # Ⓣ Ⓔ ① Ⓐ

The Reference type is used to explain the behaviour of such operators as delete, typeof, and the assignment operators. For example, the left-hand operand of an assignment is expected to produce a reference. The behaviour of assignment could, instead, be explained entirely in terms of a case analysis on the syntactic form of the left-hand operand of an assignment operator, but for one difficulty: function calls are permitted to return references. This possibility is admitted purely for the sake of host objects. No built-in ECMAScript function defined by this specification returns a reference and there is no provision for a user-defined function to return a reference. (Another reason not to use a syntactic case analysis is that it would be lengthy and awkward, affecting many parts of the specification.)

A Reference is a resolved name binding. A Reference consists of three components, the base value, the referenced name and the Boolean valued strict reference flag. The base value is either undefined, an Object, a Boolean, a String, a Number, or an environment record (10.2.1). A base value of undefined indicates that the reference could not be resolved to a binding. The referenced name is a String.

The following abstract operations are used in this specification to access the components of references:

• GetBase(V). Returns the base value component of the reference V.

• GetReferencedName(V). Returns the referenced name component of the reference V.

• IsStrictReference(V). Returns the strict reference component of the reference V.

• HasPrimitiveBase(V). Returns true if the base value is a Boolean, String, or Number.

• IsPropertyReference(V). Returns true if either the base value is an object or HasPrimitiveBase(V) is true; otherwise returns false.

• IsUnresolvableReference(V). Returns true if the base value is undefined and false otherwise.