This is not the official ECMAScript Language Specification.
The official specification is the PDF document located at http://www.ecma-international.org/publications/files/ECMA-ST/ECMA-262.pdf.
This HTML version was created from the original PDF version by a preposterous concatenation of hacks. It very likely contains errors and the official standard and errata are of course definitive. For copyright information, see ECMA's legal disclaimer in the document itself.
This version was created by Jason Orendorff and incorporates the official errata as of 31 July 2010.
Last updated: 5 November 2010.
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 published1 as the fourth edition of ECMAScript, it informs continuing evolution of the language. The present fifth edition of ECMAScript (published as ECMA-262 5th 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.
This Ecma Standard has been adopted by the General Assembly of December 2009.
1Note: Please note that for ECMAScript Edition 4 the Ecma standard number “ECMA-262 Edition 4” was reserved but not used in the Ecma publication process. Therefore “ECMA-262 Edition 4” as an Ecma International publication does not exist.
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This Standard defines the ECMAScript scripting language.
A conforming implementation of ECMAScript must provide and support all the types, values, objects, properties, functions, and program syntax and semantics described in this specification.
A conforming implementation of 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 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.
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
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.
PDF page 12 / printed page 2 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.
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.
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 PDF page 13 / printed page 3 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.
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.
PDF page 14 / printed page 4 All objects that do not directly contain a particular property that their prototype contains share that property and its value. Figure 1 illustrates this:
CF is a constructor (and also an object). Five objects have been
created by using new expressions: cf1,
cf2, cf3, cf4, and
cf5. Each of these objects contains properties named
q1 and q2. The
dashed lines represent the implicit prototype relationship; so, for example,
cf3’s prototype is CFp. The
constructor, CF, has two properties itself, named
P1 and P2, which
are not visible to CFp, cf1,
cf2, cf3, cf4, or
cf5. The property named CFP1 in
CFp is shared by cf1,
cf2, cf3, cf4, and
cf5 (but not by CF), as are any properties found in
CFp’s implicit prototype chain that are not named
q1, q2, or
CFP1. Notice that there is no implicit prototype
link between CF and CFp.
Unlike class-based object languages, properties can be added to objects dynamically by assigning values to them. That is, constructors are not required to name or assign values to all or any of the constructed object’s properties. In the above diagram, one could add a new shared property for cf1, cf2, cf3, cf4, and cf5 by assigning a new value to the property in CFp.
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.
For the purposes of this document, the following terms and definitions apply.
set of data values as defined in Clause 8 of this specification.
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.
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.
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.
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.
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.
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.
object supplied by the host environment to complete the execution environment of ECMAScript.
NOTE Any object that is not native is a host object.
primitive value used when a variable has not been assigned a value.
type whose sole value is the undefined value.
primitive value that represents the intentional absence of any object value.
type whose sole value is the null value.
member of the Boolean type.
NOTE There are only two Boolean values, true and false.
type consisting of the primitive values true and false.
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.
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.
set of all possible String values.
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).
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.
set of all possible Number values including the special “Not-a-Number” (NaN) values, positive infinity, and negative infinity.
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).
Number value that is the positive infinite Number value.
Number value that is a IEEE 754 “Not-a-Number” value.
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.
built-in object that is a function.
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.
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.
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.
internal value that defines some characteristic of a property.
property that is directly contained by its object.
property of an object that is not an own property but is a property (either own or inherited) of the object’s prototype.
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.
PDF page 18 / printed page 8 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.
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.
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.
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.
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.
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:
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:
,
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:
is a convenient abbreviation for:
PDF page 20 / printed page 10 and that:
for (
ExpressionNoInopt
;
Expressionopt ;
Expressionopt )
Statement
is a convenient abbreviation for:
for (
;
Expressionopt ;
Expressionopt )
Statement
for (
ExpressionNoIn ;
Expressionopt ;
Expressionopt )
Statement
which in turn is an abbreviation for:
for (
; ;
Expressionopt )
Statement
for (
;
Expression ;
Expressionopt )
Statement
for (
ExpressionNoIn ;
;
Expressionopt )
Statement
for (
ExpressionNoIn ;
Expression ;
Expressionopt )
Statement
which in turn is an abbreviation for:
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
0
1
2
3
4
5
6
7
8
9
the definition
n
[lookahead ∉ {1,
3, 5,
7, 9}]
DecimalDigits
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.
PDF page 21 / printed page 11 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:
return [no
LineTerminator here]
Expressionopt ;
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:
1 2 3 4 5 6 7 8 9
which is merely a convenient abbreviation for:
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:
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:
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.
PDF page 22 / printed page 12 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:
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.
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 PDF page 23 / printed page 13 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 converted to UTF-16.
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.
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);
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.
| Code Unit Value | Name | Formal Name | Usage |
|---|---|---|---|
\u200C |
Zero width non-joiner | <ZWNJ> | IdentifierPart |
\u200D |
Zero width joiner | <ZWJ> | IdentifierPart |
\uFEFF |
Byte Order Mark | <BOM> | Whitespace |
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.
PDF page 25 / printed page 15| Code Unit Value | Name | Formal Name |
|---|---|---|
| \u0009 | Tab | <TAB> |
| \u000B | Vertical Tab | <VT> |
| \u000C | Form Feed | <FF> |
| \u0020 | Space | <SP> |
| \u00A0 | No-break space | <NBSP> |
| \uFEFF | Byte Order Mark | <BOM> |
| Other category “Zs” | Any other Unicode “space separator” | <USP> |
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.
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.
| Code Unit Value | Name | Formal Name |
|---|---|---|
\u000A |
Line Feed | <LF> |
\u000D |
Carriage Return | <CR> |
\u2028 |
Line separator | <LS> |
\u2029 |
Paragraph separator | <PS> |
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.
PDF page 26 / printed page 16Comments 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.
/*
MultiLineCommentCharsopt
*/
*
PostAsteriskCommentCharsopt
*
PostAsteriskCommentCharsopt
*
/
or
asterisk *
//
SingleLineCommentCharsopt
NOTE The DivPunctuator and RegularExpressionLiteral productions define tokens, but are not included in the Token production.
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.
$
_
\ UnicodeEscapeSequence
A reserved word is an IdentifierName that cannot be used as an Identifier.
The following tokens are ECMAScript keywords and may not be used as Identifiers in ECMAScript programs.
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
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.
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:
implements
let
private
public
yield
interface
package
protected
static
{
}
(
)
[
]
.
;
,
<
>
<=
>=
==
!=
===
!==
+
-
*
%
++
--
<<
>>
>>>
&
|
^
!
~
&&
||
?
:
=
+=
-=
*=
%=
<<=
>>=
>>>=
&=
|=
^=
/ /=
null
The value of the null literal null is the sole value
of the Null type, namely null.
true
false
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.
. DecimalDigitsopt ExponentPartopt
. DecimalDigits ExponentPartopt
0
0 1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
e E
+ DecimalDigits
- DecimalDigits
0x HexDigit
0X HexDigit
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.
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.
. is the MV of
DecimalIntegerLiteral.. DecimalDigits is
the MV of DecimalIntegerLiteral plus (the MV of
DecimalDigits times
10−n), where n is the number of
characters in DecimalDigits.. ExponentPart is
the MV of DecimalIntegerLiteral times
10e, where e is the MV of
ExponentPart.. DecimalDigits
ExponentPart is (the MV of
DecimalIntegerLiteral plus (the MV of
DecimalDigits times
10−n)) times 10e, where
n is the number of characters in
DecimalDigits and e is the MV of
ExponentPart..
DecimalDigits is the MV of
DecimalDigits times
10−n, where n is the number of
characters in DecimalDigits..
DecimalDigits
ExponentPart is the MV of
DecimalDigits times
10e−n, where n is the
number of characters in DecimalDigits and
e is the MV of ExponentPart.0 is 0.+
DecimalDigits is the MV of
DecimalDigits.-
DecimalDigits is the negative of the MV of
DecimalDigits.0 or of
HexDigit
:: 0 is 0.1 or of
NonZeroDigit
:: 1 or of
HexDigit
:: 1 is 1.2 or of
NonZeroDigit
:: 2 or of
HexDigit
:: 2 is 2.3 or of
NonZeroDigit
:: 3 or of
HexDigit
:: 3 is 3.4 or of
NonZeroDigit
:: 4 or of
HexDigit
:: 4 is 4.5 or of
NonZeroDigit
:: 5 or of
HexDigit
:: 5 is 5.6 or of
NonZeroDigit
:: 6 or of
HexDigit
:: 6 is 6.7 or of
NonZeroDigit
:: 7 or of
HexDigit
:: 7 is 7.8 or of
NonZeroDigit
:: 8 or of
HexDigit
:: 8 is 8.9 or of
NonZeroDigit
:: 9 or of
HexDigit
:: 9 is 9.a or of
HexDigit
:: A is 10.b or of
HexDigit
:: B is 11.c or of
HexDigit
:: C is 12.d or of
HexDigit
:: D is 13.e or of
HexDigit
:: E is 14.f or of
HexDigit
:: F is 15.0x
HexDigit is the MV of
HexDigit.0X
HexDigit is 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
0; orA 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.
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.
" DoubleStringCharactersopt "
' SingleStringCharactersopt '
" or backslash \ or LineTerminator
\ EscapeSequence
' or backslash \ or LineTerminator
\ EscapeSequence
\ LineTerminatorSequence
0 [lookahead ∉ DecimalDigit]
' " \ b f n r t v
x
u
x HexDigit HexDigit
u HexDigit HexDigit HexDigit HexDigit
The definition of the nonterminal HexDigit is given in 7.8.3. SourceCharacter is defined in clause 6.
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.
"" is the empty
character sequence.'' is the empty
character sequence."
DoubleStringCharacters
" is the SV of
DoubleStringCharacters.'
SingleStringCharacters
' is the SV of
SingleStringCharacters.\
LineTerminatorSequence is the empty
character sequence."
or backslash
\ or
LineTerminator is the
SourceCharacter character itself.\
EscapeSequence is the CV of the
EscapeSequence.'
or backslash
\ or
LineTerminator is the
SourceCharacter character itself.\
EscapeSequence is the CV of the
EscapeSequence.0
[lookahead
∉ DecimalDigit] is a
<NUL> character (Unicode value 0000).| 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 | \ |
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.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.
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.
/ RegularExpressionBody / RegularExpressionFlags
* or \ or / or [
\ or / or [
\ RegularExpressionNonTerminator
[ RegularExpressionClassChars ]
] or \
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:
/(?:)/.
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).
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.
There are three basic rules of semicolon insertion:
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:
}.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.
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:
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.
An Identifier in a break or
continue statement should be on the same line as the
break or continue token.
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()PDF page 38 / printed page 28
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.
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.
The Undefined type has exactly one value, called undefined. Any variable that has not been assigned a value has the value undefined.
The Null type has exactly one value, called null.
The Boolean type represents a logical entity having two values, called true and false.
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 PDF page 39 / printed page 29 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.
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 × 2e
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 × 2e
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.)
PDF page 40 / printed page 30Some 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.
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.
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:
| 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.
PDF page 41 / printed page 31| 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.
| Attribute Name | Default Value |
|---|---|
| [[Value]] | undefined |
| [[Get]] | undefined |
| [[Set]] | undefined |
| [[Writable]] | false |
| [[Enumerable]] | false |
| [[Configurable]] | false |
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 PDF page 42 / printed page 32 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.
| Internal Property | Value Type Domain | Description |
|---|---|---|
| [[Prototype]] | Object or Null | The prototype of this object. |
| [[Class]] | String | A String value indicating a specification defined classification of objects. |
| [[Extensible]] | Boolean | If true, own properties may be added to the object. |
| [[Get]] | SpecOp(propertyName) → any | Returns the value of the named property. |
| [[GetOwnProperty]] | SpecOp (propertyName) → Undefined or Property Descriptor | Returns the Property Descriptor of the named own property of this object, or undefined if absent. |
| [[GetProperty]] | SpecOp (propertyName) → Undefined or Property Descriptor | Returns the fully populated Property Descriptor of the named property of this object, or undefined if absent. |
| [[Put]] | SpecOp (propertyName, any, Boolean) | Sets the specified named property to the value of the second parameter. The flag controls failure handling. |
| [[CanPut]] | SpecOp (propertyName) → Boolean | Returns a Boolean value indicating whether a [[Put]] operation with PropertyName can be performed. |
| [[HasProperty]] | SpecOp (propertyName) → Boolean | Returns a Boolean value indicating whether the object already has a property with the given name. |
| [[Delete]] | SpecOp (propertyName, Boolean) → Boolean | Removes the specified named own property from the object. The flag controls failure handling. |
| [[DefaultValue]] | SpecOp (Hint) → primitive | Hint is a String. Returns a default value for the object. |
| [[DefineOwnProperty]] | SpecOp (propertyName, PropertyDescriptor, Boolean) → Boolean | Creates or alters the named own property to have the state described by a Property Descriptor. The flag controls failure handling. |
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. PDF page 43 / printed page 33 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.
PDF page 44 / printed page 34| Internal Property | Value Type Domain | Description |
|---|---|---|
| [[PrimitiveValue]] | primitive | Internal state information associated with this object. Of the standard built-in ECMAScript objects, only Boolean, Date, Number, and String objects implement [[PrimitiveValue]]. |
| [[Construct]] | SpecOp(a List of any) → Object | Creates an object. Invoked via the new operator.
The arguments to the SpecOp are the arguments passed to the
new operator. Objects that implement this
internal method are called constructors. |
| [[Call]] | SpecOp(any, a List of any) → any or Reference | Executes code associated with the object. Invoked via a function call expression. The arguments to the SpecOp are a this object and a list containing the arguments passed to the function call expression. Objects that implement this internal method are callable. Only callable objects that are host objects may return Reference values. |
| [[HasInstance]] | SpecOp(any) → Boolean | Returns a Boolean value indicating whether the argument is likely an Object that was constructed by this object. Of the standard built-in ECMAScript objects, only Function objects implement [[HasInstance]]. |
| [[Scope]] | Lexical Environment | A lexical environment that defines the environment in which a Function object is executed. Of the standard built-in ECMAScript objects, only Function objects implement [[Scope]]. |
| [[FormalParameters]] | List of Strings | A possibly empty List containing the identifier Strings of a Function’s FormalParameterList. Of the standard built-in ECMAScript objects, only Function objects implement [[FormalParameters]]. |
| [[Code]] | ECMAScript code | The ECMAScript code of a function. Of the standard built-in ECMAScript objects, only Function objects implement [[Code]]. |
| [[TargetFunction]] | Object | The target function of a function object created using the standard built-in Function.prototype.bind method. Only ECMAScript objects created using Function.prototype.bind have a [[TargetFunction]] internal property. |
| [[BoundThis]] | any | The pre-bound this value of a function Object created using the standard built-in Function.prototype.bind method. Only ECMAScript objects created using Function.prototype.bind have a [[BoundThis]] internal property. |
| [[BoundArguments]] | List of any | The pre-bound argument values of a function Object created using the standard built-in Function.prototype.bind method. Only ECMAScript objects created using Function.prototype.bind have a [[BoundArguments]] internal property. |
| [[Match]] | SpecOp(String, index) → MatchResult | Tests for a regular expression match and returns a MatchResult value (see 15.10.2.1). Of the standard built-in ECMAScript objects, only RegExp objects implement [[Match]]. |
| [[ParameterMap]] | Object | Provides a mapping between the properties of an arguments object (see 10.6) and the formal parameters of the associated function. Only ECMAScript objects that are arguments objects have a [[ParameterMap]] internal property. |
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 PDF page 45 / printed
page 35 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.
The following abstract operations are used in this specification to operate on references:
The following [[Get]] internal method is used by GetValue when V is a property reference with a primitive base value. It is called using base as its this value and with property P as its argument. The following steps are taken:
NOTE The object that may be created in step 1 is not accessible outside of the above method. An implementation might choose to avoid the actual creation of the object. The only situation where such an actual property access that uses this internal method can have visible effect is when it invokes an accessor function.
The following [[Put]] internal method is used by PutValue when V is a property reference with a primitive base value. It is called using base as its this value and with property P, value W, and Boolean flag Throw as arguments. The following steps are taken:
NOTE The object that may be created in step 1 is not accessible outside of the above method. An implementation might choose to avoid the actual creation of that transient object. The only situations where such an actual property assignment that uses this internal method can have visible effect are when it either invokes an accessor function or is in violation of a Throw predicated error check. When Throw is true any property assignment that would create a new property on the transient object throws an error.
The List type is used to explain the evaluation of argument lists
(see 11.2.4) in new
expressions, in function calls, and in other algorithms where a simple list of
values is needed. Values of the List type are simply ordered sequences of
values. These sequences may be of any length.
The Completion type is used to explain the behaviour of statements (break, continue, return and throw) that perform nonlocal transfers of control. Values of the Completion type are triples of the form (type, value, target), where type is one of normal, break, continue, return, or throw, value is any ECMAScript language value or empty, and target is any ECMAScript identifier or empty.
The term “abrupt completion” refers to any completion with a type other than normal.
The Property Descriptor type is used to explain the manipulation and reification of named property attributes. Values of the Property Descriptor type are records composed of named fields where each field’s name is an attribute name and its value is a corresponding attribute value as specified in 8.6.1. In addition, any field may be present or absent.
Property Descriptor values may be further classified as data property descriptors and accessor property descriptors based upon the existence or use of certain fields. A data property descriptor is one that includes any fields named either [[Value]] or [[Writable]]. An accessor property descriptor is one that includes any fields named either [[Get]] or [[Set]]. Any property descriptor may have fields named [[Enumerable]] and [[Configurable]]. A Property Descriptor value may not be both a data property descriptor and an accessor property descriptor; however, it may be neither. A generic property descriptor is a Property Descriptor value that is neither a data property descriptor nor an accessor property descriptor. A fully populated property descriptor is one that is either an accessor property descriptor or a data property descriptor and that has all of the fields that correspond to the property attributes defined in either 8.6.1 Table 5 or Table 6.
For notational convenience within this specification, an object literal-like syntax can be used to define a property descriptor value. For example, Property Descriptor {[[Value]]: 42, [[Writable]]: false, [[Configurable]]: true} defines a data property descriptor. Field name order is not significant. Any fields that are not explicitly listed are considered to be absent.
In specification text and algorithms, dot notation may be used to refer to a specific field of a Property Descriptor. For example, if D is a property descriptor then D.[[Value]] is shorthand for “the field of D named [[Value]]”.
The Property Identifier type is used to associate a property name with a Property Descriptor. Values of the Property Identifier type are pairs of the form (name, descriptor), where name is a String and descriptor is a Property Descriptor value.
The following abstract operations are used in this specification to operate upon Property Descriptor values:
When the abstract operation IsAccessorDescriptor is called with property descriptor Desc, the following steps are taken:
When the abstract operation IsDataDescriptor is called with property descriptor Desc, the following steps are taken:
When the abstract operation IsGenericDescriptor is called with property descriptor Desc, the following steps are taken:
When the abstract operation FromPropertyDescriptor is called with property descriptor Desc, the following steps are taken:
The following algorithm assumes that Desc is a fully populated Property Descriptor, such as that returned from [[GetOwnProperty]] (see 8.12.1).
new Object() where
Object is the standard built-in constructor
with that name.When the abstract operation ToPropertyDescriptor is called with object Desc, the following steps are taken:
The Lexical Environment and Environment Record types are used to explain the behaviour of name resolution in nested functions and blocks. These types and the operations upon them are defined in Clause 10.
In the following algorithm descriptions, assume O is a native ECMAScript object, P is a String, Desc is a Property Description record, and Throw is a Boolean flag.
When the [[GetOwnProperty]] internal method of O is called with property name P, the following steps are taken:
However, if O is a String object it has a more elaborate [[GetOwnProperty]] internal method defined in 15.5.5.2.
When the [[GetProperty]] internal method of O is called with property name P, the following steps are taken:
When the [[Get]] internal method of O is called with property name P, the following steps are taken:
When the [[CanPut]] internal method of O is called with property name P, the following steps are taken:
Host objects may define additional constraints upon [[Put]] operations. If possible, host objects should not allow [[Put]] operations in situations where this definition of [[CanPut]] returns false.
When the [[Put]] internal method of O is called with property P, value V, and Boolean flag Throw, the following steps are taken:
When the [[HasProperty]] internal method of O is called with property name P, the following steps are taken:
When the [[Delete]] internal method of O is called with property name P and the Boolean flag Throw, the following steps are taken:
When the [[DefaultValue]] internal method of O is called with hint String, the following steps are taken:
When the [[DefaultValue]] internal method of O is called with hint Number, the following steps are taken:
When the [[DefaultValue]] internal method of O is called with no hint, then it behaves as if the hint were Number, unless O is a Date object (see 15.9.6), in which case it behaves as if the hint were String.
The above specification of [[DefaultValue]] for native objects can return only primitive values. If a host object implements its own [[DefaultValue]] internal method, it must ensure that its [[DefaultValue]] internal method can return only primitive values.
In the following algorithm, the term “Reject” means “If Throw is true, then throw a TypeError exception, otherwise return false”. The algorithm contains steps that test various fields of the Property Descriptor Desc for specific values. The fields that are tested in this manner need not actually exist in Desc. If a field is absent then its value is considered to be false.
When the [[DefineOwnProperty]] internal method of O is called with property name P, property descriptor Desc, and Boolean flag Throw, the following steps are taken:
NOTE Step 10.b allows any field of Desc to be different from the corresponding field of current if current’s [[Configurable]] field is true. This even permits changing the [[Value]] of a property whose [[Writable]] attribute is false. This is allowed because a true [[Configurable]] attribute would permit an equivalent sequence of calls where [[Writable]] is first set to true, a new [[Value]] is set, and then [[Writable]] is set to false.
The ECMAScript runtime system performs automatic type conversion as needed. To clarify the semantics of certain constructs it is useful to define a set of conversion abstract operations. These abstract operations are not a part of the language; they are defined here to aid the specification of the semantics of the language. The conversion abstract operations are polymorphic; that is, they can accept a value of any ECMAScript language type, but not of specification types.
The abstract operation ToPrimitive takes an input argument and an optional argument PreferredType. The abstract operation ToPrimitive converts its input argument to a non-Object type. If an object is capable of converting to more than one primitive type, it may use the optional hint PreferredType to favour that type. Conversion occurs according to Table 10:
| Input Type | Result |
|---|---|
| Undefined | The result equals the input argument (no conversion). |
| Null | The result equals the input argument (no conversion). |
| Boolean | The result equals the input argument (no conversion). |
| Number | The result equals the input argument (no conversion). |
| String | The result equals the input argument (no conversion). |
| Object | Return a default value for the Object. The default value of an object is retrieved by calling the [[DefaultValue]] internal method of the object, passing the optional hint PreferredType. The behaviour of the [[DefaultValue]] internal method is defined by this specification for all native ECMAScript objects in 8.12.8. |
The abstract operation ToBoolean converts its argument to a value of type Boolean according to Table 11:
| Argument Type | Result |
|---|---|
| Undefined | false |
| Null | false |
| Boolean | The result equals the input argument (no conversion). |
| Number | The result is false if the argument is +0, −0, or NaN; otherwise the result is true. |
| String | The result is false if the argument is the empty String (its length is zero); otherwise the result is true. |
| Object | true |
The abstract operation ToNumber converts its argument to a value of type Number according to Table 12:
PDF page 54 / printed page 44| Argument Type | Result |
|---|---|
| Undefined | NaN |
| Null | +0 |
| Boolean | The result is 1 if the argument is true. The result is +0 if the argument is false. |
| Number | The result equals the input argument (no conversion). |
| String | See grammar and note below. |
| Object | Apply the following steps:
|
ToNumber applied to Strings applies the following grammar to the input String. If the grammar cannot interpret the String as an expansion of StringNumericLiteral, then the result of ToNumber is NaN.
+ StrUnsignedDecimalLiteral
- StrUnsignedDecimalLiteral
Infinity
. DecimalDigitsopt ExponentPartopt
. DecimalDigits ExponentPartopt
0 1 2 3 4 5 6 7 8 9
e E
+ DecimalDigits
- DecimalDigits
0x HexDigit
0X HexDigit
0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F
Some differences should be noted between the syntax of a StringNumericLiteral and a NumericLiteral (see 7.8.3):
0 digits.+ or - to
indicate its sign.The conversion of a String to a Number value is similar overall to the determination of the Number value for a numeric literal (see 7.8.3), but some of the details are different, so the process for converting a String numeric literal to a value of Number type is given here in full. This value is determined in two steps: first, a mathematical value (MV) is derived from the String numeric literal; second, this mathematical value is rounded as described below.
+
StrUnsignedDecimalLiteral is the MV of
StrUnsignedDecimalLiteral.-
StrUnsignedDecimalLiteral is the negative
of the MV of StrUnsignedDecimalLiteral. (Note
that if the MV of StrUnsignedDecimalLiteral is 0,
the negative of this MV is also 0. The rounding rule described below
handles the conversion of this signless mathematical zero to a
floating-point +0 or
−0 as appropriate.)Infinity is
1010000 (a value so large that it will round to
+∞).. is the MV of
DecimalDigits.. DecimalDigits is
the MV of the first DecimalDigits plus (the MV of
the second DecimalDigits times
10−n), where n is the number of
characters in the second DecimalDigits.. ExponentPart is
the MV of DecimalDigits times
10e, where e is the MV of
ExponentPart.. DecimalDigits
ExponentPart is (the MV of the first
DecimalDigits plus (the MV of the second
DecimalDigits times
10−n)) times 10e, where
n is the number of characters in the second
DecimalDigits and e is the MV of
ExponentPart..
DecimalDigits is the MV of
DecimalDigits times
10−n, where n is the number of
characters in DecimalDigits.+
DecimalDigits is the MV of
DecimalDigits.-
DecimalDigits is the negative of the MV of
DecimalDigits.0 or of
HexDigit
::: 0 is 0.1 or of
HexDigit
::: 1 is 1.2 or of
HexDigit
::: 2 is 2.3 or of
HexDigit
::: 3 is 3.4 or of
HexDigit
::: 4 is 4.5 or of
HexDigit
::: 5 is 5.6 or of
HexDigit
::: 6 is 6.7 or of
HexDigit
::: 7 is 7.8 or of
HexDigit
::: 8 is 8.9 or of
HexDigit
::: 9 is 9.a or of
HexDigit
::: A is 10.b or of
HexDigit
::: B is 11.c or of
HexDigit
::: C is 12.d or of
HexDigit
::: D is 13.e or of
HexDigit
::: E is 14.f or of
HexDigit
::: F is 15.0x
HexDigit is the MV of
HexDigit.0X
HexDigit is the MV of
HexDigit.Once the exact MV for a String 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 unless the first non white space character in the String numeric
literal is ‘-’, in which case the rounded
value is −0. Otherwise, the rounded value must be the Number value for
the MV (in the sense defined in 8.5), unless the literal
includes a StrUnsignedDecimalLiteral 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 digit position. A digit
is significant if it is not part of an
ExponentPart and
0; orThe abstract operation ToInteger converts its argument to an integral numeric value. This abstract operation functions as follows:
The abstract operation ToInt32 converts its argument to one of 232 integer values in the range −231 through 231−1, inclusive. This abstract operation functions as follows:
PDF page 57 / printed page 47NOTE Given the above definition of ToInt32:
The abstract operation ToUint32 converts its argument to one of 232 integer values in the range 0 through 232−1, inclusive. This abstraction operation functions as follows:
NOTE Given the above definition of ToUInt32:
The abstract operation ToUint16 converts its argument to one of 216 integer values in the range 0 through 216−1, inclusive. This abstract operation functions as follows:
NOTE Given the above definition of ToUint16:
The abstract operation ToString converts its argument to a value of type String according to Table 13:
PDF page 58 / printed page 48| Argument Type | Result |
|---|---|
| Undefined | "undefined" |
| Null | "null" |
| Boolean | If the argument is true, then the result is "true". If the argument is false, then the result is "false". |
| Number | See 9.8.1. |
| String | Return the input argument (no conversion) |
| Object | Apply the following steps:
|
The abstract operation ToString converts a Number m to String format as follows:
0’..’, followed by the remaining
k−n digits of the decimal representation of
s.0’, followed by a decimal
point ‘.’, followed by
−n occurrences of the character
‘0’, followed by the k
digits of the decimal representation of s.e’, followed by a plus sign
‘+’ or minus sign
‘-’ according to whether
n−1 is positive or negative, followed by the decimal
representation of the integer
abs(n−1) (with no leading
zeros)..’, followed by the remaining
k−1 digits of the decimal representation of
s, followed by the lowercase character
‘e’, followed by a plus sign
‘+’ or minus sign
‘-’ according to whether
n−1 is positive or negative, followed by the decimal
representation of the integer
abs(n−1) (with no leading
zeros).NOTE 1 The following observations may be useful as guidelines for implementations, but are not part of the normative requirements of this Standard:
NOTE 2 For implementations that provide more accurate conversions than required by the rules above, it is recommended that the following alternative version of step 5 be used as a guideline:
Otherwise, let n, k, and s be integers such that k ≥ 1, 10k−1 ≤ s < 10k, the Number value for s × 10n−k is m, and k is as small as possible. If there are multiple possibilities for s, choose the value of s for which s × 10n−k is closest in value to m. If there are two such possible values of s, choose the one that is even. Note that k is the number of digits in the decimal representation of s and that s is not divisible by 10.
NOTE 3 Implementers of ECMAScript may find useful the paper and code written by David M. Gay for binary-to-decimal conversion of floating-point numbers:
PDF page 59 / printed page 49 Gay, David M. Correctly Rounded Binary-Decimal and Decimal-Binary Conversions. Numerical Analysis, Manuscript 90-10. AT&T Bell Laboratories (Murray Hill, New Jersey). November 30, 1990. Available as http://cm.bell-labs.com/cm/cs/doc/90/4-10.ps.gz. Associated code available as http://cm.bell-labs.com/netlib/fp/dtoa.c.gz and as http://cm.bell-labs.com/netlib/fp/g_fmt.c.gz and may also be found at the various netlib mirror sites.
The abstract operation ToObject converts its argument to a value of type Object according to Table 14:
| Argument Type | Result |
|---|---|
| Undefined | Throw a TypeError exception. |
| Null | Throw a TypeError exception. |
| Boolean | Create a new Boolean object whose [[PrimitiveValue]] internal property is set to the value of the argument. See 15.6 for a description of Boolean objects. |
| Number | Create a new Number object whose [[PrimitiveValue]] internal property is set to the value of the argument. See 15.7 for a description of Number objects. |
| String | Create a new String object whose [[PrimitiveValue]] internal property is set to the value of the argument. See 15.5 for a description of String objects. |
| Object | The result is the input argument (no conversion). |
The abstract operation CheckObjectCoercible throws an error if its argument is a value that cannot be converted to an Object using ToObject. It is defined by Table 15:
| Argument Type | Result |
|---|---|
| Undefined | Throw a TypeError exception. |
| Null | Throw a TypeError exception. |
| Boolean | Return |
| Number | Return |
| String | Return |
| Object | Return |
The abstract operation IsCallable determines if its argument, which must be an ECMAScript language value, is a callable function Object according to Table 16:
PDF page 60 / printed page 50| Argument Type | Result |
|---|---|
| Undefined | Return false. |
| Null | Return false. |
| Boolean | Return false. |
| Number | Return false. |
| String | Return false. |
| Object | If the argument object has a [[Call]] internal method, then return true, otherwise return false. |
The internal comparison abstract operation SameValue(x, y), where x and y are ECMAScript language values, produces true or false. Such a comparison is performed as follows:
There are three types of ECMAScript executable code:
Global code is source text that is treated as an ECMAScript Program. The global code of a particular Program does not include any source text that is parsed as part of a FunctionBody.
Eval code is the source text supplied to the built-in eval function. More precisely, if the parameter to the built-in eval function is a String, it is treated as an ECMAScript Program. The eval code for a particular invocation of eval is the global code portion of that Program.
Function code is source text that is parsed as part of a FunctionBody. The function code of a particular FunctionBody does not include any source text that is parsed as part of a nested FunctionBody. Function code also denotes the source text supplied when using the built-in Function object as a constructor. More precisely, the last parameter provided to the Function constructor is converted to a String and treated as the FunctionBody. If more than one parameter is provided to the Function constructor, all parameters except the last one are converted to Strings and concatenated together, separated by commas. The resulting String is interpreted as the FormalParameterList for the FunctionBody defined by the last parameter. The function code for a particular instantiation of a Function does not include any source text that is parsed as part of a nested FunctionBody.
An ECMAScript Program syntactic unit may be processed using either unrestricted or strict mode syntax and semantics. When processed using strict mode the three types of ECMAScript code are referred to as strict global code, strict eval code, and strict function code. Code is interpreted as strict mode code in the following situations:
Global code is strict global code if it begins with a Directive Prologue that contains a Use Strict Directive (see 14.1).
Eval code is strict eval code if it begins with a Directive Prologue that contains a Use Strict Directive or if the call to eval is a direct call (see 15.1.2.1.1) to the eval function that is contained in strict mode code.
Function code that is part of a FunctionDeclaration, FunctionExpression, or accessor PropertyAssignment is strict function code if its FunctionDeclaration, FunctionExpression, or PropertyAssignment is contained in strict mode code or if the function code begins with a Directive Prologue that contains a Use Strict Directive.
Function code that is supplied as the last argument to the built-in Function constructor is strict function code if the last argument is a String that when processed as a FunctionBody begins with a Directive Prologue that contains a Use Strict Directive.
A Lexical Environment is a specification type used to define the association of Identifiers to specific variables and functions based upon the lexical nesting structure of ECMAScript code. A Lexical Environment consists of an Environment Record and a possibly null reference to an outer Lexical Environment. Usually a Lexical Environment is associated with some specific syntactic structure of ECMAScript code such as a FunctionDeclaration, a WithStatement, or a Catch clause of a TryStatement and a new Lexical Environment is created each time such code is evaluated.
An Environment Record records the identifier bindings that are created within the scope of its associated Lexical Environment.
The outer environment reference is used to model the logical nesting of Lexical Environment values. The outer reference of a (inner) Lexical Environment is a reference to the Lexical Environment that logically surrounds the inner Lexical Environment. An outer Lexical Environment may, of course, have its own outer Lexical Environment. A Lexical Environment may serve as the outer environment for multiple inner Lexical Environments. For example, if a FunctionDeclaration contains two nested FunctionDeclarations then the Lexical Environments of each of the nested functions will have as their outer Lexical Environment the Lexical Environment of the current execution of the surrounding function.
Lexical Environments and Environment Record values are purely specification mechanisms and need not correspond to any specific artefact of an ECMAScript implementation. It is impossible for an ECMAScript program to directly access or manipulate such values.
There are two kinds of Environment Record values used in this specification: declarative environment records and object environment records. Declarative environment records are used to define the effect of ECMAScript language syntactic elements such as FunctionDeclarations, VariableDeclarations, and Catch clauses that directly associate identifier bindings with ECMAScript language values. Object environment records are used to define the effect of ECMAScript elements such as Program and WithStatement that associate identifier bindings with the properties of some object.
For specification purposes Environment Record values can be thought of as existing in a simple object-oriented hierarchy where Environment Record is an abstract class with two concrete subclasses, declarative environment record and object environment record. The abstract class includes the abstract specification PDF page 62 / printed page 52 methods defined in Table 17. These abstract methods have distinct concrete algorithms for each of the concrete subclasses.
| Method | Purpose |
|---|---|
| HasBinding(N) | Determine if an environment record has a binding for an identifier. Return true if it does and false if it does not. The String value N is the text of the identifier. |
| CreateMutableBinding(N, D) | Create a new mutable binding in an environment record. The String value N is the text of the bound name. If the optional Boolean argument D is true the binding is may be subsequently deleted. |
| SetMutableBinding(N, V, S) | Set the value of an already existing mutable binding in an environment record. The String value N is the text of the bound name. V is the value for the binding and may be a value of any ECMAScript language type. S is a Boolean flag. If S is true and the binding cannot be set throw a TypeError exception. S is used to identify strict mode references. |
| GetBindingValue(N, S) | Returns the value of an already existing binding from an environment record. The String value N is the text of the bound name. S is used to identify strict mode references. If S is true and the binding does not exist or is uninitialized throw a ReferenceError exception. |
| DeleteBinding(N) | Delete a binding from an environment record. The String value N is the text of the bound name If a binding for N exists, remove the binding and return true. If the binding exists but cannot be removed return false. If the binding does not exist return true. |
| ImplicitThisValue() | Returns the value to use as the this value on calls to function objects that are obtained as binding values from this environment record. |
Each declarative environment record is associated with an ECMAScript program scope containing variable and/or function declarations. A declarative environment record binds the set of identifiers defined by the declarations contained within its scope.
In addition to the mutable bindings supported by all Environment Records, declarative environment records also provide for immutable bindings. An immutable binding is one where the association between an identifier and a value may not be modified once it has been established. Creation and initialization of immutable binding are distinct steps so it is possible for such bindings to exist in either an initialized or uninitialized state. Declarative environment records support the methods listed in Table 18 in addition to the Environment Record abstract specification methods:
| Method | Purpose |
|---|---|
| CreateImmutableBinding(N) | Create a new but uninitialized immutable binding in an environment record. The String value N is the text of the bound name. |
| InitializeImmutableBinding(N, V) | Set the value of an already existing but uninitialized immutable binding in an environment record. The String value N is the text of the bound name. V is the value for the binding and is a value of any ECMAScript language type. |
PDF page 63 / printed page 53 The behaviour of the concrete specification methods for Declarative Environment Records are defined by the following algorithms.
The concrete environment record method HasBinding for declarative environment records simply determines if the argument identifier is one of the identifiers bound by the record:
The concrete Environment Record method CreateMutableBinding for declarative environment records creates a new mutable binding for the name N that is initialized to the value undefined. A binding must not already exist in this Environment Record for N. If Boolean argument D is provided and has the value true the new binding is marked as being subject to deletion.
The concrete Environment Record method SetMutableBinding for declarative environment records attempts to change the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. A binding for N must already exist. If the binding is an immutable binding, a TypeError is thrown if S is true.
The concrete Environment Record method GetBindingValue for declarative environment records simply returns the value of its bound identifier whose name is the value of the argument N. The binding must already exist. If S is true and the binding is an uninitialized immutable binding throw a ReferenceError exception.
The concrete Environment Record method DeleteBinding for declarative environment records can only delete bindings that have been explicitly designated as being subject to deletion.
Declarative Environment Records always return undefined as their ImplicitThisValue.
The concrete Environment Record method CreateImmutableBinding for declarative environment records creates a new immutable binding for the name N that is initialized to the value undefined. A binding must not already exist in this environment record for N.
The concrete Environment Record method InitializeImmutableBinding for declarative environment records is used to set the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. An uninitialized immutable binding for N must already exist.
Each object environment record is associated with an object called its binding object. An object environment record binds the set of identifier names that directly correspond to the property names of its binding object. Property names that are not an IdentifierName are not included in the set of bound identifiers. Both own and inherited properties are included in the set regardless of the setting of their [[Enumerable]] attribute. Because properties can be dynamically added and deleted from objects, the set of identifiers bound by an object environment record may potentially change as a side-effect of any operation that adds or deletes properties. Any bindings that are created as a result of such a side-effect are considered to be a mutable binding even if the Writable attribute of the corresponding property has the value false. Immutable bindings do not exist for object environment records.
Object environment records can be configured to provide their binding object as an implicit this value for use in function calls. This capability is used to specify the behaviour of With Statement (12.10) induced bindings. The capability is controlled by a provideThis Boolean value that is associated with each object environment record. By default, the value of provideThis is false for any object environment record.
The behaviour of the concrete specification methods for Object Environment Records is defined by the following algorithms.
The concrete Environment Record method HasBinding for object environment records determines if its associated binding object has a property whose name is the value of the argument N:
The concrete Environment Record method CreateMutableBinding for object environment records creates in an environment record’s associated binding object a property whose name is the String value and initializes it to the value undefined. A property named N must not already exist in the binding object. If Boolean argument D is provided and has the value true the new property’s [[Configurable]] attribute is set to true, otherwise it is set to false.
The concrete Environment Record method SetMutableBinding for object environment records attempts to set the value of the environment record’s associated binding object’s property whose name is the value of the argument N to the value of argument V. A property named N should already exist but if it does not or is not currently writable, error handling is determined by the value of the Boolean argument S.
The concrete Environment Record method GetBindingValue for object environment records returns the value of its associated binding object’s property whose name is the String value of the argument identifier N. The property should already exist but if it does not the result depends upon the value of the S argument:
The concrete Environment Record method DeleteBinding for object environment records can only delete bindings that correspond to properties of the environment object whose [[Configurable]] attribute have the value true.
Object Environment Records return undefined as their ImplicitThisValue unless their provideThis flag is true.
The following abstract operations are used in this specification to operate upon lexical environments:
The abstract operation GetIdentifierReference is called with a Lexical Environment lex, an identifier String name, and a Boolean flag strict. The value of lex may be null. When called, the following steps are performed:
When the abstract operation NewDeclarativeEnvironment is called with either a Lexical Environment or null as argument E the following steps are performed:
When the abstract operation NewObjectEnvironmentis called with an Object O and a Lexical Environment E (or null) as arguments, the following steps are performed:
The global environment is a unique Lexical Environment which is created before any ECMAScript code is executed. The global environment’s Environment Record is an object environment record whose binding object is the global object (15.1). The global environment’s outer environment reference is null.
As ECMAScript code is executed, additional properties may be added to the global object and the initial properties may be modified.
When control is transferred to ECMAScript executable code, control is entering an execution context. Active execution contexts logically form a stack. The top execution context on this logical stack is the running execution context. A new execution context is created whenever control is transferred from the executable code associated with the currently running execution context to executable code that is not associated with PDF page 67 / printed page 57 that execution context. The newly created execution context is pushed onto the stack and becomes the running execution context.
An execution context contains whatever state is necessary to track the execution progress of its associated code. In addition, each execution context has the state components listed in Table 19.
| Component | Purpose |
|---|---|
| LexicalEnvironment | Identifies the Lexical Environment used to resolve identifier references made by code within this execution context. |
| VariableEnvironment | Identifies the Lexical Environment whose environment record holds bindings created by VariableStatements and FunctionDeclarations within this execution context. |
| ThisBinding | The value associated with the this keyword
within ECMAScript code associated with this execution context. |
The LexicalEnvironment and VariableEnvironment components of an execution context are always Lexical Environments. When an execution context is created its LexicalEnvironment and VariableEnvironment components initially have the same value. The value of the VariableEnvironment component never changes while the value of the LexicalEnvironment component may change during execution of code within an execution context.
In most situations only the running execution context (the top of the execution context stack) is directly manipulated by algorithms within this specification. Hence when the terms “LexicalEnvironment”, “VariableEnvironment” and “ThisBinding” are used without qualification they are in reference to those components of the running execution context.
An execution context is purely a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation. It is impossible for an ECMAScript program to access an execution context.
Identifier resolution is the process of determining the binding of an Identifier using the LexicalEnvironment of the running execution context. During execution of ECMAScript code, the syntactic production PrimaryExpression : Identifier is evaluated using the following algorithm:
The result of evaluating an identifier is always a value of type Reference with its referenced name component equal to the Identifier String.
Evaluation of global code or code using the eval function (15.1.2.1) establishes and enters a new execution context. Every invocation of an ECMAScript code function (13.2.1) also establishes and enters a new execution context, even if a function is calling itself recursively. Every return exits an execution context. A thrown exception may also exit one or more execution contexts.
When control enters an execution context, the execution context’s ThisBinding is set, its VariableEnvironment and initial LexicalEnvironment are defined, and declaration binding instantiation (10.5) is performed. The exact manner in which these actions occur depend on the type of code being entered.
PDF page 68 / printed page 58The following steps are performed when control enters the execution context for global code:
The following steps are performed to initialize a global execution context for ECMAScript code C:
The following steps are performed when control enters the execution context for eval code:
The eval code cannot instantiate variable or function bindings in the variable environment of the calling context that invoked the eval if either the code of the calling context or the eval code is strict code. Instead such bindings are instantiated in a new VariableEnvironment that is only accessible to the eval code.
The following steps are performed when control enters the execution context for function code contained in function object F, a caller provided thisArg, and a caller provided argumentsList:
Every execution context has an associated VariableEnvironment. Variables and functions declared in ECMAScript code evaluated in an execution context are added as bindings in that VariableEnvironment’s Environment Record. For function code, parameters are also added as bindings to that Environment Record.
Which Environment Record is used to bind a declaration and its kind depends upon the type of ECMAScript code executed by the execution context, but the remainder of the behaviour is generic. On entering an execution context, bindings are created in the VariableEnvironment as follows using the caller provided code and, if it is function code, argument List args:
When control enters an execution context for function code, an arguments
object is created unless (as specified in 10.5) the
identifier arguments occurs as an
Identifier in the function’s
FormalParameterList or occurs as the
Identifier of a
VariableDeclaration or
FunctionDeclaration contained in the function code.
The arguments object is created by calling the abstract operation CreateArgumentsObject with arguments func the function object whose code is to be evaluated, names a List containing the function’s formal parameter names, args the actual arguments passed to the [[Call]] internal method, env the variable environment for the function code, and strict a Boolean that indicates whether or not the function code is strict code. When CreateArgumentsObject is called the following steps are performed:
new Object() where
Object is the standard built-in constructor
with that name.The abstract operation MakeArgGetter called with String name and environment record env creates a function object that when executed returns the value bound for name in env. It performs the following steps:
The abstract operation MakeArgSetter called with String name and environment record env creates a function object that when executed sets the value bound for name in env. It performs the following steps:
The [[Get]] internal method of an arguments object for a non-strict mode function with formal parameters when called with a property name P performs the following steps:
The [[GetOwnProperty]] internal method of an arguments object for a non-strict mode function with formal parameters when called with a property name P performs the following steps:
The [[DefineOwnProperty]] internal method of an arguments object for a non-strict mode function with formal parameters when called with a property name P, Property Descriptor Desc, and Boolean flag Throw performs the following steps:
The [[Delete]] internal method of an arguments object for a non-strict mode function with formal parameters when called with a property name P and Boolean flag Throw performs the following steps:
NOTE 1 For non-strict mode functions the array index (defined in 15.4) named data properties of an arguments object whose numeric name values are less than the number of formal parameters of the corresponding function object initially share their values with the corresponding argument bindings in the function’s execution context. This means that changing the property changes the corresponding value of the argument binding and vice-versa. This correspondence is broken if such a property is deleted and then redefined or if the property is changed into an accessor property. For strict mode functions, the values of the arguments object’s properties are simply a copy of the arguments passed to the function and there is no dynamic linkage between the property values and the formal parameter values.
NOTE 2 The ParameterMap object and its property values are used as a device for specifying the arguments object correspondence to argument bindings. The ParameterMap object and the objects that are the values of its properties are not directly accessible from ECMAScript code. An ECMAScript implementation does not need to actually create or use such objects to implement the specified semantics.
NOTE 3 Arguments objects for strict mode functions
define non-configurable accessor properties named
“caller” and
“callee” which throw a
TypeError exception on access. The
“callee” property has a more
specific meaning for non-strict mode functions and a
“caller” property has historically
been provided as an implementation-defined extension by some ECMAScript
implementations. The strict mode definition of these properties exists to
ensure that neither of them is defined in any other manner by conforming
ECMAScript implementations.
this
( Expression )
The this keyword evaluates to the value of the
ThisBinding of the current execution context.
An Identifier is evaluated by performing Identifier Resolution as specified in 10.3.1. The result of evaluating an Identifier is always a value of type Reference.
A Literal is evaluated as described in 7.8.
An array initialiser is an expression describing the initialisation of an Array object, written in a form of a literal. It is a list of zero or more expressions, each of which represents an array element, enclosed in square brackets. The elements need not be literals; they are evaluated each time the array initialiser is evaluated.
Array elements may be elided at the beginning, middle or end of the element list. Whenever a comma in the element list is not preceded by an AssignmentExpression (i.e., a comma at the beginning or after another comma), the missing array element contributes to the length of the Array and increases the index of subsequent elements. Elided array elements are not defined. If an element is elided at the end of an array, that element does not contribute to the length of the Array.
[ Elisionopt ]
[ ElementList ]
[ ElementList , Elisionopt ]
, Elisionopt AssignmentExpression
,
,
The production ArrayLiteral
: [
Elisionopt ]
is evaluated as follows:
new Array() where
Array is the standard built-in constructor with
that name.The production ArrayLiteral
: [
ElementList ] is
evaluated as follows:
The production ArrayLiteral
: [
ElementList ,
Elisionopt ]
is evaluated as follows:
The production ElementList : Elisionopt AssignmentExpression is evaluated as follows:
new Array() where
Array is the standard built-in constructor with
that name.The production ElementList
: ElementList
, Elisionopt
AssignmentExpression is evaluated as
follows:
The production Elision
: , is evaluated as
follows:
The production Elision
: Elision
, is evaluated as follows:
NOTE [[DefineOwnProperty]] is used to ensure that own properties are defined for the array even if the standard built-in Array prototype object has been modified in a manner that would preclude the creation of new own properties using [[Put]].
An object initialiser is an expression describing the initialisation of an Object, written in a form resembling a literal. It is a list of zero or more pairs of property names and associated values, enclosed in curly braces. The values need not be literals; they are evaluated each time the object initialiser is evaluated.
{ }
{ PropertyNameAndValueList }
{ PropertyNameAndValueList , }
, PropertyAssignment
: AssignmentExpression
get PropertyName ( ) { FunctionBody }
set PropertyName ( PropertySetParameterList ) { FunctionBody }
The production ObjectLiteral
: {
} is evaluated as follows:
new Object() where
Object is the standard built-in constructor
with that name.The productions ObjectLiteral
: {
PropertyNameAndValueList
} and
ObjectLiteral
: {
PropertyNameAndValueList ,
} are evaluated as follows:
The production PropertyNameAndValueList : PropertyAssignment is evaluated as follows:
new Object() where
Object is the standard built-in constructor
with that name.The production
PropertyNameAndValueList
: PropertyNameAndValueList
, PropertyAssignment is
evaluated as follows:
If the above steps would throw a SyntaxError then an implementation must treat the error as an early error (Clause 16).
The production PropertyAssignment
: PropertyName
: AssignmentExpression is
evaluated as follows:
PDF page 76 / printed page 66 The
production PropertyAssignment
: get
PropertyName (
) {
FunctionBody } is
evaluated as follows:
The production PropertyAssignment
: set
PropertyName (
PropertySetParameterList )
{ FunctionBody
} is evaluated as follows:
It is a SyntaxError if the Identifier "eval" or the Identifier "arguments" occurs as the Identifier in a PropertySetParameterList of a PropertyAssignment that is contained in strict code or if its FunctionBody is strict code.
The production PropertyName : IdentifierName is evaluated as follows:
The production PropertyName : StringLiteral is evaluated as follows:
The production PropertyName : NumericLiteral is evaluated as follows:
The production PrimaryExpression
: (
Expression ) is evaluated
as follows:
NOTE This algorithm does not apply
GetValue to the result of evaluating Expression. The
principal motivation for this is so that operators such as
delete and typeof may be
applied to parenthesised expressions.
[ Expression ]
. IdentifierName
new MemberExpression Arguments
new NewExpression
[ Expression ]
. IdentifierName
( )
( ArgumentList )
, AssignmentExpression
Properties are accessed by name, using either the dot notation:
.
IdentifierName
. IdentifierName
or the bracket notation:
[
Expression ]
[
Expression ]
The dot notation is explained by the following syntactic conversion:
.
IdentifierName
is identical in its behaviour to
[
<identifier-name-string> ]
and similarly
.
IdentifierName
is identical in its behaviour to
[
<identifier-name-string> ]
where <identifier-name-string> is a string literal containing the same sequence of characters after processing of Unicode escape sequences as the IdentifierName.
The production MemberExpression
: MemberExpression
[ Expression
] is evaluated as follows:
The production CallExpression
: CallExpression
[ Expression
] is evaluated in exactly the same manner, except
that the contained CallExpression is evaluated in step
1.
The production NewExpression
: new
NewExpression is evaluated as follows:
The production MemberExpression
: new
MemberExpression
Arguments is evaluated as follows:
The production CallExpression : MemberExpression Arguments is evaluated as follows:
The production CallExpression : CallExpression Arguments is evaluated in exactly the same manner, except that the contained CallExpression is evaluated in step 1.
NOTE The returned result will never be of type Reference if func is a native ECMAScript object. Whether calling a host object can return a value of type Reference is implementation-dependent. If a value of type Reference is returned, it must be a non-strict Property Reference.
The evaluation of an argument list produces a List of values (see 8.8).
The production Arguments
: (
) is evaluated as follows:
The production Arguments
: (
ArgumentList ) is
evaluated as follows:
The production ArgumentList : AssignmentExpression is evaluated as follows:
The production ArgumentList
: ArgumentList
, AssignmentExpression is
evaluated as follows:
The production MemberExpression : FunctionExpression is evaluated as follows:
The production PostfixExpression
: LeftHandSideExpression
[no
LineTerminator here]
++ is evaluated as follows:
+ operator (see
11.6.3).The production PostfixExpression
: LeftHandSideExpression
[no
LineTerminator here]
-- is evaluated as follows:
- operator
(11.6.3).delete UnaryExpression
void UnaryExpression
typeof UnaryExpression
++ UnaryExpression
-- UnaryExpression
+ UnaryExpression
- UnaryExpression
~ UnaryExpression
! UnaryExpression
delete OperatorThe production UnaryExpression
: delete
UnaryExpression is evaluated as follows:
NOTE When a delete operator
occurs within strict mode code, a
SyntaxError exception is thrown if its
UnaryExpression is a direct reference to a variable,
function argument, or function name. In addition, if a
delete operator occurs within
strict mode code and the property to be deleted has
the attribute { [[Configurable]]: false }, a
TypeError exception is thrown.
void OperatorThe production UnaryExpression
: void
UnaryExpression is evaluated as follows:
NOTE GetValue must be called even though its value is not used because it may have observable side-effects.
typeof OperatorThe production UnaryExpression
: typeof
UnaryExpression is evaluated as follows:
| Type of val | Result |
|---|---|
| Undefined | "undefined" |
| Null | "object" |
| Boolean | "boolean" |
| Number | "number" |
| String | "string" |
| Object (native and does not implement [[Call]]) | "object" |
| Object (native or host and does implement [[Call]]) | "function" |
| Object (host and does not implement [[Call]]) | Implementation-defined except may not be "undefined", "boolean", "number", or "string". |
The production UnaryExpression
: ++
UnaryExpression is evaluated as follows:
+ operator (see
11.6.3).The production UnaryExpression
: --
UnaryExpression is evaluated as follows:
- operator
(see 11.6.3).+ OperatorThe unary + operator converts its operand to Number type.
The production UnaryExpression
: +
UnaryExpression is evaluated as follows:
- OperatorThe unary - operator converts its operand to Number
type and then negates it. Note that negating +0
produces −0, and negating
−0 produces +0.
The production UnaryExpression
: -
UnaryExpression is evaluated as follows:
~ )The production UnaryExpression
: ~
UnaryExpression is evaluated as follows:
! )The production UnaryExpression
: !
UnaryExpression is evaluated as follows:
* UnaryExpression
/ UnaryExpression
% UnaryExpression
The production
MultiplicativeExpression
: MultiplicativeExpression
@
UnaryExpression, where @ stands for one of the operators in the above
definitions, is evaluated as follows:
* OperatorThe * operator performs multiplication, producing the
product of its operands. Multiplication is commutative. Multiplication is not
always associative in ECMAScript, because of finite precision.
The result of a floating-point multiplication is governed by the rules of IEEE 754 binary double-precision arithmetic:
/ OperatorThe / operator performs division, producing the
quotient of its operands. The left operand is the dividend and the right
operand is the divisor. ECMAScript does not perform integer division. The
operands and result of all division operations are double-precision
floating-point numbers. The result of division is determined by the
specification of IEEE 754 arithmetic:
% OperatorThe % operator yields the remainder of its operands
from an implied division; the left operand is the dividend and the right
operand is the divisor.
NOTE In C and C++, the remainder operator accepts only integral operands; in ECMAScript, it also accepts floating-point operands.
The result of a floating-point remainder operation as computed by the
% operator is not the same as the
“remainder” operation defined by IEEE 754. The IEEE 754
“remainder” operation computes the remainder from a rounding
division, not a truncating division, and so its behaviour is not analogous to
that of the usual integer remainder operator. Instead the ECMAScript language
defines % on floating-point operations to behave in a
manner analogous to that of the Java integer remainder operator; this may be
compared with the C library function fmod.
The result of an ECMAScript floating-point remainder operation is determined by the rules of IEEE arithmetic:
+ MultiplicativeExpression
- MultiplicativeExpression
+ )The addition operator either performs string concatenation or numeric addition.
The production AdditiveExpression
: AdditiveExpression
+
MultiplicativeExpression is evaluated as
follows:
NOTE 1 No hint is provided in the calls to ToPrimitive in steps 5 and 6. All native ECMAScript objects except Date objects handle the absence of a hint as if the hint Number were given; Date objects handle the absence of a hint as if the hint String were given. Host objects may handle the absence of a hint in some other manner.
PDF page 85 / printed page 75 NOTE 2 Step 7 differs from step 3 of the comparison algorithm for the relational operators (11.8.5), by using the logical-or operation instead of the logical-and operation.
- )The production AdditiveExpression
: AdditiveExpression
-
MultiplicativeExpression is evaluated as
follows:
The + operator performs addition when applied to two
operands of numeric type, producing the sum of the operands. The
- operator performs subtraction, producing the
difference of two numeric operands.
Addition is a commutative operation, but not always associative.
The result of an addition is determined using the rules of IEEE 754 binary double-precision arithmetic:
The - operator performs subtraction when applied to
two operands of numeric type, producing the difference of its operands; the
left operand is the minuend and the right operand is the subtrahend. Given
numeric operands a and b, it is always the case that
a-b produces the same result as
a +(-b).
<< AdditiveExpression
>> AdditiveExpression
>>> AdditiveExpression
<< )Performs a bitwise left shift operation on the left operand by the amount specified by the right operand.
PDF page 86 / printed page 76 The
production ShiftExpression
: ShiftExpression
<<
AdditiveExpression is evaluated as follows:
>> )Performs a sign-filling bitwise right shift operation on the left operand by the amount specified by the right operand.
The production ShiftExpression
: ShiftExpression
>>
AdditiveExpression is evaluated as follows:
>>> )Performs a zero-filling bitwise right shift operation on the left operand by the amount specified by the right operand.
The production ShiftExpression
: ShiftExpression
>>>
AdditiveExpression is evaluated as follows:
< ShiftExpression
> ShiftExpression
<= ShiftExpression
>= ShiftExpression
instanceof ShiftExpression
in ShiftExpression
< ShiftExpression
> ShiftExpression
<= ShiftExpression
>= ShiftExpression
instanceof ShiftExpression
NOTE The “NoIn” variants are needed to
avoid confusing the in operator in a relational
expression with the in operator in a
for statement.
The result of evaluating a relational operator is always of type Boolean, reflecting whether the relationship named by the operator holds between its two operands.
The RelationalExpressionNoIn productions are evaluated in the same manner as the RelationalExpression productions except that the contained RelationalExpressionNoIn is evaluated instead of the contained RelationalExpression.
< )The production
RelationalExpression
: RelationalExpression
< ShiftExpression is
evaluated as follows:
> )The production RelationalExpression
: RelationalExpression
> ShiftExpression is
evaluated as follows:
<= )The production RelationalExpression
: RelationalExpression
<= ShiftExpression is
evaluated as follows:
>= )The production RelationalExpression
: RelationalExpression
>= ShiftExpression is
evaluated as follows:
The comparison x < y, where x and y are values, produces true, false, or undefined (which indicates that at least one operand is NaN). In addition to x and y the algorithm takes a Boolean flag named LeftFirst as a parameter. The flag is used to control the order in which operations with potentially visible side-effects are performed upon x and y. It is necessary because ECMAScript specifies left to right evaluation of expressions. The default value of LeftFirst is true and indicates that the x parameter corresponds to an expression that occurs to the left of the y parameter’s corresponding expression. If LeftFirst is false, the reverse is the case and operations must be performed upon y before x. Such a comparison is performed as follows:
NOTE 1 Step 3 differs from step 7 in the algorithm
for the addition operator +
(11.6.1) in using and instead of or.
NOTE 2 The comparison of Strings uses a simple lexicographic ordering on sequences of code unit values. There is no attempt to use the more complex, semantically oriented definitions of character or string equality and collating order defined in the Unicode specification. Therefore String values that are canonically equal according to the Unicode standard could test as unequal. In effect this algorithm assumes that both Strings are already in normalised form. Also, note that for strings containing supplementary characters, lexicographic ordering on sequences of UTF-16 code unit values differs from that on sequences of code point values.
instanceof operatorThe production
RelationalExpression:
RelationalExpression instanceof
ShiftExpression is evaluated as follows:
in operatorThe production
RelationalExpression
: RelationalExpression
in ShiftExpression is
evaluated as follows:
== RelationalExpression
!= RelationalExpression
=== RelationalExpression
!== RelationalExpression
== RelationalExpressionNoIn
!= RelationalExpressionNoIn
=== RelationalExpressionNoIn
!== RelationalExpressionNoIn
The result of evaluating an equality operator is always of type Boolean, reflecting whether the relationship named by the operator holds between its two operands.
The EqualityExpressionNoIn productions are evaluated in the same manner as the EqualityExpression productions except that the contained EqualityExpressionNoIn and RelationalExpressionNoIn are evaluated instead of the contained EqualityExpression and RelationalExpression, respectively.
== )The production EqualityExpression :
EqualityExpression ==
RelationalExpression is evaluated as
follows:
!= )The production EqualityExpression
: EqualityExpression
!= RelationalExpression
is evaluated as follows:
The comparison x == y, where x and y are values, produces true or false. Such a comparison is performed as follows:
NOTE 1 Given the above definition of equality:
"" + a == "" +
b.+a ==
+b.!a ==
!b.NOTE 2 The equality operators maintain the following invariants:
A != B is equivalent to
!(A == B).A == B is equivalent to
B == A, except in the
order of evaluation of A and B.NOTE 3 The equality operator is not always
transitive. For example, there might be two distinct String objects, each
representing the same String value; each String object would be considered
equal to the String value by the == operator, but the
two String objects would not be equal to each other.
NOTE 4 Comparison of Strings uses a simple equality test on sequences of code unit values. There is no attempt to use the more complex, semantically oriented definitions of character or string equality and collating order defined in the Unicode specification. Therefore Strings values that are canonically equal according to the Unicode standard could test as unequal. In effect this algorithm assumes that both Strings are already in normalised form.
=== )The production EqualityExpression
: EqualityExpression
=== RelationalExpression
is evaluated as follows:
!== )The production EqualityExpression
: EqualityExpression
!== RelationalExpression
is evaluated as follows:
The comparison x === y, where x and y are values, produces true or false. Such a comparison is performed as follows:
NOTE This algorithm differs from the SameValue Algorithm (9.12) in its treatment of signed zeroes and NaNs.
& EqualityExpression
& EqualityExpressionNoIn
^ BitwiseANDExpression
^ BitwiseANDExpressionNoIn
| BitwiseXORExpression
| BitwiseXORExpressionNoIn
The production A :
A @ B,
where @ is one of the bitwise operators
in the productions above, is evaluated as follows:
&& BitwiseORExpression
&& BitwiseORExpressionNoIn
|| LogicalANDExpression
|| LogicalANDExpressionNoIn
The production
LogicalANDExpression
: LogicalANDExpression
&&
BitwiseORExpression is evaluated as follows:
The production
LogicalORExpression
: LogicalORExpression
|| LogicalANDExpression
is evaluated as follows:
The LogicalANDExpressionNoIn and LogicalORExpressionNoIn productions are evaluated in the same manner as the LogicalANDExpression and LogicalORExpression productions except that the contained LogicalANDExpressionNoIn, BitwiseORExpressionNoIn and LogicalORExpressionNoIn are evaluated instead of the contained LogicalANDExpression, BitwiseORExpression and LogicalORExpression, respectively.
NOTE The value produced by a
&& or || operator is
not necessarily of type Boolean. The value produced will always be the value
of one of the two operand expressions.
? : )? AssignmentExpression : AssignmentExpression
? AssignmentExpression : AssignmentExpressionNoIn
The production
ConditionalExpression
: LogicalORExpression
? AssignmentExpression
: AssignmentExpression is
evaluated as follows:
PDF page 94 / printed page 84 The ConditionalExpressionNoIn production is evaluated in the same manner as the ConditionalExpression production except that the contained LogicalORExpressionNoIn, AssignmentExpression and AssignmentExpressionNoIn are evaluated instead of the contained LogicalORExpression, first AssignmentExpression and second AssignmentExpression, respectively.
NOTE The grammar for a ConditionalExpression in ECMAScript is a little bit different from that in C and Java, which each allow the second subexpression to be an Expression but restrict the third expression to be a ConditionalExpression. The motivation for this difference in ECMAScript is to allow an assignment expression to be governed by either arm of a conditional and to eliminate the confusing and fairly useless case of a comma expression as the centre expression.
= *= /= %= += -= <<= >>= >>>= &= ^= |=
The AssignmentExpressionNoIn productions are evaluated in the same manner as the AssignmentExpression productions except that the contained ConditionalExpressionNoIn and AssignmentExpressionNoIn are evaluated instead of the contained ConditionalExpression and AssignmentExpression, respectively.
= )The production
AssignmentExpression
: LeftHandSideExpression
= AssignmentExpression is
evaluated as follows:
NOTE When an assignment occurs within strict mode code, its LeftHandSide must not evaluate to an unresolvable reference. If it does a ReferenceError exception is thrown upon assignment. The LeftHandSide also may not be a reference to a data property with the attribute value {[[Writable]]:false}, to an accessor property with the attribute value {[[Set]]:undefined}, nor to a non-existent property of an object whose [[Extensible]] internal property has the value false. In these cases a TypeError exception is thrown.
op= )The production
AssignmentExpression
: LeftHandSideExpression
@ =
AssignmentExpression, where @
represents one of the operators indicated above, is evaluated as follows:
NOTE See NOTE 11.13.1.
, ), AssignmentExpression
, AssignmentExpressionNoIn
The production Expression
: Expression
, AssignmentExpression is
evaluated as follows:
The ExpressionNoIn production is evaluated in the same manner as the Expression production except that the contained ExpressionNoIn and AssignmentExpressionNoIn are evaluated instead of the contained Expression and AssignmentExpression, respectively.
NOTE GetValue must be called even though its value is not used because it may have observable side-effects.