15   Standard Built-in ECMAScript Objects

There are certain built-in objects available whenever an ECMAScript program begins execution. One, the global object, is part of the lexical environment of the executing program. Others are accessible as initial properties of the global object.

Unless specified otherwise, the [[Class]] internal property of a built-in object is "Function" if that built-in object has a [[Call]] internal property, or "Object" if that built-in object does not have a [[Call]] internal property. Unless specified otherwise, the [[Extensible]] internal property of a built-in object initially has the value true.

Many built-in objects are functions: they can be invoked with arguments. Some of them furthermore are constructors: they are functions intended for use with the new operator. For each built-in function, this

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specification describes the arguments required by that function and properties of the Function object. For each built-in constructor, this specification furthermore describes properties of the prototype object of that constructor and properties of specific object instances returned by a new expression that invokes that constructor.

Unless otherwise specified in the description of a particular function, if a function or constructor described in this clause is given fewer arguments than the function is specified to require, the function or constructor shall behave exactly as if it had been given sufficient additional arguments, each such argument being the undefined value.

Unless otherwise specified in the description of a particular function, if a function or constructor described in this clause is given more arguments than the function is specified to allow, the extra arguments are evaluated by the call and then ignored by the function. However, an implementation may define implementation specific behaviour relating to such arguments as long as the behaviour is not the throwing of a TypeError exception that is predicated simply on the presence of an extra argument.
Every built-in function and every built-in constructor has the Function prototype object, which is the initial value of the expression Function.prototype (15.3.4), as the value of its [[Prototype]] internal property.

Unless otherwise specified every built-in prototype object has the Object prototype object, which is the initial value of the expression Object.prototype (15.2.4), as the value of its [[Prototype]] internal property, except the Object prototype object itself.

None of the built-in functions described in this clause that are not constructors shall implement the [[Construct]] internal method unless otherwise specified in the description of a particular function. None of the built-in functions described in this clause shall have a prototype property unless otherwise specified in the description of a particular function.

This clause generally describes distinct behaviours for when a constructor is "called as a function" and for when it is "called as part of a new expression". The "called as a function" behaviour corresponds to the invocation of the constructor’s [[Call]] internal method and the "called as part of a new expression" behaviour corresponds to the invocation of the constructor’s [[Construct]] internal method.

Every built-in Function object described in this clause—whether as a constructor, an ordinary function, or both—has a length property whose value is an integer. Unless otherwise specified, this value is equal to the largest number of named arguments shown in the subclause headings for the function description, including optional parameters.



In every case, the length property of a built-in Function object described in this clause has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }. Every other property described in this clause has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true } unless otherwise specified.

15.1   The Global Object

The unique global object is created before control enters any execution context.

Unless otherwise specified, the standard built-in properties of the global object have attributes {[[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true}.

The global object does not have a [[Construct]] internal property; it is not possible to use the global object as a constructor with the new operator.

The global object does not have a [[Call]] internal property; it is not possible to invoke the global object as a function.

The values of the [[Prototype]] and [[Class]] internal properties of the global object are implementation-dependent.

In addition to the properties defined in this specification the global object may have additional host defined properties. This may include a property whose value is the global object itself; for example, in the HTML document object model the window property of the global object is the global object itself.

15.1.1   Value Properties of the Global Object

15.1.1.1   NaN

The value of NaN is NaN (see 8.5). This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.1.1.2   Infinity

The value of Infinity is +∞ (see 8.5). This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.1.1.3   undefined

The value of undefined is undefined (see 8.1). This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.1.2   Function Properties of the Global Object

15.1.2.1   eval (x)

When the eval function is called with one argument x, the following steps are taken:

1. If Type(x) is not String, return x.
2. Let prog be the ECMAScript code that is the result of parsing x as a Program. If the parse fails, throw a SyntaxError exception (but see also clause 16).
3. Let evalCtx be the result of establishing a new execution context (10.4.2) for the eval code prog.
4. Let result be the result of evaluating the program prog.
5. Exit the running execution context evalCtx, restoring the previous execution context.
6. If result.type is normal and its completion value is a value V, then return the value V.
7. If result.type is normal and its completion value is empty, then return the value undefined.
8. Otherwise, result.type must be throw. Throw result.value as an exception.

15.1.2.1.1   Direct Call to Eval

A direct call to the eval function is one that is expressed as a CallExpression that meets the following two conditions:

The Reference that is the result of evaluating the MemberExpression in the CallExpression has an environment record as its base value and its reference name is "eval".

The result of calling the abstract operation GetValue with that Reference as the argument is the standard builtin function defined in 15.1.2.1.

15.1.2.2   parseInt (string , radix)

The parseInt function produces an integer value dictated by interpretation of the contents of the string argument according to the specified radix. Leading white space in string is ignored. If radix is undefined or 0, it is assumed to be 10 except when the number begins with the character pairs 0x or 0X, in which case a radix of 16 is assumed. If radix is 16, number may also optionally begin with the character pairs 0x or 0X.

When the parseInt function is called, the following steps are taken:

1. Let inputString be ToString(string).
2. Let S be a newly created substring of inputString consisting of the first character that is not a StrWhiteSpaceChar and all characters following that character. (In other words, remove leading white space.)
3. Let sign be 1.
4. If S is not empty and the first character of S is a minus sign -, let sign be −1.
5. If S is not empty and the first character of S is a plus sign + or a minus sign -, then remove the first character from S.
6. Let R = ToInt32(radix).
7. Let stripPrefix be true.
8. If R ≠ 0, then
   1. If R < 2 or R > 36, then return NaN.
   2. If R ≠ 16, let stripPrefix be false.
9. Else, R = 0
   1. Let R = 10.
10. If stripPrefix is true, then
    1. If the length of S is at least 2 and the first two characters of S are either "0x" or "0X", then remove the first two characters from S and let R = 16.
11. If S contains any character that is not a radix-R digit, then let Z be the substring of S consisting of all characters before the first such character; otherwise, let Z be S.
12. If Z is empty, return NaN.
13. Let mathInt be the mathematical integer value that is represented by Z in radix-R notation, using the letters A-Z and a-z for digits with values 10 through 35. (However, if R is 10 and Z contains more than 20 significant digits, every significant digit after the 20th may be replaced by a 0 digit, at the option of the implementation; and if R is not 2, 4, 8, 10, 16, or 32, then mathInt may be an implementation-dependent approximation to the mathematical integer value that is represented by Z in radix-R notation.)
14. Let number be the Number value for mathInt.
15. Return sign × number.

15.1.2.3   parseFloat (string)

The parseFloat function produces a Number value dictated by interpretation of the contents of the string argument as a decimal literal.

When the parseFloat function is called, the following steps are taken:

1. Let inputString be ToString(string).
2. Let trimmedString be a substring of inputString consisting of the leftmost character that is not a StrWhiteSpaceChar and all characters to the right of that character.(In other words, remove leading white space.)
3. If neither trimmedString nor any prefix of trimmedString satisfies the syntax of a StrDecimalLiteral (see 9.3.1), return NaN.
4. Let numberString be the longest prefix of trimmedString, which might be trimmedString itself, that satisfies the syntax of a StrDecimalLiteral.
5. Return the Number value for the MV of numberString.

15.1.2.4   isNaN (number)

Returns true if the argument coerces to NaN, and otherwise returns false.

1. If ToNumber(number) is NaN, return true.
2. Otherwise, return false.

NOTE A reliable way for ECMAScript code to test if a value X is a NaN is an expression of the form ! ==  X. The result will be true if and only if X is a NaN.

15.1.2.5   isFinite (number)

Returns false if the argument coerces to NaN, +∞, or −∞, and otherwise returns true.

1. If ToNumber(number) is NaN, +∞, or −∞, return false.
2. Otherwise, return true.

15.1.3   URI Handling Function Properties

Uniform Resource Identifiers, or URIs, are Strings that identify resources (e.g. web pages or files) and transport protocols by which to access them (e.g. HTTP or FTP) on the Internet. The ECMAScript language itself does not provide any support for using URIs except for functions that encode and decode URIs as described in 15.1.3.1, 15.1.3.2, 15.1.3.3 and 15.1.3.4. A URI is composed of a sequence of components separated by component separators. The general form is:

Scheme : First  /  Second ; Third ? Fourth

where the italicised names represent components and the " : ", "/", ";" and “?” are reserved characters used as separators. The encodeURI and decodeURI functions are intended to work with complete URIs; they assume that any reserved characters in the URI are intended to have special meaning and so are not encoded. The encodeURIComponent and decodeURIComponent functions are intended to work with the individual component parts of a URI; they assume that any reserved characters represent text and so must be encoded so that they are not interpreted as reserved characters when the component is part of a complete URI.

The following lexical grammar specifies the form of encoded URIs.

uri :::

uriCharacters_opt

See 15.1.3

uriCharacters :::

uriCharacter uriCharacters_opt

See 15.1.3

uriCharacter :::

uriReserved
uriUnescaped
uriEscaped

See 15.1.3

uriReserved ::: one of

;  /  ?  :  @  &  =  +  $  ,

uriUnescaped :::

uriAlpha
DecimalDigit
uriMark

uriEscaped :::

% HexDigit HexDigit

uriAlpha ::: one of

a b c d e f g h i j k l m n o p q r s t u v w x y z
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

uriMark ::: one of

-  _  .  !  ~  *  ‘  (  )

When a character to be included in a URI is not listed above or is not intended to have the special meaning sometimes given to the reserved characters, that character must be encoded. The character is transformed into its UTF-8 encoding, with surrogate pairs first converted from UTF-16 to the corresponding code point value. (Note that for code units in the range [0,127] this results in a single octet with the same value.) The resulting sequence of octets is then transformed into a String with each octet represented by an escape sequence of the form "%xx".

The encoding and escaping process is described by the abstract operation Encode taking two String arguments string and unescapedSet.

1. Let strLen be the number of characters in string.
2. Let R be the empty String.
3. Let k be 0.
4. Repeat
   1. If k equals strLen, return R.
   2. Let C be the character at position k within string.
   3. If C is in unescapedSet, then
      1. Let S be a String containing only the character C.
      2. Let R be a new String value computed by concatenating the previous value of R and S.
   4. Else, C is not in unescapedSet
      1. If the code unit value of C is not less than 0xDC00 and not greater than 0xDFFF, throw a URIError exception.
      2. If the code unit value of C is less than 0xD800 or greater than 0xDBFF, then
         1. Let V be the code unit value of C.
      3. Else,
         1. Increase k by 1.
         2. If k equals strLen, throw a URIError exception.
         3. Let kChar be the code unit value of the character at position k within string.
         4. If kChar is less than 0xDC00 or greater than 0xDFFF, throw a URIError exception.
         5. Let V be (((the code unit value of C) – 0xD800) * 0x400 + (kChar – 0xDC00) + 0x10000).
      4. Let Octets be the array of octets resulting by applying the UTF-8 transformation to V, and let L be the array size.
      5. Let j be 0.
      6. Repeat, while j < L
         1. Let jOctet be the value at position j within Octets.
         2. Let S be a String containing three characters "%XY" where XY are two uppercase hexadecimal digits encoding the value of jOctet.
         3. Let R be a new String value computed by concatenating the previous value of R and S.
         4. Increase j by 1.
   5. Increase k by 1.

The unescaping and decoding process is described by the abstract operation Decode taking two String arguments string and reservedSet.

1. Let strLen be the number of characters in string.
2. Let R be the empty String.
3. Let k be 0.
4. Repeat
   1. If k equals strLen, return R.
   2. Let C be the character at position k within string.
   3. If C is not ‘%’, then
      1. Let S be the String containing only the character C.
   4. Else, C is ‘%’
      1. Let start be k.
      2. If k + 2 is greater than or equal to strLen, throw a URIError exception.
      3. If the characters at position (k+1) and (k + 2) within string do not represent hexadecimal digits, throw a URIError exception.
      4. Let B be the 8-bit value represented by the two hexadecimal digits at position (k + 1) and (k + 2).
      5. Increment k by 2.
      6. If the most significant bit in B is 0, then
         1. Let C be the character with code unit value B.
         2. If C is not in reservedSet, then
            1. Let S be the String containing only the character C.
         3. Else, C is in reservedSet
            1. Let S be the substring of string from position start to position k included.
      7. Else, the most significant bit in B is 1
         1. Let n be the smallest non-negative number such that (B << n) & 0x80 is equal to 0.
         2. If n equals 1 or n is greater than 4, throw a URIError exception.
         3. Let Octets be an array of 8-bit integers of size n.
         4. Put B into Octets at position 0.
         5. If k + (3 * (n – 1)) is greater than or equal to strLen, throw a URIError exception.
         6. Let j be 1.
         7. Repeat, while j < n
            1. Increment k by 1.
            2. If the character at position k is not ‘%’, throw a URIError exception.
            3. If the characters at position (k +1) and (k + 2) within string do not represent hexadecimal digits, throw a URIError exception.
            4. Let B be the 8-bit value represented by the two hexadecimal digits at position (k + 1) and (k + 2).
            5. If the two most significant bits in B are not 10, throw a URIError exception.
            6. Increment k by 2.
            7. Put B into Octets at position j.
            8. Increment j by 1.
         8. Let V be the value obtained by applying the UTF-8 transformation to Octets, that is, from an array of octets into a 32-bit value. If Octets does not contain a valid UTF-8 encoding of a Unicode code point throw a URIError exception.
         9. If V is less than 0x10000, then
            1. Let C be the character with code unit value V.
            2. If C is not in reservedSet, then
               1. Let S be the String containing only the character C.
            3. Else, C is in reservedSet
               1. Let S be the substring of string from position start to position k included.
         10. Else, V is ≥ 0x10000
             1. If V is greater than 0x10FFFF, throw a URIError exception.
             2. Let L be (((V – 0x10000) & 0x3FF) + 0xDC00).
             3. Let H be ((((V – 0x10000) >> 10) & 0x3FF) + 0xD800).
             4. Let S be the String containing the two characters with code unit values H and L.

15.1.3.1   decodeURI (encodedURI)

The decodeURI function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the sort that might be introduced by the encodeURI function is replaced with the character that it represents. Escape sequences that could not have been introduced by encodeURI are not replaced.

When the decodeURI function is called with one argument encodedURI, the following steps are taken:

1. Let uriString be ToString(encodedURI).
2. Let reservedURISet be a String containing one instance of each character valid in uriReserved plus "#".
3. Return the result of calling Decode(uriString, reservedURISet)

15.1.3.2   decodeURIComponent (encodedURIComponent)

The decodeURIComponent function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the sort that might be introduced by the encodeURIComponent function is replaced with the character that it represents.

When the decodeURIComponent function is called with one argument encodedURIComponent, the following steps are taken:

1. Let componentString be ToString(encodedURIComponent).
2. Let reservedURIComponentSet be the empty String.
3. Return the result of calling Decode(componentString, reservedURIComponentSet)

15.1.3.3   encodeURI (uri)

The encodeURI function computes a new version of a URI in which each instance of certain characters is replaced by one, two or three escape sequences representing the UTF-8 encoding of the character.

When the encodeURI function is called with one argument uri, the following steps are taken:

1. Let uriString be ToString(uri).
2. Let unescapedURISet be a String containing one instance of each character valid in uriReserved and uriUnescaped plus "#".
3. Return the result of calling Encode(uriString, unescapedURISet)

15.1.3.4   encodeURIComponent (uriComponent)

The encodeURIComponent function computes a new version of a URI in which each instance of certain characters is replaced by one, two or three escape sequences representing the UTF-8 encoding of the character.

When the encodeURIComponent function is called with one argument uriComponent, the following steps are taken:

1. Let componentString be ToString(uriComponent).
2. Let unescapedURIComponentSet be a String containing one instance of each character valid in uriUnescaped.
3. Return the result of calling Encode(componentString, unescapedURIComponentSet)

15.1.4   Constructor Properties of the Global Object

15.1.4.1   Object ( . . . )

See 15.2.1 and 15.2.2

15.1.4.2   Function ( . . . )

See 15.3.1 and 15.3.2

15.1.4.3   Array ( . . . )

See 15.4.1 and 15.4.2

15.1.4.4   String ( . . . )

See 15.5.1 and 15.5.2.

15.1.4.5   Boolean ( . . . )

See 15.6.1 and 15.6.2.

15.1.4.6   Number ( . . . )

See 15.7.1 and 15.7.2.

15.1.4.7   Date ( . . . )

See 15.9.2.

15.1.4.8   RegExp ( . . . )

See 15.10.3 and 15.10.4.

15.1.4.9   Error ( . . . )

See 15.11.1 and 15.11.2.

15.1.4.10   EvalError ( . . . )

See 15.11.6.1.

15.1.4.11   RangeError ( . . . )

See 15.11.6.2.

15.1.4.12   ReferenceError ( . . . )

See 15.11.6.3.

15.1.4.13   SyntaxError ( . . . )

See 15.11.6.4.

15.1.4.14   TypeError ( . . . )

See 15.11.6.5.

15.1.4.15   URIError ( . . . )

See 15.11.6.6.

15.1.5   Other Properties of the Global Object

15.1.5.1   Math

See 15.8.

15.1.5.2   JSON

See 15.12.

15.2   Object Objects

15.2.1   The Object Constructor Called as a Function

When Object is called as a function rather than as a constructor, it performs a type conversion.

15.2.1.1   Object ( [ value ] )

When the Object function is called with no arguments or with one argument value, the following steps are taken:

1. If value is null, undefined or not supplied, create and return a new Object object exactly as if the standard
   built-in Object constructor had been called with the same arguments (15.2.2.1).
2. Return ToObject(value).

15.2.2   The Object Constructor

When Object is called as part of a new expression, it is a constructor that may create an object.

15.2.2.1   new Object ( [ value ] )

When the Object constructor is called with no arguments or with one argument value, the following steps are taken:

1. If value is supplied, then
   1. If Type(value) is Object, then
      1. If the value is a native ECMAScript object, do not create a new object but simply return value.
      2. If the value is a host object, then actions are taken and a result is returned in an implementation-dependent manner that may depend on the host object.
   2. If Type(value) is String, return ToObject(value).
   3. If Type(value) is Boolean, return ToObject(value).
   4. If Type(value) is Number, return ToObject(value).
2. Asset: The argument value was not supplied or its type was Null or Undefined.
3. Let obj be a newly created native ECMAScript object.
4. Set the [[Prototype]] internal property of obj t to the standard built-in Object prototype object (15.2.4).
5. Set the [[Class]] internal property of obj to "Object".
6. Set the [[Extensible]] internal property of obj to true.
7. Set the all the internal methods of obj as specified in 8.12.
8. Return obj.

15.2.3   Properties of the Object Constructor

The value of the [[Prototype]] internal property of the Object constructor is the standard built-in Function prototype object.

Besides the internal properties and the length property (whose value is 1), the Object constructor has the following properties:

15.2.3.1   Object.prototype

The initial value of Object.prototype is the standard built-in Object prototype object (15.2.4).

This property has the attributes {[[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.2.3.2   Object.getPrototypeOf ( O )

When the getPrototypeOf function is called with argument O, the following steps are taken:

1. If Type(O) is not Object throw a TypeError exception.
2. Return the value of the [[Prototype]] internal property of O.

15.2.3.3   Object.getOwnPropertyDescriptor ( O, P )

When the getOwnPropertyDescriptor function is called, the following steps are taken:

1. If Type(O) is not Object throw a TypeError exception.
2. Let name be ToString(P).
3. Let desc be the result of calling the [[GetOwnProperty]] internal method of O with argument name.
4. Return the result of calling FromPropertyDescriptor(desc) (8.10.4).

15.2.3.4   Object.getOwnPropertyNames ( O )

When the getOwnPropertyNames function is called, the following steps are taken:

1. If Type(O) is not Object throw a TypeError exception.
2. Let array be the result of creating a new object as if by the expression new Array () where Array is the standard built-in constructor with that name.
3. Let n be O.
4. For each named own property P of O
   1. Let name be the String value that is the name of P.
   2. Call the [[DefineOwnProperty]] internal method of array with arguments ToString(n), the PropertyDescriptor {[[Value]]: name, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
   3. Increment n by 1.
5. Return array.

15.2.3.5   Object.create ( O [, Properties] )

The create function creates a new object with a specified prototype. When the create function is called, the following steps are taken:

1. If Type(O) is not Object or Null throw a TypeError exception.
2. Let obj be the result of creating a new object as if by the expression new Object() where Object is the standard built-in constructor with that name
3. Set the [[Prototype]] internal property of obj to O.
4. If the argument Properties is present and not undefined, add own properties to obj as if by calling the standard built-in function Object.defineProperties with arguments obj and Properties.
5. Return obj.

15.2.3.6   Object.defineProperty ( O, P, Attributes )

The defineProperty function is used to add an own property and/or update the attributes of an existing own property of an object. When the defineProperty function is called, the following steps are taken:

1. If Type(O) is not Object throw a TypeError exception.
2. Let name be ToString(P).
3. Let desc be the result of calling ToPropertyDescriptor with Attributes as the argument.
4. Call the [[DefineOwnProperty]] internal method of O with arguments name, desc, and true.
5. Return O.

15.2.3.7   Object.defineProperties ( O, Properties )

The defineProperties function is used to add own properties and/or update the attributes of existing own properties of an object. When the defineProperties function is called, the following steps are taken:

1. If Type(O) is not Object throw a TypeError exception.
2. Let props be ToObject(Properties).
3. Let names be an internal list containing the names of each enumerable own property of props.
4. Let descriptors be an empty internal List.
5. For each element P of names in list order,
   1. Let descObj be the result of calling the [[Get]] internal method of props with P as the argument.
   2. Let desc be the result of calling ToPropertyDescriptor with descObj as the argument.
   3. Append desc to the end of descriptors.
6. For each element desc of descriptors in list order,
   1. Call the [[DefineOwnProperty]] internal method of O with arguments P, desc, and true.
7. Return O.

If an implementation defines a specific order of enumeration for the for-in statement, that same enumeration order must be used to order the list elements in step 3 of this algorithm.

15.2.3.8   Object.seal ( O )

When the seal function is called, the following steps are taken:

1. If Type(O) is not Object throw a TypeError exception.
2. For each named own property name P of O,
   1. Let desc be the result of calling the [[GetOwnProperty]] internal method of O with P.
   2. If desc.[[Configurable]] is true, set desc.[[Configurable]] to false.
   3. Call the [[DefineOwnProperty]] internal method of O with P, desc, and true as arguments.
3. Set the [[Extensible]] internal property of O to false.
4. Return O.

15.2.3.9   Object.freeze ( O )

When the freeze function is called, the following steps are taken:

1. If Type(O) is not Object throw a TypeError exception.
2. For each named own property name P of O,
   1. Let desc be the result of calling the [[GetOwnProperty]] internal method of O with P.
   2. If IsDataDescriptor(desc) is true, then
      1. If desc.[[Writable]] is true, set desc.[[Writable]] to false.
   3. If desc.[[Configurable]] is true, set desc.[[Configurable]] to false.
   4. Call the [[DefineOwnProperty]] internal method of O with P, desc, and true as arguments.
3. Set the [[Extensible]] internal property of O to false.
4. Return O.

15.2.3.10   Object.preventExtensions ( O )

When the preventExtensions function is called, the following steps are taken:

1. If Type(O) is not Object throw a TypeError exception.
2. Set the [[Extensible]] internal property of O to false.
3. Return O.

15.2.3.11   Object.isSealed ( O )

When the isSealed function is called with argument O, the following steps are taken:

1. If Type(O) is not Object throw a TypeError exception.
2. For each named own property name P of O,

2. 1. Let desc be the result of calling the [[GetOwnProperty]] internal method of O with P.
   2. If desc.[[Configurable]] is true, then return false.
3. If the [[Extensible]] internal property of O is false, then return true.
4. Otherwise, return false.

15.2.3.12   Object.isFrozen ( O )

When the isFrozen function is called with argument O, the following steps are taken:

1. If Type(O) is not Object throw a TypeError exception.
2. For each named own property name P of O,
   1. Let desc be the result of calling the [[GetOwnProperty]] internal method of O with P.
   2. If IsDataDescriptor(desc) is true then
      1. If desc.[[Writable]] is true, return false.
   3. If desc.[[Configurable]] is true, then return false.
3. If the [[Extensible]] internal property of O is false, then return true.
4. Otherwise, return false.

15.2.3.13   Object.isExtensible ( O )

When the isExtensible function is called with argument O, the following steps are taken:

1. If Type(O) is not Object throw a TypeError exception.
2. Return the Boolean value of the [[Extensible]] internal property of O.

15.2.3.14   Object.keys ( O )

When the keys function is called with argument O, the following steps are taken:

1. If the Type(O) is not Object, throw a TypeError exception.
2. Let n be the number of own enumerable properties of O
3. Let array be the result of creating a new Object as if by the expression new Array(n) where Array is the standard built-in constructor with that name.
4. Let index be 0.
5. For each own enumerable property of O whose name String is P
   1. Call the [[DefineOwnProperty]] internal method of array with arguments ToString(index), the PropertyDescriptor {[[Value]]: P, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
   2. Increment index by 1.
6. Return array.

If an implementation defines a specific order of enumeration for the for-in statement, that same enumeration order must be used in step 5 of this algorithm.

15.2.4   Properties of the Object Prototype Object

The value of the [[Prototype]] internal property of the Object prototype object is null, the value of the [[Class]] internal property is "Object", and the initial value of the [[Extensible]] internal property is true.

15.2.4.1   Object.prototype.constructor

The initial value of Object.prototype.constructor is the standard built-in Object constructor.

15.2.4.2   Object.prototype.toString ( )

When the toString method is called, the following steps are taken:

1. Let O be the result of calling ToObject passing the this value as the argument.
2. Let class be the value of the [[Class]] internal property of O.

15.2.4.3   Object.prototype.toLocaleString ( )

When the toLocaleString method is called, the following steps are taken:

1. Let O be the result of calling ToObject passing the this value as the argument.
2. Let toString be the result of calling the [[Get]] internal method of O passing "toString" as the argument.
3. If IsCallable(toString) is false, throw a TypeError exception.
4. Return the result of calling the [[Call]] internal method of toString passing O as the this value and no arguments.

15.2.4.4   Object.prototype.valueOf ( )

When the valueOf method is called, the following steps are taken:

1. Let O be the result of calling ToObject passing the this value as the argument.
2. If O is the result of calling the Object constructor with a host object (15.2.2.1), then
   1. Return either O or another value such as the host object originally passed to the constructor. The specific result that is returned is implementation-defined.
3. Return O.

15.2.4.5   Object.prototype.hasOwnProperty (V)

When the hasOwnProperty method is called with argument V, the following steps are taken:

1. Let P be ToString(V).
2. Let O be the result of calling ToObject passing the this value as the argument.
3. Let desc be the result of calling the [[GetOwnProperty]] internal method of O passing P as the argument.
4. If desc is undefined, return false.
5. Return true.

15.2.4.6   Object.prototype.isPrototypeOf (V)

When the isPrototypeOf method is called with argument V, the following steps are taken:

1. If V is not an object, return false.
2. Let O be the result of calling ToObject passing the this value as the argument.
3. Repeat
   1. Let V be the value of the [[Prototype]] internal property of V.
   2. if V is null, return false
   3. If O and V refer to the same object, return true.

15.2.4.7   Object.prototype.propertyIsEnumerable (V)

When the propertyIsEnumerable method is called with argument V, the following steps are taken:

1. Let P be ToString(V).
2. Let O be the result of calling ToObject passing the this value as the argument.
3. Let desc be the result of calling the [[GetOwnProperty]] internal method of O passing P as the argument.
4. If desc is undefined, return false.
5. Return the value of desc.[[Enumerable]].

15.2.5   Properties of Object Instances

Object instances have no special properties beyond those inherited from the Object prototype object.

15.3   Function Objects

15.3.1   The Function Constructor Called as a Function

When Function is called as a function rather than as a constructor, it creates and initialises a new Function object. Thus the function call Function(…) is equivalent to the object creation expression new Function(…) with the same arguments.

15.3.1.1   Function (p1, p2, … , pn, body)

When the Function function is called with some arguments p1, p2, … , pn, body (where n might be 0, that is, there are no “p” arguments, and where body might also not be provided), the following steps are taken:

1. Create and return a new Function object as if the standard built-in constructor Function was used in a new expression with the same arguments (15.3.2.1).

15.3.2   The Function Constructor

When Function is called as part of a new expression, it is a constructor: it initialises the newly created object.

15.3.2.1   new Function (p1, p2, … , pn, body)

The last argument specifies the body (executable code) of a function; any preceding arguments specify formal parameters.

When the Function constructor is called with some arguments p1, p2, … , pn, body (where n might be 0, that is, there are no “p” arguments, and where body might also not be provided), the following steps are taken:

1. Let argCount be the total number of arguments passed to this function invocation.
2. Let P be the empty String.
3. If argCount = 0, let body be the empty String.
4. Else if argCount = 1, let body be that argument.
5. Else, argCount > 1
   1. Let firstArg be the first argument.
   2. Let P be ToString(firstArg).
   3. Let k be 2.
   4. Repeat, while k < argCount
      1. Let nextArg be the k’th argument.

2. Let P be the result of concatenating the previous value of P, the String "," (a comma), and ToString(nextArg).
3. Increase k by 1.

6. Let body be ToString(body).
7. If P is not parsable as a FormalParameterList_opt then throw a SyntaxError exception.
8. If body is not parsable as FunctionBody then throw a SyntaxError exception.
9. If body is strict mode code (see 10.1.1) then let strict be true, else let strict be false.
10. If strict is true, throw any exceptions specified in 13.1 that apply.
11. Return a new Function object created as specified in 13.2 passing P as the FormalParameterList and body as the FunctionBody. Pass in the Global Environment as the Scope parameter and strict as the Strict flag.

A prototype property is automatically created for every function, to provide for the possibility that the function will be used as a constructor.

15.3.3   Properties of the Function Constructor

The Function constructor is itself a Function object and its [[Class]] is "Function". The value of the [[Prototype]] internal property of the Function constructor is the standard built-in Function prototype object (15.3.4).

The value of the [[Extensible]] internal property of the Function constructor is true.

The Function constructor has the following properties:

15.3.3.1   Function.prototype

The initial value of Function.prototype is the standard built-in Function prototype object (15.3.4). This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.3.3.2   Function.length

This is a data property with a value of 1. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.3.4   Properties of the Function Prototype Object

The Function prototype object is itself a Function object (its [[Class]] is "Function") that, when invoked, accepts any arguments and returns undefined.

The value of the [[Prototype]] internal property of the Function prototype object is the standard built-in Object prototype object (15.2.4). The initial value of the [[Extensible]] internal property of the Function prototype object is true.

The Function prototype object does not have a valueOf property of its own; however, it inherits the valueOf property from the Object prototype Object.

The length property of the Function prototype object is 0.

15.3.4.1   Function.prototype.constructor

The initial value of Function.prototype.constructor is the built-in Function constructor.

15.3.4.2   Function.prototype.toString ( )

An implementation-dependent representation of the function is returned. This representation has the syntax of a FunctionDeclaration. Note in particular that the use and placement of white space, line terminators, and semicolons within the representation String is implementation-dependent.

The toString function is not generic; it throws a TypeError exception if its this value is not a Function object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

15.3.4.3   Function.prototype.apply (thisArg, argArray)

When the apply method is called on an object func with arguments thisArg and argArray, the following steps are taken:

1. If IsCallable(func) is false, then throw a TypeError exception.
2. If argArray is null or undefined, then
   1. Return the result of calling the [[Call]] internal method of func, providing thisArg as the this value and an empty list of arguments.
3. If Type(argArray) is not Object, then throw a TypeError exception.
4. Let len be the result of calling the [[Get]] internal method of argArray with argument "length".
5. If len is null or undefined, then throw a TypeError exception.
6. Let n be ToUint32(len).
7. If n is not equal to ToNumber(len), then throw a TypeError exception.
8. Let argList be an empty List.
9. Let index be 0.
10. Repeat while index < n
    1. Let indexName be ToString(index).
    2. Let nextArg be the result of calling the [[Get]] internal method of argArray with indexName as the argument.
    3. Append nextArg as the last element of argList.
    4. Set index to index + 1.
11. Return the result of calling the [[Call]] internal method of func, providing thisArg as the this value and argList as the list of arguments.

The length property of the apply method is 2.

15.3.4.4   Function.prototype.call (thisArg [ , arg1 [ , arg2, … ] ] )

When the call method is called on an object func with argument thisArg and optional arguments arg1, arg2 etc, the following steps are taken:

1. If IsCallable(func) is false, then throw a TypeError exception.
2. Let argList be an empty List.
3. If this method was called with more than one argument then in left to right order starting with arg1 append each argument as the last element of argList
4. Return the result of calling the [[Call]] internal method of func, providing thisArg as the this value and argList as the list of arguments.

The length property of the call method is 1.

15.3.4.5   Function.prototype.bind (thisArg [, arg1 [, arg2, …]])

The bind method takes one or more arguments, thisArg and (optionally) arg1, arg2, etc, and returns a new function object by performing the following steps:

1. Let Target be the this value.
2. If IsCallable(Target) is false, throw a TypeError exception.
3. Let A be a new (possibly empty) internal list of all of the argument values provided after thisArg (arg1, arg2 etc), in order.
4. Let F be a new native ECMAScript object .
5. Set all the internal methods, except for [[Get]], of F as specified in 8.12.
6. Set the [[Get]] internal property of F as specified in 15.3.5.4.
7. Set the [[TargetFunction]] internal property of F to Target.
8. Set the [[BoundThis]] internal property of F to the value of thisArg.
9. Set the [[BoundArgs]] internal property of F to A.
10. Set the [[Class]] internal property of F to "Function".
11. Set the [[Prototype]] internal property of F to the standard built-in Function prototype object as specified in 15.3.3.1.
12. Set the [[Call]] internal property of F as described in 15.3.4.5.1.
13. Set the [[Construct]] internal property of F as described in 15.3.4.5.2.
14. Set the [[HasInstance]] internal property of F as described in 15.3.4.5.3.
15. If the [[Class]] internal property of Target is "Function", then
    1. Let L be the length property of Target minus the length of A.
    2. Set the length own property of F to either 0 or L, whichever is larger.
16. Else set the length own property of F to 0.
17. Set the attributes of the length own property of F to the values specified in 15.3.5.1.
18. Set the [[Extensible]] internal property of F to true.
19. Let thrower be the [[ThrowTypeError]] function Object (13.2.3).
20. Call the [[DefineOwnProperty]] internal method of F with arguments "caller", PropertyDescriptor {[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]: false}, and false.
21. Call the [[DefineOwnProperty]] internal method of F with arguments "arguments", PropertyDescriptor {[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]: false}, and false.
22. Return F.

The length property of the bind method is 1.

15.3.4.5.1   [[Call]]

When the [[Call]] internal method of a function object, F, which was created using the bind function is called with a this value and a list of arguments ExtraArgs, the following steps are taken:

1. Let boundArgs be the value of F’s [[BoundArgs]] internal property.
2. Let boundThis be the value of F’s [[BoundThis]] internal property.
3. Let target be the value of F’s [[TargetFunction]] internal property.
4. Let args be a new list containing the same values as the list boundArgs in the same order followed by the same values as the list ExtraArgs in the same order.
5. Return the result of calling the [[Call]] internal method of target providing boundThis as the this value and providing args as the arguments.

15.3.4.5.2   [[Construct]]

When the [[Construct]] internal method of a function object, F that was created using the bind function is called with a list of arguments ExtraArgs, the following steps are taken:

1. Let target be the value of F’s [[TargetFunction]] internal property.
2. If target has no [[Construct]] internal method, a TypeError exception is thrown.
3. Let boundArgs be the value of F’s [[BoundArgs]] internal property.
4. Let args be a new list containing the same values as the list boundArgs in the same order followed by the same values as the list ExtraArgs in the same order.
5. Return the result of calling the [[Construct]] internal method of target providing args as the arguments.

15.3.4.5.3   [[HasInstance]] (V)

When the [[HasInstance]] internal method of a function object F, that was created using the bind function is called with argument V, the following steps are taken:

1. Let target be the value of F’s [[TargetFunction]] internal property.
2. If target has no [[HasInstance]] internal method, a TypeError exception is thrown.
3. Return the result of calling the [[HasInstance]] internal method of target providing V as the argument.

15.3.5   Properties of Function Instances

In addition to the required internal properties, every function instance has a [[Call]] internal property and in most cases use a different version of the [[Get]] internal property. Depending on how they are created (see 8.6.2 ,13.2, 15, and 15.3.4.5), function instances may have a [[HasInstance]] internal property, a [[Scope]] internal property, a [[Construct]] internal property, a [[FormalParameters]] internal property, a [[Code]] internal property, a [[TargetFunction]] internal property, a [[BoundThis]] internal property, and a [[BoundArgs]] internal property.

The value of the [[Class]] internal property is "Function".

Function instances that correspond to strict mode functions (13.2) and function instances created using the Function.prototype.bind method (15.3.4.5) have properties named "caller" and "arguments" that throw a TypeError exception. An ECMAScript implementation must not associate any implementation specific behaviour with accesses of these properties from strict mode function code.

15.3.5.1   length

The value of the length property is an integer that indicates the “typical” number of arguments expected by the function. However, the language permits the function to be invoked with some other number of arguments. The behaviour of a function when invoked on a number of arguments other than the number specified by its length property depends on the function. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.3.5.2   prototype

The value of the prototype property is used to initialise the [[Prototype]] internal property of a newly created object before the Function object is invoked as a constructor for that newly created object. This property has the attribute { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.

15.3.5.3   [[HasInstance]] (V)

Assume F is a Function object.

When the [[HasInstance]] internal method of F is called with value V, the following steps are taken:

1. If V is not an object, return false.
2. Let O be the result of calling the [[Get]] internal method of F with property name "prototype".
3. If Type(O) is not Object, throw a TypeError exception.
4. Repeat
   1. Let V be the value of the [[Prototype]] internal property of V.

2. If V is null, return false.
3. If O and V refer to the same object, return true.

15.3.5.4   [[Get]] (P)

Function objects use a variation of the [[Get]] internal method used for other native ECMAScript objects (8.12.3). Assume F is a Function object. When the [[Get]] internal method of F is called with property name P, the following steps are taken:

1. Let v be the result of calling the default [[Get]] internal method (8.12.3) on F passing P as the property name argument.
2. If P is "caller" and v is a strict mode Function object, throw a TypeError exception.
3. Return v.

15.4   Array Objects

Array objects give special treatment to a certain class of property names. A property name P (in the form of a String value) is an array index if and only if ToString(ToUint32(P)) is equal to P and ToUint32(P) is not equal to 2³²−1. A property whose property name is an array index is also called an element. Every Array object has a length property whose value is always a nonnegative integer less than 2³². The value of the length property is numerically greater than the name of every property whose name is an array index; whenever a property of an Array object is created or changed, other properties are adjusted as necessary to maintain this invariant. Specifically, whenever a property is added whose name is an array index, the length property is changed, if necessary, to be one more than the numeric value of that array index; and whenever the length property is changed, every property whose name is an array index whose value is not smaller than the new length is automatically deleted. This constraint applies only to own properties of an Array object and is unaffected by length or array index properties that may be inherited from its prototypes.

An object, O, is said to be sparse if the following algorithm returns true:

1. Let len be the result of calling the [[Get]] internal method of O with argument "length".
2. For each integer i in the range 0≤iToUint32(len)
   1. Let elem be the result of calling the [[GetOwnProperty]] internal method of O with argument
      ToString(i).
   2. If elem is undefined, return true.
3. Return false.

15.4.1   The Array Constructor Called as a Function

When Array is called as a function rather than as a constructor, it creates and initialises a new Array object. Thus the function call Array(…) is equivalent to the object creation expression new Array(…) with the same arguments.

15.4.1.1   Array ( [ item1 [ , item2 [ , … ] ] ] )

When the Array function is called the following steps are taken:

1. Create and return a new Array object exactly as if the standard built-in constructor Array was used in a new expression with the same arguments (15.4.2).

15.4.2   The Array Constructor

When Array is called as part of a new expression, it is a constructor: it initialises the newly created object.

15.4.2.1   new Array ( [ item0 [ , item1 [ , … ] ] ] )

This description applies if and only if the Array constructor is given no arguments or at least two arguments.

The [[Prototype]] internal property of the newly constructed object is set to the original Array prototype object, the one that is the initial value of Array.prototype (15.4.3.1).

The [[Class]] internal property of the newly constructed object is set to "Array".

The [[Extensible]] internal property of the newly constructed object is set to true.

The length property of the newly constructed object is set to the number of arguments.

The 0 property of the newly constructed object is set to item0 (if supplied); the 1 property of the newly constructed object is set to item1 (if supplied); and, in general, for as many arguments as there are, the k property of the newly constructed object is set to argument k, where the first argument is considered to be argument number 0. These properties all have the attributes {[[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.

15.4.2.2   new Array (len)

The [[Prototype]] internal property of the newly constructed object is set to the original Array prototype object, the one that is the initial value of Array.prototype (15.4.3.1). The [[Class]] internal property of the newly constructed object is set to "Array". The [[Extensible]] internal property of the newly constructed object is set to true.

If the argument len is a Number and ToUint32(len) is equal to len, then the length property of the newly constructed object is set to ToUint32(len). If the argument len is a Number and ToUint32(len) is not equal to len, a RangeError exception is thrown.

If the argument len is not a Number, then the length property of the newly constructed object is set to 1 and the 0 property of the newly constructed object is set to len with attributes {[[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}..

15.4.3   Properties of the Array Constructor

The value of the [[Prototype]] internal property of the Array constructor is the Function prototype object (15.3.4).

Besides the internal properties and the length property (whose value is 1), the Array constructor has the following properties:

15.4.3.1   Array.prototype

The initial value of Array.prototype is the Array prototype object (15.4.4).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.4.3.2   Array.isArray ( arg )

The isArray function takes one argument arg, and returns the Boolean value true if the argument is an object whose class internal property is "Array"; otherwise it returns false. The following steps are taken:

1. If Type(arg) is not Object, return false.
2. If the value of the [[Class]] internal property of arg is "Array", then return true.
3. Return false.

15.4.4   Properties of the Array Prototype Object

The value of the [[Prototype]] internal property of the Array prototype object is the standard built-in Object prototype object (15.2.4).

The Array prototype object is itself an array; its [[Class]] is "Array", and it has a length property (whose initial value is +0) and the special [[DefineOwnProperty]] internal method described in 15.4.5.1.

In following descriptions of functions that are properties of the Array prototype object, the phrase “this object” refers to the object that is the this value for the invocation of the function. It is permitted for the this to be an object for which the value of the [[Class]] internal property is not "Array".

15.4.4.1   Array.prototype.constructor

The initial value of Array.prototype.constructor is the standard built-in Array constructor.

15.4.4.2   Array.prototype.toString ( )

When the toString method is called, the following steps are taken:

1. Let array be the result of calling ToObject on the this value.
2. Let func be the result of calling the [[Get]] internal method of array with argument "join".
3. If IsCallable(func) is false, then let func be the standard built-in method Object.prototype.toString (15.2.4.2).
4. Return the result of calling the [[Call]] internal method of func providing array as the this value and an empty arguments list.

15.4.4.3   Array.prototype.toLocaleString ( )

The elements of the array are converted to Strings using their toLocaleString methods, and these Strings are then concatenated, separated by occurrences of a separator String that has been derived in an implementation-defined locale-specific way. The result of calling this function is intended to be analogous to the result of toString, except that the result of this function is intended to be locale-specific.

The result is calculated as follows:

1. Let O be the result of calling ToObject passing the this value as the argument.
2. Let arrayLen be the result of calling the [[Get]] internal method of array with argument "length".
3. Let len be ToUint32(arrayLen).
4. Let separator be the String value for the list-separator String appropriate for the host environment’s current locale (this is derived in an implementation-defined way).
5. If len is zero, return the empty String.
6. Let firstElement be the result of calling the [[Get]] internal method of array with argument "0".
7. If firstElement is undefined or null, then
   1. Let R be the empty String.
8. Else
   1. Let elementObj be ToObject(firstElement).

1. 1. Let func be the result of calling the [[Get]] internal method of elementObj with argument "toLocaleString".
   2. If IsCallable(func) is false, throw a TypeError exception.
   3. Let R be the result of calling the [[Call]] internal method of func providing elementObj as the this value and an empty arguments list.

1. 9.     Let k be 1
2. 10.    Repeat, while k < len
   1. Let S be a String value produced by concatenating R and separator.
   2. Let nextElement be the result of calling the [[Get]] internal method of array with argument ToString(k).
   3. If nextElement is undefined or null, then
      1. Let R be the empty String.
   4. Else
      1. Let elementObj be ToObject(nextElement).
      2. Let func be the result of calling the [[Get]] internal method of elementObj with argument "toLocaleString".
      3. If IsCallable(func) is false, throw a TypeError exception.
      4. Let R be the result of calling the [[Call]] internal method of func providing elementObj as the this value and an empty arguments list.
   5. Let R be a String value produced by concatenating S and R.
   6. Increase k by 1.
3. 11.    Return R.

15.4.4.4   Array.prototype.concat ( [ item1 [ , item2 [ , … ] ] ] )

When the concat method is called with zero or more arguments item1, item2, etc., it returns an array containing the array elements of the object followed by the array elements of each argument in order.

The following steps are taken:

1. Let O be the result of calling ToObject passing the this value as the argument.
2. Let A be a new array created as if by the expression new Array() where Array is the standard built-in constructor with that name.
3. Let n be 0.
4. Let items be an internal List whose first element is O and whose subsequent elements are, in left to right order, the arguments that were passed to this function invocation.
5. Repeat, while items is not empty
   1. Remove the first element from items and let E be the value of the element.
   2. If the value of the [[Class]] internal property of E is "Array", then
      1. Let k be 0.
      2. Let len be the result of calling the [[Get]] internal method of E with argument "length".
      3. Repeat, while k < len
         1. Let P be ToString(k).
         2. Let exists be the result of calling the [[HasProperty]] internal method of E with P.
         3. If exists is true, then
            1. Let subElement be the result of calling the [[Get]] internal method of E with argument P.
            2. Call the [[DefineOwnProperty]] internal method of A with arguments ToString(n), Property Descriptor {[[Value]]: subElement, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
         4. Increase n by 1.
         5. Increase k by 1.

3. Else, E is not an Array
   1. Call the [[DefineOwnProperty]] internal method of A with arguments ToString(n), Property Descriptor {[[Value]]: E, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
   2. Increase n by 1.

The "length" property of the "concat" method is 1.

15.4.4.5   Array.prototype.join (separator)

The elements of the array are converted to Strings, and these Strings are then concatenated, separated by occurrences of the separator. If no separator is provided, a single comma is used as the separator.

The join method takes one argument, separator, and performs the following steps:

1. Let O be the result of calling ToObject passing the this value as the argument.
2. Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
3. Let len be ToUint32(lenVal).
4. If separator is undefined, let separator be the single-character String ",".
5. Let sep be ToString(separator).
6. If len is zero, return the empty String.
7. Let element0 be the result of calling the [[Get]] internal method of O with argument "0".
8. If element0 is undefined or null, let R be the empty String; otherwise, Let R be ToString(element0).
9. Let k be 1.
10. Repeat, while k < len
    1. Let S be the String value produced by concatenating R and sep.
    2. Let element be the result of calling the [[Get]] internal method of O with argument ToString(k).
    3. If element is undefined or null, Let next be the empty String; otherwise, let next be ToString(element).
    4. Let R be a String value produced by concatenating S and next.
    5. Increase k by 1.
11. Return R.

The "length" property of the "join" method is 1.

15.4.4.6   Array.prototype.pop ( )

The last element of the array is removed from the array and returned.

1. Let O be the result of calling ToObject passing the this value as the argument.
2. Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
3. Let len be ToUint32(lenVal).
4. If len is zero,
   1. Call the [[Put]] internal method of O with arguments "length", 0, and true.
   2. Return undefined.
5. Else, len > 0
   1. Let indx be ToString(len–1).
   2. Let element be the result of calling the [[Get]] internal method of O with argument indx.
   3. Call the [[Delete]] internal method of O with arguments indx and true.
   4. Call the [[Put]] internal method of O with arguments "length", indx, and true.

15.4.4.7   Array.prototype.push ( [ item1 [ , item2 [ , … ] ] ] )

The arguments are appended to the end of the array, in the order in which they appear. The new length of the array is returned as the result of the call.

When the push method is called with zero or more arguments item1, item2, etc., the following steps are taken:

1. Let O be the result of calling ToObject passing the this value as the argument.
2. Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
3. Let n be ToUint32(lenVal).
4. Let items be an internal List whose elements are, in left to right order, the arguments that were passed to this function invocation.
5. Repeat, while items is not empty
   1. Remove the first element from items and let E be the value of the element.
   2. Call the [[Put]] internal method of O with arguments ToString(n), E, and true.
   3. Increase n by 1.
6. Call the [[Put]] internal method of O with arguments "length", n, and true.
7. Return n.

The "length" property of the "push" method is 1.

15.4.4.8   Array.prototype.reverse ( )

The elements of the array are rearranged so as to reverse their order. The object is returned as the result of the call.

1. Let O be the result of calling ToObject passing the this value as the argument.
2. Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
3. Let len be ToUint32(lenVal).
4. Let middle be floor(len/2).
5. Let lower be 0.
6. Repeat, while lower ≠ middle
   1. Let upper be len − lower − 1.
   2. Let upperP be ToString(upper).
   3. Let lowerP be ToString(lower).
   4. Let lowerValue be the result of calling the [[Get]] internal method of O with argument lowerP.
   5. Let upperValue be the result of calling the [[Get]] internal method of O with argument upperP.
   6. Let lowerExists be the result of calling the [[HasProperty]] internal method of O with argument lowerP.
   7. Let upperExists be the result of calling the [[HasProperty]] internal method of O with argument upperP.
   8. If lowerExists is true and upperExists is true, then
      1. Call the [[Put]] internal method of O with arguments lowerP, upperValue, and true.
      2. Call the [[Put]] internal method of O with arguments upperP, lowerValue, and true.
   9. Else if lowerExists is false and upperExists is true, then
      1. Call the [[Put]] internal method of O with arguments lowerP, upperValue, and true.
      2. Call the [[Delete]] internal method of O, with arguments upperP and true.
   10. Else if lowerExists is true and upperExists is false, then
       1. Call the [[Delete]] internal method of O, with arguments lowerP and true.

15.4.4.9   Array.prototype.shift ( )

The first element of the array is removed from the array and returned.

1. Let O be the result of calling ToObject passing the this value as the argument.
2. Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
3. Let len be ToUint32(lenVal).
4. If len is zero, then
   1. Call the [[Put]] internal method of O with arguments "length", 0, and true.
   2. Return undefined.
5. Let first be the result of calling the [[Get]] internal method of O with argument "0".
6. Let k be 1.
7. Repeat, while k < len
   1. Let from be ToString(k).
   2. Let to be ToString(k–1).
   3. Let fromPresent be the result of calling the [[HasProperty]] internal method of O with argument from.
   4. If fromPresent is true, then
      1. Let fromVal be the result of calling the [[Get]] internal method of O with argument from.
      2. Call the [[Put]] internal method of O with arguments to, fromVal, and true.
   5. Else, fromPresent is false
      1. Call the [[Delete]] internal method of O with arguments to and true.
   6. Increase k by 1.
8. Call the [[Delete]] internal method of O with arguments ToString(len–1) and true.
9. Call the [[Put]] internal method of O with arguments "length", (len–1) , and true.
10. Return first.

15.4.4.10   Array.prototype.slice (start, end)

The slice method takes two arguments, start and end, and returns an array containing the elements of the array from element start up to, but not including, element end (or through the end of the array if end is undefined). If start is negative, it is treated as length+start where length is the length of the array. If end is negative, it is treated as length+end where length is the length of the array. The following steps are taken:

1. Let O be the result of calling ToObject passing the this value as the argument.
2. Let A be a new array created as if by the expression new Array() where Array is the standard built-in constructor with that name.
3. Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
4. Let len be ToUint32(lenVal).
5. Let relativeStart be ToInteger(start).
6. If relativeStart is negative, let k be max((len + relativeStart),0); else let k be min(relativeStart, len).
7. If end is undefined, let relativeEnd be len; else let relativeEnd be ToInteger(end).
8. If relativeEnd is negative, let final be max((len + relativeEnd),0); else let final be min(relativeEnd, len).
9. Let n be 0.
10. Repeat, while k < final

1. Let Pk be ToString(k).
2. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
3. If kPresent is true, then
   1. Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
   2. Call the [[DefineOwnProperty]] internal method of A with arguments ToString(n), Property Descriptor {[[Value]]: kValue, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
4. Increase k by 1.
5. Increase n by 1.

The length property of the slice method is 2.

15.4.4.11   Array.prototype.sort (comparefn)

The elements of this array are sorted. The sort is not necessarily stable (that is, elements that compare equal do not necessarily remain in their original order). If comparefn is not undefined, it should be a function that accepts two arguments x and y and returns a negative value if x < y, zero if x = y, or a positive value if x > y.

Let obj be the result of calling ToObject passing the this value as the argument.

Let len be the result of applying Uint32 to the result of calling the [[Get]] internal method of obj with argument "length".

If comparefn is not undefined and is not a consistent comparison function for the elements of this array (see below), the behaviour of sort is implementation-defined.

Let proto be the value of the [[Prototype]] internal property of obj. If proto is not null and there exists an integer j such that all of the conditions below are satisfied then the behaviour of sort is implementation-defined:

• obj is sparse (15.4)
• 0 ≤ j < len
• The result of calling the [[HasProperty]] internal method of proto with argument ToString(j) is true.

The behaviour of sort is also implementation defined if obj is sparse and any of the following conditions are true:

• The [[Extensible]] internal property of obj is false.
• Any array index property of obj whose name is a nonnegative integer less than len is a data property whose [[Configurable]] attribute is false.

The behaviour of sort is also implementation defined if any array index property of obj whose name is a nonnegative integer less than len is an accessor property or is a data property whose [[Writable]] attribute is false.

Otherwise, the following steps are taken.

1. Perform an implementation-dependent sequence of calls to the [[Get]] , [[Put]], and [[Delete]] internal methods of obj and to SortCompare (described below), where the first argument for each call to [[Get]], [[Put]], or [[Delete]] is a nonnegative integer less than len and where the arguments for calls to SortCompare are results of previous calls to the [[Get]] internal method. The throw argument to the [[Put]] and [[Delete]] internal methods will be the value true. If obj is not sparse then [[Delete]] must not be called.
2. Return obj.

The returned object must have the following two properties.

• There must be some mathematical permutation π of the nonnegative integers less than len, such that for every nonnegative integer j less than len, if property old[j] existed, then new[π(j)] is exactly the same value as old[j],. But if property old[j] did not exist, then new[π(j)] does not exist.
• Then for all nonnegative integers j and k, each less than len, if SortCompare(j,k) < 0 (see SortCompare below), then π(j) < π(k).

Here the notation old[j] is used to refer to the hypothetical result of calling the [[Get]] internal method of obj with argument j before this function is executed, and the notation new[j] to refer to the hypothetical result of calling the [[Get]] internal method of obj with argument j after this function has been executed.

• Calling comparefn(a,b) always returns the same value v when given a specific pair of values a and b as its two arguments. Furthermore, Type(v) is Number, and v is not NaN. Note that this implies that exactly one of a <_CF b,
  a  =_CF b, and a >_CF b will be true for a given pair of a and b.
• Calling comparefn(a,b) does not modify the this object.
• a =_CF a (reflexivity)
• If  a =_CF b, then b =_CF a (symmetry)
• If  a =_CF b and b =_CF c, then a =_CF c  (transitivity of  =_CF)
• If  a <_CF b and b <_CF c, then a <_CF c  (transitivity of  <_CF)
• If  a >_CF b and b >_CF c, then a >_CF c  (transitivity of  >_CF)
• If  a >_CF b and b >_CF c, then a >_CF c (transitivity of >_CF)

When the SortCompare abstract operation is called with two arguments j and k, the following steps are taken:

1. Let jString be ToString(j).
2. Let kString be ToString(k).
3. Let hasj be the result of calling the [[HasProperty]] internal method of obj with argument jString.
4. Let hask be the result of calling the [[HasProperty]] internal method of obj with argument kString.
5. If hasj and hask are both false, then return +0.
6. If hasj is false, then return 1.
7. If hask is false, then return –1.
8. Let x be the result of calling the [[Get]] internal method of obj with argument jString.
9. Let y be the result of calling the [[Get]] internal method of obj with argument kString.
10. If x and y are both undefined, return +0.
11. If x is undefined, return 1.
12. If y is undefined, return −1.
13. If the argument comparefn is not undefined, then
    1. If IsCallable(comparefn) is false, throw a TypeError exception.
    2. Return the result of calling the [[Call]] internal method of comparefn passing undefined as the this value and with arguments x and y.
14. Let xString be ToString(x).
15. Let yString be ToString(y).
16. If xString < yString, return −1.
17. If xString > yString, return 1.
18. Return +0.

15.4.4.12  Array.prototype.splice (start, deleteCount [ , item1 [ , item2 [ , … ] ] ] )

When the splice method is called with two or more arguments start, deleteCount and (optionally) item1, item2, etc., the deleteCount elements of the array starting at array index start are replaced by the arguments item1, item2, etc. An Array object containing the deleted elements (if any) is returned. The following steps are taken:

1. Let O be the result of calling ToObject passing the this value as the argument.
2. Let A be a new array created as if by the expression new Array() where Array is the standard built-in constructor with that name.
3. Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
4. Let len be ToUint32(lenVal).
5. Let relativeStart be ToInteger(start).
6. If relativeStart is negative, let actualStart be max((len + relativeStart),0); else let actualStart be min(relativeStart, len).
7. Let actualDeleteCount be min(max(ToInteger(deleteCount),0), len – actualStart).
8. Let k be 0.
9. Repeat, while k < actualDeleteCount
   1. Let from be ToString(relativeStart+k).
   2. Let fromPresent be the result of calling the [[HasProperty]] internal method of O with argument from.
   3. If fromPresent is true, then
      1. Let fromValue be the result of calling the [[Get]] internal method of O with argument from.
      2. Call the [[DefineOwnProperty]] internal method of A with arguments ToString(k), Property Descriptor {[[Value]]: fromValue, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
   4. Increment k by 1.
10. Let item be an internal List whose elements are, in left to right order, the portion of the actual argument list starting with item1. The list will be empty if no such items are present.
11. Let itemCount be the number of elements in items.
12. If itemCount < actualDeleteCount, then
    1. Let k be actualStart.
    2. Repeat, while k < (len – actualDeleteCount)
       1. Let from be ToString(k+actualDeleteCount).
       2. Let to be ToString(k+itemCount).
       3. Let fromPresent be the result of calling the [[HasProperty]] internal method of O with
          argument from.
       4. If fromPresent is true, then
          1. Let fromValue be the result of calling the [[Get]] internal method of O with
             argument from.
          2. Call the [[Put]] internal method of O with arguments to, fromValue, and true.
       5. Else, fromPresent is false
          1. Call the [[Delete]] internal method of O with arguments to and true.
       6. Increase k by 1.
    3. Let k be len.
    4. Repeat, while k > (len – actualDeleteCount + itemCount)
       1. Call the [[Delete]] internal method of O with arguments ToString(k–1) and true.
       2. Decrease k by 1.
13. Else if itemCount > actualDeleteCount, then
    1. Let k be (len – actualDeleteCount).
    2. Repeat, while k > actualStart
       1. Let from be ToString(k + actualDeleteCount – 1).
       2. Let to be ToString(k + itemCount – 1)
       3. Let fromPresent be the result of calling the [[HasProperty]] internal method of O with
          argument from.
       4. If fromPresent is true, then
          1. Let fromValue be the result of calling the [[Get]] internal method of O with
             argument from.
          2. Call the [[Put]] internal method of O with arguments to, fromValue, and true.
       5. Else, fromPresent is false
          1. Call the [[Delete]] internal method of O with argument to and true.

The length property of the splice method is 2.

15.4.4.13   Array.prototype.unshift ( [ item1 [ , item2 [ , … ] ] ] )

The arguments are prepended to the start of the array, such that their order within the array is the same as the order in which they appear in the argument list.

When the unshift method is called with zero or more arguments item1, item2, etc., the following steps are taken:

1. Let O be the result of calling ToObject passing the this value as the argument.
2. Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
3. Let len be ToUint32(lenVal).
4. Let argCount be the number of actual arguments.
5. Let k be len.
6. Repeat, while k > 0,
   1. Let from be ToString(k–1).
   2. Let to be ToString(k+argCount –1).
   3. Let fromPresent be the result of calling the [[HasProperty]] internal method of O with argument from.
   4. If fromPresent is true, then
      1. Let fromValue be the result of calling the [[Get]] internal method of O with argument from.
      2. Call the [[Put]] internal method of O with arguments to, fromValue, and true.
   5. Else, fromPresent is false
      1. Call the [[Delete]] internal method of O with arguments to, and true.
   6. Decrease k by 1.
7. Let j be 0.
8. Let items be an internal List whose elements are, in left to right order, the arguments that were passed to this function invocation.
9. Repeat, while items is not empty
   1. Remove the first element from items and let E be the value of that element.
   2. Call the [[Put]] internal method of O with arguments ToString(j), E, and true.
   3. Increase j by 1.
10. Call the [[Put]] internal method of O with arguments "length", len+argCount, and true.
11. Return len+argCount.

The length property of the unshift method is 1.

15.4.4.14   Array.prototype.indexOf ( searchElement [ , fromIndex ] )

indexOf compares searchElement to the elements of the array, in ascending order, using the internal Strict Equality Comparison Algorithm (11.9.6), and if found at one or more positions, returns the index of the first such position; otherwise, -1 is returned.

The optional second argument fromIndex defaults to 0 (i.e. the whole array is searched). If it is greater than or equal to the length of the array, -1 is returned, i.e. the array will not be searched. If it is negative, it is used as the offset from the end of the array to compute fromIndex. If the computed index is less than 0, the whole array will be searched.

When the indexOf method is called with one or two arguments, the following steps are taken:

1. Let O be the result of calling ToObject passing the this value as the argument.
2. Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".
3. Let len be ToUint32(lenValue).
4. If len is 0, return -1.
5. If argument fromIndex was passed let n be ToInteger(fromIndex); else let n be 0.
6. If n ≥ len, return -1.
7. If n ≥ 0, then
   1. Let k be n.
8. Else, n<0
   1. Let k be len - abs(n).
   2. If k is less than 0, then let k be 0.
9. Repeat, while k<len
   1. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument ToString(k).
   2. If kPresent is true, then
      1. Let elementK be the result of calling the [[Get]] internal method of O with the argument ToString(k).
      2. Let same be the result of applying the Strict Equality Comparison Algorithm to searchElement and elementK.
      3. If same is true, return k.
   3. Increase k by 1.
10. Return -1.

The length property of the indexOf method is 1.

15.4.4.15   Array.prototype.lastIndexOf ( searchElement [ , fromIndex ] )

lastIndexOf compares searchElement to the elements of the array in descending order using the internal Strict Equality Comparison Algorithm (11.9.6), and if found at one or more positions, returns the index of the last such position; otherwise, -1 is returned.

The optional second argument fromIndex defaults to the array's length (i.e. the whole array is searched). If it is greater than or equal to the length of the array, the whole array will be searched. If it is negative, it is used as the offset from the end of the array to compute fromIndex. If the computed index is less than 0, -1 is returned.

When the lastIndexOf method is called with one or two arguments, the following steps are taken:

1. Let O be the result of calling ToObject passing the this value as the argument.
2. Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".
3. Let len be ToUint32(lenValue).
4. If len is 0, return -1.
5. If argument fromIndex was passed let n be ToInteger(fromIndex); else let n be len.

6. If n ≥ 0, then let k be min(n, len – 1).
7. Else, n < 0
   1. Let k be len - abs(n).
8. Repeat, while k≥ 0
   1. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument ToString(k).
   2. If kPresent is true, then
      1. Let elementK be the result of calling the [[Get]] internal method of O with the argument ToString(k).
      2. Let same be the result of applying the Strict Equality Comparision Algorithm to searchElement and elementK.
      3. If same is true, return k.
   3. Decrease k by 1.
9. Return -1.

The length property of the lastIndexOf method is 1.

15.4.4.16   Array.prototype.every ( callbackfn [ , thisArg ] )

callbackfn should be a function that accepts three arguments and returns a value that is coercible to the Boolean value true or false. every calls callbackfn once for each element present in the array, in ascending order, until it finds one where callbackfn returns false. If such an element is found, every immediately returns false. Otherwise, if callbackfn returned true for all elements, every will return true. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

every does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by every is set before the first call to callbackfn. Elements which are appended to the array after the call to every begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time every visits them; elements that are deleted after the call to every begins and before being visited are not visited. every acts like the "for all" quantifier in mathematics. In particular, for an empty array, it returns true.

When the every method is called with one or two arguments, the following steps are taken:

1. Let O be the result of calling ToObject passing the this value as the argument.
2. Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".
3. Let len be ToUint32(lenValue).
4. If IsCallable(callbackfn) is false, throw a TypeError exception.
5. If thisArg was supplied, let T be thisArg; else let T be undefined.
6. Let k be 0.
7. Repeat, while k < len
   1. Let Pk be ToString(k).
   2. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
   3. If kPresent is true, then
      1. Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
      2. Let testResult be the result of calling the [[Call]] internal method of callbackfn with T as the this value and argument list containing kValue, k, and O.

The length property of the every method is 1.

15.4.4.17   Array.prototype.some ( callbackfn [ , thisArg ] )

callbackfn should be a function that accepts three arguments and returns a value that is coercible to the Boolean value true or false. some calls callbackfn once for each element present in the array, in ascending order, until it finds one where callbackfn returns true. If such an element is found, some immediately returns true. Otherwise, some returns false. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

some does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by some is set before the first call to callbackfn. Elements that are appended to the array after the call to some begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time that some visits them; elements that are deleted after the call to some begins and before being visited are not visited. some acts like the "exists" quantifier in mathematics. In particular, for an empty array, it returns false.

When the some method is called with one or two arguments, the following steps are taken:

1. Let O be the result of calling ToObject passing the this value as the argument.
2. Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".
3. Let len be ToUint32(lenValue).
4. If IsCallable(callbackfn) is false, throw a TypeError exception.
5. If thisArg was supplied, let T be thisArg; else let T be undefined.
6. Let k be 0.
7. Repeat, while k < len
   1. Let Pk be ToString(k).
   2. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
   3. If kPresent is true, then
      1. Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
      2. Let testResult be the result of calling the [[Call]] internal method of callbackfn with T as the this value and argument list containing kValue, k, and O.
      3. If ToBoolean(testResult) is true, return true.
   4. Increase k by 1.
8. Return false.

The length property of the some method is 1.

15.4.4.18   Array.prototype.forEach ( callbackfn [ , thisArg ] )

callbackfn should be a function that accepts three arguments. forEach calls callbackfn once for each element present in the array, in ascending order. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

forEach does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by forEach is set before the first call to callbackfn. Elements which are appended to the array after the call to forEach begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callback will be the value at the time forEach visits them; elements that are deleted after the call to forEach begins and before being visited are not visited.

When the forEach method is called with one or two arguments, the following steps are taken:

1. Let O be the result of calling ToObject passing the this value as the argument.
2. Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".
3. Let len be ToUint32(lenValue).
4. If IsCallable(callbackfn) is false, throw a TypeError exception.
5. If thisArg was supplied, let T be thisArg; else let T be undefined.
6. Let k be 0.
7. Repeat, while k < len
   1. Let Pk be ToString(k).
   2. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
   3. If kPresent is true, then
      1. Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
      2. Call the [[Call]] internal method of callbackfn with T as the this value and argument list containing kValue, k, and O.
   4. Increase k by 1.
8. Return.

The length property of the forEach method is 1.

15.4.4.19   Array.prototype.map ( callbackfn [ , thisArg ] )

callbackfn should be a function that accepts three arguments. map calls callbackfn once for each element in the array, in ascending order, and constructs a new Array from the results. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

map does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by map is set before the first call to callbackfn. Elements which are appended to the array after the call to map begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time map visits them; elements that are deleted after the call to map begins and before being visited are not visited.

When the map method is called with one or two arguments, the following steps are taken:

1. Let O be the result of calling ToObject passing the this value as the argument.
2. Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".
3. Let len be ToUint32(lenValue).
4. If IsCallable(callbackfn) is false, throw a TypeError exception.
5. If thisArg was supplied, let T be thisArg; else let T be undefined.
6. Let A be a new array created as if by the expression new Array new Array(len) where Array is the standard builtin constructor with that name and len is the value of len.
7. Let k be 0.
8. Repeat, while k < len
   1. Let Pk be ToString(k).
   2. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
   3. If kPresent is true, then
      1. Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
      2. Let mappedValue be the result of calling the [[Call]] internal method of callbackfn with T as the this value and argument list containing kValue, k, and O.
      3. Call the [[DefineOwnProperty]] internal method of A with arguments Pk, Property Descriptor {[[Value]]: mappedValue, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
   4. Increase k by 1.
9. Return A.

The length property of the map method is 1.

15.4.4.20   Array.prototype.filter ( callbackfn [ , thisArg ] )

callbackfn should be a function that accepts three arguments and returns a value that is coercible to the Boolean value true or false. filter calls callbackfn once for each element in the array, in ascending order, and constructs a new array of all the values for which callbackfn returns true. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is not provided, undefined is used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

filter does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by filter is set before the first call to callbackfn. Elements which are appended to the array after the call to filter begins will not be visited by callbackfn. If existing elements of the array are changed their value as passed to callbackfn will be the value at the time filter visits them; elements that are deleted after the call to filter begins and before being visited are not visited.

When the filter method is called with one or two arguments, the following steps are taken:

1. Let O be the result of calling ToObject passing the this value as the argument.
2. Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".

3. Let len be ToUint32(lenValue).
4. If IsCallable(callbackfn) is false, throw a TypeError exception.
5. If thisArg was supplied, let T be thisArg; else let T be undefined.
6. Let A be a new array created as if by the expression new Array() where Array is the standard built-in constructor with that name.
7. Let k be 0.
8. Let to be 0.
9. Repeat, while k < len
   1. Let Pk be ToString(k).
   2. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
   3. If kPresent is true, then
      1. Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
      2. Let selected be the result of calling the [[Call]] internal method of callbackfn with T as the this value and argument list containing kValue, k, and O.
      3. If ToBoolean(selected) is true, then
         1. Call the [[DefineOwnProperty]] internal method of A with arguments ToString(to), Property Descriptor {[[Value]]: kValue, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
         2. Increase to by 1.
   4. Increase k by 1.
10. Return A.

The length property of the filter method is 1.

15.4.4.21   Array.prototype.reduce ( callbackfn [ , initialValue ] )

callbackfn should be a function that takes four arguments. reduce calls the callback, as a function, once for each element present in the array, in ascending order.

callbackfn is called with four arguments: the previousValue (or value from the previous call to callbackfn), the currentValue (value of the current element), the currentIndex, and the object being traversed. The first time that callback is called, the previousValue and currentValue can be one of two values. If an initialValue was provided in the call to reduce, then previousValue will be equal to initialValue and currentValue will be equal to the first value in the array. If no initialValue was provided, then previousValue will be equal to the first value in the array and currentValue will be equal to the second. It is a TypeError if the array contains no elements and initialValue is not provided.

reduce does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by reduce is set before the first call to callbackfn. Elements that are appended to the array after the call to reduce begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time reduce visits them; elements that are deleted after the call to filter begins and before being visited are not visited.

When the reduce method is called with one or two arguments, the following steps are taken:

1. Let O be the result of calling ToObject passing the this value as the argument.
2. Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".
3. Let len be ToUint32(lenValue ).
4. If IsCallable(callbackfn) is false, throw a TypeError exception.
5. If len is 0 and initialValue is not present, throw a TypeError exception.
6. Let k be 0.
7. If initialValue is present, then
   1. Set accumulator to initialValue.
8. Else, initialValue is not present

8. 1. Let kPresent be false.
   2. Repeat, while kPresent is false and k < len
      1. Let Pk be ToString(k).
      2. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
      3. If kPresent is true, then
         1. Let accumulator be the result of calling the [[Get]] internal method of O with
            argument Pk.
      4. Increase k by 1.
   3. If kPresent is false, throw a TypeError exception.
9. Repeat, while k < len
   1. Let Pk be ToString(k).
   2. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
   3. If kPresent is true, then
      1. Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
      2. Let accumulator be the result of calling the [[Call]] internal method of callbackfn with undefined as the this value and argument list containing accumulator, kValue, k, and O.
   4. Increase k by 1.
10. Return accumulator.

The length property of the reduce method is 1.

15.4.4.22   Array.prototype.reduceRight ( callbackfn [ , initialValue ] )

callbackfn should be a function that takes four arguments. reduceRight calls the callback, as a function, once for each element present in the array, in descending order.

callbackfn is called with four arguments: the previousValue (or value from the previous call to callbackfn), the currentValue (value of the current element), the currentIndex and the object being traversed. The first time the function is called, the previousValue and currentValue can be one of two values. If an initialValue was provided in the call to reduceRight, then previousValue will be equal to initialValue and currentValue will be equal to the last value in the array. If no initialValue was provided, then previousValue will be equal to the last value in the array and currentValue will be equal to the second-to-last value. It is a TypeError if the array contains no elements and initialValue is not provided.

reduceRight does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by reduceRight is set before the first call to callbackfn. Elements that are appended to the array after the call to reduceRight begins will not be visited by callbackfn. If existing elements of the array are changed by callbackfn, their value as passed to callbackfn will be the value at the time reduceRight visits them; elements that are deleted after the call to filter begins and before being visited are not visited.

When the reduceRight method is called with one or two arguments, the following steps are taken:

1. Let O be the result of calling ToObject passing the this value as the argument.
2. Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".
3. Let len be ToUint32(lenValue ).
4. If IsCallable(callbackfn) is false, throw a TypeError exception.
5. If len is 0 and initialValue is not present, throw a TypeError exception.
6. Let k be len-1.
7. If initialValue is present, then
   1. Set accumulator to initialValue.
8. Else, initialValue is not present
   1. Let kPresent be false.

8. 2. Repeat, while kPresent is false and k ≥ 0
      1. Let Pk be ToString(k).
      2. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
      3. If kPresent is true, then
         1. Let accumulator be the result of calling the [[Get]] internal method of O with
            argument Pk.
      4. Decrease kby 1.
   3. If kPresent is false, throw a TypeError exception.
9. Repeat, while k ≥ 0
   1. Let Pk be ToString(k).
   2. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
   3. If kPresent is true, then
      1. Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
      2. Let accumulator be the result of calling the [[Call]] internal method of callbackfn with null as the this value and argument list containing accumulator, kValue, k, and O.
   4. Decrease k by 1.
10. Return accumulator.

The length property of the reduceRight method is 1.

15.4.5   Properties of Array Instances

Array instances inherit properties from the Array prototype object and their [[Class]] internal property value is "Array". Array instances also have the following properties.

15.4.5.1   [[DefineOwnProperty]] ( P, Desc, Throw )

Array objects use a variation of the [[DefineOwnProperty]] internal method used for other native ECMAScript objects (8.12.9).

Assume A is an Array object, Desc is a Property Descriptor, and Throw is a Boolean flag.

In the following algorithm, the term “Reject” means "If Throw is true, then throw a TypeError exception, otherwise return false."

When the [[DefineOwnProperty]] internal method of A is called with property P, Property Descriptor Desc, and Boolean flag Throw, the following steps are taken:

1. Let oldLenDesc be the result of calling the [[GetOwnProperty]] internal method of A passing "length" as the argument. The result will never be undefined or an accessor descriptor because Array objects are created with a length data property that cannot be deleted or reconfigured.
2. Let oldLen be oldLenDesc.[[Value]].
3. If P is "length", then
   1. If the [[Value]] field of Desc is absent, then
      1. Return the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) on A passing "length", Desc, and Throw as arguments.
   2. Let newLenDesc be a copy of Desc.
   3. Let newLen be ToUint32(Desc.[[Value]]).
   4. If newLen is not equal to ToNumber( Desc.[[Value]]), throw a RangeError exception.
   5. Set newLenDesc.[[Value] to newLen .
   6. If newLen ≥ oldLen, then
      1. Return the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) on A passing "length", newLenDesc, and Throw as arguments.
   7. Reject if oldLenDesc.[[Writable]] is false.
   8. If newLenDesc.[[Writable]] is absent or has the value true, let newWritable be true.

3. 9. Else,
      1. Need to defer setting the [[Writable]] attribute to false in case any elements cannot be deleted.
      2. Let newWritable be false.
      3. Set newLenDesc.[[Writable] to true.
   10. Let succeeded be the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) on A passing "length", newLenDesc, and Throw as arguments.
   11. If succeeded is false, return false.
   12. While newLen < oldLen repeat,
       1. Set oldLen to oldLen – 1.
       2. Let cannotDelete be the result of calling the [[Delete]] internal method of A passing ToString(oldLen) and false as arguments.
       3. If cannotDelete is true, then
          1. Set newLenDesc.[[Value] to oldLen+1.
          2. If newWritable is false, set newLenDesc.[[Writable] to false.
          3. Call the default [[DefineOwnProperty]] internal method (8.12.9) on A passing "length", newLenDesc, and false as arguments.
          4. Reject.
   13. If newWritable is false, then
       1. Call the default [[DefineOwnProperty]] internal method (8.12.9) on A passing "length", Property Descriptor{[[Writable]]: false}, and false as arguments. This call will always
          return true.
   14. Return true.
4. Else if P is an array index (15.4), then
   1. Let index be ToUint32(P).
   2. Reject if index ≥ oldLen and oldLenDesc.[[Writable]] is false.
   3. Let succeeded be the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) on A passing P, Desc, and false as arguments.
   4. Reject if succeeded is false.
   5. If index ≥ oldLen
      1. Set oldLenDesc.[[Value]] to index + 1.
      2. Call the default [[DefineOwnProperty]] internal method (8.12.9) on A passing "length", oldLenDesc, and false as arguments. This call will always return true.
   6. Return true.
5. Return the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) on A passing P, Desc, and Throw as arguments.

15.4.5.2   length

The length property of this Array object is a data property whose value is always numerically greater than the name of every deletable property whose name is an array index.

The length property initially has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.

15.5   String Objects

15.5.1   The String Constructor Called as a Function

When String is called as a function rather than as a constructor, it performs a type conversion.

15.5.1.1   String ( [ value ] )

Returns a String value (not a String object) computed by ToString(value). If value is not supplied, the empty
String " " is returned.

15.5.2   The String Constructor

When String is called as part of a new expression, it is a constructor: it initialises the newly created object.

15.5.2.1   new String ( [ value ] )

The [[Prototype]] internal property of the newly constructed object is set to the standard built-in String prototype object that is the initial value of String.prototype (15.5.3.1).

The [[Class]] internal property of the newly constructed object is set to "String".

The [[Extensible]] internal property of the newly constructed object is set to true.

The [[PrimitiveValue]] internal property of the newly constructed object is set to ToString(value), or to the empty String if value is not supplied.

15.5.3   Properties of the String Constructor

The value of the [[Prototype]] internal property of the String constructor is the standard built-in Function prototype object (15.3.4).

Besides the internal properties and the length property (whose value is 1), the String constructor has the following properties:

15.5.3.1   String.prototype

The initial value of String.prototype is the standard built-in String prototype object (15.5.4).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.5.3.2   String.fromCharCode ( [ char0 [ , char1 [ , … ] ] ] )

Returns a String value containing as many characters as the number of arguments. Each argument specifies one character of the resulting String, with the first argument specifying the first character, and so on, from left to right. An argument is converted to a character by applying the operation ToUint16 (9.7) and regarding the resulting 16-bit integer as the code unit value of a character. If no arguments are supplied, the result is the empty String.

The length property of the fromCharCode function is 1.

15.5.4   Properties of the String Prototype Object

The String prototype object is itself a String object (its [[Class]] is "String") whose value is an empty String.

The value of the [[Prototype]] internal property of the String prototype object is the standard built-in Object prototype object (15.2.4).

15.5.4.1   String.prototype.constructor

The initial value of String.prototype.constructor is the built-in String constructor.

15.5.4.2   String.prototype.toString ( )

Returns this String value. (Note that, for a String object, the toString method happens to return the same thing as the valueOf method.)

The toString function is not generic; it throws a TypeError exception if its this value is not a String or a String object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

15.5.4.3   String.prototype.valueOf ( )

Returns this String value.

The valueOf function is not generic; it throws a TypeError exception if its this value is not a String or String object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

15.5.4.4   String.prototype.charAt (pos)

Returns a String containing the character at position pos in the String resulting from converting this object to a String. If there is no character at that position, the result is the empty String. The result is a String value, not a String object.

If pos is a value of Number type that is an integer, then the result of x.charAt(pos) is equal to the result of x.substring(pos, pos+1).

When the charAt method is called with one argument pos, the following steps are taken:

1. Call CheckObjectCoercible passing the this value as its argument.
2. Let S be the result of calling ToString, giving it the this value as its argument.
3. Let position be ToInteger(pos).
4. Let size be the number of characters in S.
5. If position < 0 or position ≥ size, return the empty String.
6. Return a String of length 1, containing one character from S, namely the character at position position, where the first (leftmost) character in S is considered to be at position 0, the next one at position 1, and so on.

15.5.4.5   String.prototype.charCodeAt (pos)

Returns a Number (a nonnegative integer less than 2¹⁶) representing the code unit value of the character at position pos in the String resulting from converting this object to a String. If there is no character at that position, the result is NaN.

When the charCodeAt method is called with one argument pos, the following steps are taken:

1. Call CheckObjectCoercible passing the this value as its argument.
2. Let S be the result of calling ToString, giving it the this value as its argument.
3. Let position be ToInteger(pos).
4. Let size be the number of characters in S.
5. If position < 0 or position ≥ size, return NaN.
6. Return a value of Number type, whose value is the code unit value of the character at position position in the String S, where the first (leftmost) character in S is considered to be at position 0, the next one at position 1, and so on.

15.5.4.6   String.prototype.concat ( [ string1 [ , string2 [ , … ] ] ] )

When the concat method is called with zero or more arguments string1, string2, etc., it returns a String consisting of the characters of this object (converted to a String) followed by the characters of each of string1, string2, etc. (where each argument is converted to a String). The result is a String value, not a String object.
The following steps are taken:

1. Call CheckObjectCoercible passing the this value as its argument.
2. Let S be the result of calling ToString, giving it the this value as its argument.
3. Let args be an internal list that is a copy of the argument list passed to this function.
4. Let R be S.
5. Repeat, while args is not empty
   1. Remove the first element from args and let next be the value of that element.
   2. Let R be the String value consisting of the characters in the previous value of R followed by the characters of ToString(next).
6. Return R.

The length property of the concat method is 1.

15.5.4.7   String.prototype.indexOf (searchString, position)

If searchString appears as a substring of the result of converting this object to a String, at one or more positions that are greater than or equal to position, then the index of the smallest such position is returned; otherwise, - 1 is returned. If position is undefined, 0 is assumed, so as to search all of the String.

The indexOf method takes two arguments, searchString and position, and performs the following steps:

1. Call CheckObjectCoercible passing the this value as its argument.
2. Let S be the result of calling ToString, giving it the this value as its argument.
3. Let searchStr be ToString(searchString).
4. Let pos be ToInteger(position). (If position is undefined, this step produces the value 0).
5. Let len be the number of characters in S.
6. Let start be min(max(pos, 0), len).
7. Let searchLen be the number of characters in searchStr.
8. Return the smallest possible integer k not smaller than start such that k+ searchLen is not greater than len, and for all nonnegative integers j less than searchLen, the character at position k+j of S is the same as the character at position j of searchStr); but if there is no such integer k, then return the value -1.

The length property of the indexOf method is 1.

15.5.4.8   String.prototype.lastIndexOf (searchString, position)

If searchString appears as a substring of the result of converting this object to a String at one or more positions that are smaller than or equal to position, then the index of the greatest such position is returned; otherwise, - 1 is returned. If position is undefined, the length of the String value is assumed, so as to search all of the String.

The lastIndexOf method takes two arguments, searchString and position, and performs the following steps:

1. Call CheckObjectCoercible passing the this value as its argument.
2. Let S be the result of calling ToString, giving it the this value as its argument.
3. Let searchStr be ToString(searchString).
4. Let numPos be ToNumber(position). (If position is undefined, this step produces the value NaN).
5. If numPos is NaN, let pos be +∞; otherwise, let pos be ToInteger(numPos).
6. Let len be the number of characters in S.
7. Let start min(max(pos, 0), len).
8. Let searchLen be the number of characters in searchStr.

The length property of the lastIndexOf method is 1.

15.5.4.9   String.prototype.localeCompare (that)

When the localeCompare method is called with one argument that, it returns a Number other than NaN that represents the result of a locale-sensitive String comparison of the this value (converted to a String) with that (converted to a String). The two Strings are S and That. The two Strings are compared in an implementation-defined fashion. The result is intended to order String values in the sort order specified by the system default locale, and will be negative, zero, or positive, depending on whether S comes before That in the sort order, the Strings are equal, or S comes after That in the sort order, respectively.

Before perform the comparisons the following steps are performed to prepare the Strings:

1. Call CheckObjectCoercible passing the this value as its argument.
2. Let S be the result of calling ToString, giving it the this value as its argument.
3. Let That be ToString(That).

The localeCompare method, if considered as a function of two arguments this and That, is a consistent comparison function (as defined in 15.4.4.11) on the set of all Strings. Furthermore, localeCompare returns 0 or –0 when comparing two Strings that are considered canonically equivalent by the Unicode standard.

The actual return values are implementation-defined to permit implementers to encode additional information in the value, but the function is required to define a total ordering on all Strings and to return 0 when comparing Strings that are considered canonically equivalent by the Unicode standard.

If no language-sensitive comparison at all is available from the host environment, this function may perform a bitwise comparison.

15.5.4.10   String.prototype.match (regexp)

When the match method is called with argument regexp, the following steps are taken:

1. Call CheckObjectCoercible passing the this value as its argument.
2. Let S be the result of calling ToString, giving it the this value as its argument.
3. If Type(regexp) is Object and the value of the [[Class]] internal property of regexp is "RegExp", then let rx be regexp;

4. Else, let rx be a new RegExp object created as if by the expression new RegExp(regexp) where RegExp is the standard built-in constructor with that name.
5. Let global be the result of calling the [[Get]] internal method of rx with argument "global".
6. Let exec be the standard built-in function RegExp.prototype.exec (see 15.10.6.2)
7. If global is not true, then
   1. Return the result of calling the [[Call]] internal method of exec with rx as the this value and argument list containing S.
8. Else, global is true
   1. Call the [[Put]] internal method of rx with arguments "lastIndex" and 0.
   2. Let A be a new array created as if by the expression new Array() where Array is the standard built-in constructor with that name.
   3. Let previousLastIndex be 0.
   4. Let n be 0.
   5. Let lastMatch be true.
   6. Repeat, while lastMatch is true
      1. Let result be the result of calling the [[Call]] internal method of exec with rx as the this value and argument list containing S.
      2. If result is null, then set lastMatch to false.
      3. Else, result is not null
         1. Let thisIndex be the result of calling the [[Get]] internal method of rx with argument "lastIndex".
         2. If thisIndex = previousLastIndex then
            1. Call the [[Put]] internal method of rx with arguments "lastIndex" and thisIndex+1.
            2. Set previousLastIndex to thisIndex+1.
         3. Else, set previousLastIndex to thisIndex.
         4. Let matchStr be the result of calling the [[Get]] internal method of result with argument "0".
         5. Call the [[DefineOwnProperty]] internal method of A with arguments ToString(n), the Property Descriptor {[[Value]]: matchStr, [[Writable]]: true, [[Enumerable]]: true, [[configurable]]: true}, and false.
         6. Increment n.
   7. If n = 0, then return null.
   8. Return A.

15.5.4.11   String.prototype.replace (searchValue, replaceValue)

First set string according to the following steps:

1. Call CheckObjectCoercible passing the this value as its argument.
2. Let string be the result of calling ToString, giving it the this value as its argument.

If searchValue is a regular expression (an object whose [[Class]] internal property is "RegExp"), do the following: If searchValue.global is false, then search string for the first match of the regular expression searchValue. If searchValue.global is true, then search string for all matches of the regular expression searchValue. Do the search in the same manner as in String.prototype.match, including the update of searchValue.lastIndex. Let m be the number of left capturing parentheses in searchValue (using NcapturingParens as specified in 15.10.2.1).

If searchValue is not a regular expression, let searchString be ToString(searchValue) and search string for the first occurrence of searchString. Let m be 0.

If replaceValue is a function, then for each matched substring, call the function with the following m + 3 arguments. Argument 1 is the substring that matched. If searchValue is a regular expression, the next m arguments are all of the captures in the MatchResult (see 15.10.2.1). Argument m + 2 is the offset within string where the match occurred, and argument m + 3 is string. The result is a String value derived from the original

input by replacing each matched substring with the corresponding return value of the function call, converted to a String if need be.

Otherwise, let newstring denote the result of converting replaceValue to a String. The result is a String value derived from the original input String by replacing each matched substring with a String derived from newstring by replacing characters in newstring by replacement text as specified in Table 22. These $ replacements are done left-to-right, and, once such a replacement is performed, the new replacement text is not subject to further replacements. For example, "$1,$2".replace(/(\$(\d))/g, "$$1-$1$2") returns "$1- $11,$1-$22". A $ in newstring that does not match any of the forms below is left as is.

Table 22 — Replacement Text Symbol Substitutions

Characters	Replacement text
$$	$
$&	The matched substring.
$‘	The portion of string that precedes the matched substring.
$’	The nth capture, where n is a single digit in the range 1 to 9 and $n is not followed by a decimal digit. If n ≤ m and the nth capture is undefined, use the empty String instead. If n>m, the result is implementation-defined.
$nn	The nnth capture, where nn is a two-digit decimal number in the range 01 to 99. If nn ≤ m and the nnth capture is undefined, use the empty String instead. If nn > m, the result is implementation-defined.

15.5.4.12   String.prototype.search (regexp)

When the search method is called with argument regexp, the following steps are taken:

1. Call CheckObjectCoercible passing the this value as its argument.
2. Let string be the result of calling ToString, giving it the this value as its argument.
3. If Type(regexp) is Object and the value of the [[Class]] internal property of regexp is "RegExp", then let rx be regexp;
4. Else, let rx be a new RegExp object created as if by the expression new RegExp(regexp) where RegExp is the standard built-in constructor with that name.
5. Search the value string from its beginning for an occurrence of the regular expression pattern rx. Let result be a Number indicating the offset within string where the pattern matched, or –1 if there was no match. The lastIndex and global properties of regexp are ignored when performing the search. The lastIndex property of regexp is left unchanged.
6. Return result.

15.5.4.13   String.prototype.slice (start, end)

The slice method takes two arguments, start and end, and returns a substring of the result of converting this object to a String, starting from character position start and running to, but not including, character position end (or through the end of the String if end is undefined). If start is negative, it is treated as sourceLength+start where sourceLength is the length of the String. If end is negative, it is treated as sourceLength+end where sourceLength is the length of the String. The result is a String value, not a String object. The following steps are taken:

1. Call CheckObjectCoercible passing the this value as its argument.
2. Let S be the result of calling ToString, giving it the this value as its argument.

3. Let len be the number of characters in S.
4. Let intStart be ToInteger(start).
5. If end is undefined, let intEnd be len; else let intEnd be ToInteger(end).
6. If intStart is negative, let from be max(len + intStart,0); else let from be min(intStart, len).
7. If intEnd is negative, let to be max(len + intEnd,0); else let to be min(intEnd, len).
8. Let span be max(to – from,0).
9. Return a String containing span consecutive characters from S beginning with the character at position from.

The length property of the slice method is 2.

15.5.4.14   String.prototype.split (separator, limit)

Returns an Array object into which substrings of the result of converting this object to a String have been stored. The substrings are determined by searching from left to right for occurrences of separator; these occurrences are not part of any substring in the returned array, but serve to divide up the String value. The value of separator may be a String of any length or it may be a RegExp object (i.e., an object whose [[Class]] internal property is "RegExp"; see 15.10).

The value of separator may be an empty String, an empty regular expression, or a regular expression that can match an empty String. In this case, separator does not match the empty substring at the beginning or end of the input String, nor does it match the empty substring at the end of the previous separator match. (For example, if separator is the empty String, the String is split up into individual characters; the length of the result array equals the length of the String, and each substring contains one character.) If separator is a regular expression, only the first match at a given position of the this String is considered, even if backtracking could yield a non-empty-substring match at that position. (For example, "ab".split(/a*?/) evaluates to the array ["a","b"], while "ab".split(/a*/) evaluates to the array["","b"].)

If the this object is (or converts to) the empty String, the result depends on whether separator can match the empty String. If it can, the result array contains no elements. Otherwise, the result array contains one element, which is the empty String.

If separator is a regular expression that contains capturing parentheses, then each time separator is matched the results (including any undefined results) of the capturing parentheses are spliced into the output array. For example,

"A<B>bold</B>and<CODE<coded</CODE<".split(/<(\/)?([^<>]+)>/) evaluates to the array

["A", undefined, "B", "bold", "/", "B", "and", undefined, "CODE", "coded", "/", "CODE", ""]

If separator is undefined, then the result array contains just one String, which is the this value (converted to a String). If limit is not undefined, then the output array is truncated so that it contains no more than limit elements.

When the split method is called, the following steps are taken:

1. Call CheckObjectCoercible passing the this value as its argument.
2. Let S be the result of calling ToString, giving it the this value as its argument.
3. Let A be a new array created as if by the expression new Array()where Array is the standard built-in constructor with that name.
4. Let lengthA be 0.
5. If limit is undefined, let lim = 2³²–1; else let lim = ToUint32(limit).
6. Let s be the number of characters in S.
7. Let p = 0.
8. If separator is a RegExp object (its [[Class]] is "RegExp"), let R = separator; otherwise let R = ToString(separator).

9. If lim = 0, return A.
10. If separator is undefined, then
    1. Call the [[DefineOwnProperty]] internal method of A with arguments "0", Property Descriptor {[[Value]]: S, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
    2. Return A.
11. If s = 0, then
    1. Call SplitMatch(R, S, 0) and let z be its MatchResult result.
    2. If z is not failure, return A.
    3. Call the [[DefineOwnProperty]] internal method of A with arguments "0", Property Descriptor {[[Value]]: S, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
    4. Return A.
12. Let q = p.
13. Repeat, while q ≠ s
    1. Call SplitMatch(R, S, q) and let z be its MatchResult result.
    2. If z is failure, then let q = q+1.
    3. Else, z is not failure
       1. z must be a State. Let e be z's endIndex and let cap be z's captures array.
       2. If e = p, then let q = q+1.
       3. Else, e ≠ p
          1. Let T be a String value equal to the substring of S consisting of the characters at positions p (inclusive) through q (exclusive).
          2. Call the [[DefineOwnProperty]] internal method of A with arguments ToString(lengthA), Property Descriptor {[[Value]]: T, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
          3. Increment lengthA by 1.
          4. If lengthA = lim, return A.
          5. Let p = e.
          6. Let i = 0.
          7. Repeat, while i is not equal to the number of elements in cap.
             1. Let i = i+1.
             2. Call the [[DefineOwnProperty]] internal method of A with arguments ToString(lengthA), Property Descriptor {[[Value]]: cap[i], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
             3. Increment lengthA by 1.
             4. If A.length = lim, return A.
          8. Let q = p.
14. Let T be a String value equal to the substring of S consisting of the characters at positions p (inclusive) through s (exclusive).
15. Call the [[DefineOwnProperty]] internal method of A with arguments ToString(lengthA), Property Descriptor {[[Value]]: T, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
16. Return A.

The abstract operation SplitMatch takes three parameters, a String S, an integer q, and a String or RegExp R, and performs the following in order to return a MatchResult (see 15.10.2.1):

1. If R is a RegExp object (its [[Class]] is "RegExp"), then
   1. Call the [[Match]] internal method of R giving it the arguments S and q, and return the MatchResult result.
2. Type(R) must be String. Let r be the number of characters in R.
3. Let s be the number of characters in S.
4. If q+r > s then return the MatchResult failure.
5. If there exists an integer i between 0 (inclusive) and r (exclusive) such that the character at position q+i of S is different from the character at position i of R, then return failure.
6. Let cap be an empty array of captures (see 15.10.2.1).
7. Return the State (q+r, cap). (see 15.10.2.1)

The length property of the split method is 2.

15.5.4.15   String.prototype.substring (start, end)

The substring method takes two arguments, start and end, and returns a substring of the result of converting this object to a String, starting from character position start and running to, but not including, character position end of the String (or through the end of the String is end is undefined). The result is a String value, not a String object.

If either argument is NaN or negative, it is replaced with zero; if either argument is larger than the length of the String, it is replaced with the length of the String.

If start is larger than end, they are swapped.

The following steps are taken:

1. Call CheckObjectCoercible passing the this value as its argument.
2. Let S be the result of calling ToString, giving it the this value as its argument.
3. Let len be the number of characters in S.
4. Let intStart be ToInteger(start).
5. If end is undefined, let intEnd be len; else let intEnd be ToInteger(end).
6. Let finalStart be min(max(intStart, 0), len).
7. Let finalEnd be min(max(intEnd, 0), len).
8. Let from be min(finalStart, finalEnd).
9. Let to be max(finalStart, finalEnd).
10. Return a String whose length is to - from, containing characters from S, namely the characters with indices from through to −1, in ascending order.

The length property of the substring method is 2.

15.5.4.16   String.prototype.toLowerCase ( )

The following steps are taken:

1. Call CheckObjectCoercible passing the this value as its argument.
2. Let S be the result of calling ToString, giving it the this value as its argument.
3. Let L be a String where each character of L is either the Unicode lowercase equivalent of the corresponding character of S or the actual corresponding character of S if no Unicode lowercase equivalent exists.
4. Return L.

For the purposes of this operation, the 16-bit code units of the Strings are treated as code points in the Unicode Basic Multilingual Plane. Surrogate code points are directly transferred from S to L without any mapping.

The result must be derived according to the case mappings in the Unicode character database (this explicitly includes not only the UnicodeData.txt file, but also the SpecialCasings.txt file that accompanies it in Unicode 2.1.8 and later).

15.5.4.17   String.prototype.toLocaleLowerCase ( )

This function works exactly the same as toLowerCase except that its result is intended to yield the correct result for the host environment’s current locale, rather than a locale-independent result. There will only be a difference in the few cases (such as Turkish) where the rules for that language conflict with the regular Unicode case mappings.

15.5.4.18   String.prototype.toUpperCase ( )

This function behaves in exactly the same way as String.prototype.toLowerCase, except that characters are mapped to their uppercase equivalents as specified in the Unicode Character Database.

15.5.4.19   String.prototype.toLocaleUpperCase ( )

This function works exactly the same as toUpperCase except that its result is intended to yield the correct result for the host environment’s current locale, rather than a locale-independent result. There will only be a difference in the few cases (such as Turkish) where the rules for that language conflict with the regular Unicode case mappings.

15.5.4.20   String.prototype.trim ( )

The following steps are taken:

1. Call CheckObjectCoercible passing the this value as its argument.
2. Let S be the result of calling ToString, giving it the this value as its argument.
3. Let T be a String value that is a copy of S with both leading and trailing white space removed. The definition of white space is the union of WhiteSpace and LineTerminator.
4. Return T.

15.5.5   Properties of String Instances

String instances inherit properties from the String prototype object and their [[Class]] internal property value is "String". String instances also have a [[PrimitiveValue]] internal property, a length property, and a set of enumerable properties with array index names.

The [[PrimitiveValue]] internal property is the String value represented by this String object. The array index named properties correspond to the individual characters of the String value. A special [[GetOwnProperty]] internal method is used to specify the number, values, and attributes of the array index named properties.

15.5.5.1   length

The number of characters in the String value represented by this String object.

Once a String object is created, this property is unchanging. It has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.5.5.2   [[GetOwnProperty]] ( P )

String objects use a variation of the [[GetOwnProperty]] internal method used for other native ECMAScript objects (8.12.1). This special internal method is used to specify the array index named properties of String objects.

Assume S is a String object and P is a String.

When the [[GetOwnProperty]] internal method of S is called with property name P, the following steps are taken:

1. Let desc be the result of calling the default [[GetOwnProperty]] internal method (8.12.1) on S with argument P.
2. If desc is not undefined return desc.
3. If P is not an array index (15.4), return undefined.
4. Let str be the String value of the [[PrimitiveValue]] internal property of S.
5. Let index be ToUint32(P).
6. Let len be the number of characters in str.
7. If len ≤ index, return undefined.
8. Let resultStr be a String of length 1, containing one character from str, specifically the character at position index, where the first (leftmost) character in str is considered to be at position 0, the next one at position 1, and so on.
9. Return a Property Descriptor { [[Value]]: resultStr, [[Enumerable]]: true, [[Writable]]: false, [[Configurable]]: false }

15.6   Boolean Objects

15.6.1   The Boolean Constructor Called as a Function

When Boolean is called as a function rather than as a constructor, it performs a type conversion.

15.6.1.1   Boolean (value)

Returns a Boolean value (not a Boolean object) computed by ToBoolean(value).

15.6.2   The Boolean Constructor

When Boolean is called as part of a new expression it is a constructor: it initializes the newly created object.

15.6.2.1   new Boolean (value)

The [[Prototype]] internal property of the newly constructed object is set to the original Boolean prototype object, the one that is the initial value of Boolean.prototype (15.6.3.1).

The [[Class]] internal property of the newly constructed Boolean object is set to "Boolean".

The [[PrimitiveValue]] internal property of the newly constructed Boolean object is set to ToBoolean(value).

The [[Extensible]] internal property of the newly constructed object is set to true.

15.6.3   Properties of the Boolean Constructor

The value of the [[Prototype]] internal property of the Boolean constructor is the Function prototype object (15.3.4).

Besides the internal properties and the length property (whose value is 1), the Boolean constructor has the following property:

15.6.3.1   Boolean.prototype

The initial value of Boolean.prototype is the Boolean prototype object (15.6.4).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.6.4   Properties of the Boolean Prototype Object

The Boolean prototype object is itself a Boolean object (its [[Class]] is "Boolean") whose value is false. The value of the [[Prototype]] internal property of the Boolean prototype object is the standard built-in Object prototype object (15.2.4).

15.6.4.1   Boolean.prototype.constructor

The initial value of Boolean.prototype.constructor is the built-in Boolean constructor.

15.6.4.2   Boolean.prototype.toString ( )

The following steps are taken:

1. Let B be the this value.
2. If Type(B) is Boolean, then let b be B.
3. Else if Type(B) is Object and the value of the [[Class]] internal property of B is "Boolean", then let b be the value of the [[PrimitiveValue]] internal property of B.
4. Else throw a TypeError exception.
5. If b is true, then return "true"; else return "false".

15.6.4.3   Boolean.prototype.valueOf ( )

The following steps are taken:

1. Let B be the this value.
2. If Type(B) is Boolean, then let b be B.
3. Else if Type(B) is Object and the value of the [[Class]] internal property of B is "Boolean", then let b be the value of the [[PrimitiveValue]] internal property of B.
4. Else throw a TypeError exception.
5. Return b.

15.6.5   Properties of Boolean Instances

Boolean instances inherit properties from the Boolean prototype object and their [[Class]] internal property value is "Boolean". Boolean instances also have a [[PrimitiveValue]] internal property.

The [[PrimitiveValue]] internal property is the Boolean value represented by this Boolean object.

15.7   Number Objects

15.7.1   The Number Constructor Called as a Function

When Number is called as a function rather than as a constructor, it performs a type conversion.

15.7.1.1   Number ( [ value ] )

Returns a Number value (not a Number object) computed by ToNumber(value) if value was supplied, else returns +0.

15.7.2   The Number Constructor

When Number is called as part of a new expression it is a constructor: it initializes the newly created object.

15.7.2.1   new Number ( [ value ] )

The [[Prototype]] internal property of the newly constructed object is set to the original Number prototype object, the one that is the initial value of Number.prototype (15.7.3.1).

The [[Class]] internal property of the newly constructed object is set to "Number".

The [[PrimitiveValue]] internal property of the newly constructed object is set to ToNumber(value) if value was supplied, else to +0.

The [[Extensible]] internal property of the newly constructed object is set to true.

15.7.3   Properties of the Number Constructor

The value of the [[Prototype]] internal property of the Number constructor is the Function prototype object (15.3.4).

Besides the internal properties and the length property (whose value is 1), the Number constructor has the following property:

15.7.3.1   Number.prototype

The initial value of Number.prototype is the Number prototype object (15.7.4).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.7.3.2   Number.MAX_VALUE

The value of Number.MAX_VALUE is the largest positive finite value of the Number type, which is approximately 1.7976931348623157 × 10³⁰⁸.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: fafalselse, [[Configurable]]: false }.

15.7.3.3   Number.MIN_VALUE

The value of Number.MIN_VALUE is the smallest positive value of the Number type, which is approximately
5 × 10^-324.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.7.3.4   Number.NaN

The value of Number.NaN is NaN.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.7.3.5   Number.NEGATIVE_INFINITY

The value of Number.NEGATIVE_INFINITY is −∞.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.7.3.6   Number.POSITIVE_INFINITY

The value of Number.POSITIVE_INFINITY is +∞.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.7.4   Properties of the Number Prototype Object

The Number prototype object is itself a Number object (its [[Class]] is "Number") whose value is +0.

The value of the [[Prototype]] internal property of the Number prototype object is the standard built-in Object prototype object (15.2.4).

Unless explicitly stated otherwise, the methods of the Number prototype object defined below are not generic and the this value passed to them must be either a Number value or an Object for which the value of the [[Class]] internal property is "Number".

In the following descriptions of functions that are properties of the Number prototype object, the phrase “this Number object” refers to either the object that is the this value for the invocation of the function or, if Type(this value) is Number, an object that is created as if by the expression new Number(this value) where Number is the standard built-in constructor with that name. Also, the phrase “this Number value” refers to either the Number value represented by this Number object, that is, the value of the [[PrimitiveValue]] internal property of this Number object or the this value if its type is Number. A TypeError exception is thrown if the this value is neither an object for which the value of the [[Class]] internal property is "Number" or a value whose type is Number.

15.7.4.1   Number.prototype.constructor

The initial value of Number.prototype.constructor is the built-in Number constructor.

15.7.4.2   Number.prototype.toString ( [ radix ] )

The optional radix should be an integer value in the inclusive range 2 to 36. If radix not present or is undefined the Number 10 is used as the value of radix. If ToInteger(radix) is the Number 10 then this Number value is given as an argument to the ToString abstract operation; the resulting String value is returned.

If ToInteger(radix) is not an integer between 2 and 36 inclusive throw a RangeError exception. If ToInteger(radix) is an integer from 2 to 36, but not 10, the result is a String representation of this Number value using the specified radix. Letters a-z are used for digits with values 10 through 35. The precise algorithm is implementation-dependent if the radix is not 10, however the algorithm should be a generalization of that specified in 9.8.1.

The toString function is not generic; it throws a TypeError exception if its this value is not a Number or a Number object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

15.7.4.3   Number.prototype.toLocaleString()

Produces a String value that represents this Number value formatted according to the conventions of the host environment’s current locale. This function is implementation-dependent, and it is permissible, but not encouraged, for it to return the same thing as toString.

15.7.4.4   Number.prototype.valueOf ( )

Returns this Number value. The valueOf function is not generic; it throws a TypeError exception if its this value is not a Number or a Number object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

15.7.4.5   Number.prototype.toFixed (fractionDigits)

Return a String containing this Number value represented in decimal fixed-point notation with fractionDigits digits after the decimal point. If fractionDigits is undefined, 0 is assumed. Specifically, perform the following steps:

1. Let f be ToInteger(fractionDigits). (If fractionDigits is undefined, this step produces the value 0).
2. If f < 0 or f > 20, throw a RangeError exception.
3. Let x be this Number value.
4. If x is NaN, return the String "NaN".
5. Let s be the empty String.
6. If x < 0, then
   1. Let s be "-".
   2. Let x = –x.
7. If x ≥ 10²¹, then
   1. Let m = ToString(x).
8. Else, x < 10²¹
   1. Let n be an integer for which the exact mathematical value of n ÷ 10^f – x is as close to zero as possible. If there are two such n, pick the larger n.
   2. If n = 0, let m be the String "0". Otherwise, let m be the String consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
   3. If f ≠ 0, then
      1. Let k be the number of characters in m.
      2. If k ≤ f, then
         1. Let z be the String consisting of f+1–k occurrences of the character ‘0’.
         2. Let m be the concatenation of Strings z and m.
         3. Let k = f + 1.
      3. Let a be the first k–f characters of m, and let b be the remaining f characters of m.
      4. Let m be the concatenation of the three Strings a, ".", and b.
9. Return the concatenation of the Strings s and m.

The length property of the toFixed method is 1.

If the toFixed method is called with more than one argument, then the behaviour is undefined (see clause 15).

An implementation is permitted to extend the behaviour of toFixed for values of fractionDigits less than 0 or greater than 20. In this case toFixed would not necessarily throw RangeError for such values.

(1000000000000000128).toString() returns "1000000000000000100",
while (1000000000000000128).toFixed(0) returns "1000000000000000128".

15.7.4.6   Number.prototype.toExponential (fractionDigits)

Return a String containing this Number value represented in decmal exponential notation with one digit before the significand's decimal point and fractionDigits digits after the significand's decimal point. If fractionDigits is undefined, include as many significand digits as necessary to uniquely specify the Number (just like in ToString except that in this case the Number is always output in exponential notation). Specifically, perform the following steps:

1. Let x be this Number value.
2. Let f be ToInteger(fractionDigits).
3. If x is NaN, return the String "NaN".
4. Let s be the empty String.
5. If x < 0, then
   1. Let s be "-".
   2. Let x = –x.
6. If x = +∞, then
   1. Return the concatenation of the Strings s and "Infinity".
7. If fractionDigits is not undefined and (f < 0 or f > 20), throw a RangeError exception.
8. If x = 0, then
   1. Let f = 0.
   2. Let m be the String consisting of f+1 occurrences of the character ‘0’.
   3. Let e = 0.
9. Else, x ≠ 0
   1. If fractionDigits is not undefined, then
      1. Let e and n be integers such that 10^f ≤ n < 10^f+1 and for which the exact mathematical value of n × 10^e–f – x is as close to zero as possible. If there are two such sets of e and n, pick the e and n for which n × 10^e–f is larger.
   2. Else, fractionDigits is undefined
      1. Let e, n, and f be integers such that f ≥ 0, 10^f ≤ n < 10^f+1, the number value for n × 10^e–f is x, and f is as small as possible. Note that the decimal representation of n has f+1 digits, n is not divisible by 10, and the least significant digit of n is not necessarily uniquely determined by these criteria.
   3. Let m be the String consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
10. If f ≠ 0, then
    1. Let a be the first character of m, and let b be the remaining f characters of m.
    2. Let m be the concatenation of the three Strings a, ".", and b.
11. If e = 0, then
    1. Let c = "+".
    2. Let d = "0".
12. Else
    1. If e > 0, then let c = "+".
    2. Else, e ≤ 0
       1. Let c = "-".
       2. Let e = –e.
    3. Let d be the String consisting of the digits of the decimal representation of e (in order, with no leading zeroes).
13. Let m be the concatenation of the four Strings m, "e", c, and d.
14. Return the concatenation of the Strings s and m.

The length property of the toExponential method is 1.

If the toExponential method is called with more than one argument, then the behaviour is undefined (see
clause 15).

An implementation is permitted to extend the behaviour of toExponential for values of fractionDigits less than 0 or greater than 20. In this case toExponential would not necessarily throw RangeError for such values.

15.7.4.7   Number.prototype.toPrecision (precision)

Return a String containing this Number value represented either in decimal exponential notation with one digit before the significand's decimal point and precision–1 digits after the significand's decimal point or in decimal fixed notation with precision significant digits. If precision is undefined, call ToString (9.8.1) instead. Specifically, perform the following steps:

1. Let x be this Number value.
2. If precision is undefined, return ToString(x).
3. Let p be ToInteger(precision).
4. If x is NaN, return the String "NaN".
5. Let s be the empty String.
6. If x < 0, then
   1. Let s be "-".
   2. Let x = –x.
7. If x = +∞, then
   1. Return the concatenation of the Strings s and "Infinity".
8. If p < 1 or p > 21, throw a RangeError exception.
9. If x = 0, then
   1. Let m be the String consisting of p occurrences of the character ‘0’.
   2. Let e = 0.
10. Else x ≠ 0,
    1. Let e and n be integers such that 10^p–1 ≤ n < 10^p and for which the exact mathematical value of
       n × 10^e–p+1 – x is as close to zero as possible. If there are two such sets of e and n, pick the e and n for which n × 10^e–p+1 is larger.
    2. Let m be the String consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
    3. If e < –6 or e ≥ p, then
       1. Let a be the first character of m, and let b be the remaining p1 characters of m.
       2. Let m be the concatenation of the three Strings a, ".", and b.
       3. If e = 0, then
          1. Let c = "+" and d = "0".
       4. Else e ≠ 0,
          1. If e > 0, then
             1. Let c = "+".
          2. Else e < 0,
             1. Let c = "-".
             2. Let e = –e.
          3. Let d be the String consisting of the digits of the decimal representation of e (in order, with no leading zeroes).
       5. Let m be the concatenation of the five Strings s, m, "e", c, and d.
11. If e = p–1, then return the concatenation of the Strings s and m.
12. If e ≥ 0, then
    1. Let m be the concatenation of the first e+1 characters of m, the character ‘.’, and the remaining p– (e+1) characters of m.
13. Else e < 0,
    1. Let m be the concatenation of the String "0.", –(e+1) occurrences of the character ‘0’, and the
       String m.

The length property of the toPrecision method is 1.

If the toPrecision method is called with more than one argument, then the behaviour is undefined (see clause 15).

An implementation is permitted to extend the behaviour of toPrecision for values of precision less than 1 or greater than 21. In this case toPrecision would not necessarily throw RangeError for such values.

15.7.5   Properties of Number Instances

Number instances inherit properties from the Number prototype object and their [[Class]] internal property value is "Number". Number instances also have a [[PrimitiveValue]] internal property.

The [[PrimitiveValue]] internal property is the Number value represented by this Number object.

15.8   The Math Object

The Math object is a single object that has some named properties, some of which are functions. The value of the [[Prototype]] internal property of the Math object is the standard built-in Object prototype object (15.2.4). The value of the [[Class]] internal property of the Math object is "Math".

The Math object does not have a [[Construct]] internal property; it is not possible to use the Math object as a constructor with the new operator.

The Math object does not have a [[Call]] internal property; it is not possible to invoke the Math object as a function.

15.8.1   Value Properties of the Math Object

15.8.1.1   E

The Number value for e, the base of the natural logarithms, which is approximately 2.7182818284590452354.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.8.1.2   LN10

The Number value for the natural logarithm of 10, which is approximately 2.302585092994046.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.8.1.3   LN2

The Number value for the natural logarithm of 2, which is approximately 0.6931471805599453.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.8.1.4   LOG2E

The Number value for the base-2 logarithm of e, the base of the natural logarithms; this value is approximately 1.4426950408889634.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.8.1.5   LOG10E

The Number value for the base-10 logarithm of e, the base of the natural logarithms; this value is approximately 0.4342944819032518. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.8.1.6   PI

The Number value for π, the ratio of the circumference of a circle to its diameter, which is approximately 3.1415926535897932. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.8.1.7   SQRT1_2

The Number value for the square root of ½, which is approximately 0.7071067811865476. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.8.1.8   SQRT2

The Number value for the square root of 2, which is approximately 1.4142135623730951. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.8.2   Function Properties of the Math Object

Each of the following Math object functions applies the ToNumber abstract operator to each of its arguments (in left-to-right order if there is more than one) and then performs a computation on the resulting Number value(s). In the function descriptions below, the symbols NaN, −0, +0, −∞ and +∞ refer to the Number values described in 8.5. Although the choice of algorithms is left to the implementation, it is recommended (but not specified by this standard) that implementations use the approximation algorithms for IEEE 754 arithmetic contained in fdlibm, the freely distributable mathematical library from Sun Microsystems (http://www.netlib.org/fdlibm).

15.8.2.1   abs (x)

Returns the absolute value of x; the result has the same magnitude as x but has positive sign.

• If x is NaN, the result is NaN.
• If x is −0, the result is +0.
• If x is −∞, the result is +∞.

15.8.2.2   acos (x)

Returns an implementation-dependent approximation to the arc cosine of x. The result is expressed in radians and ranges from +0 to +π.

• If x is NaN, the result is NaN.
• If x is greater than 1, the result is NaN.
• If x is less than −1, the result is NaN.
• If x is exactly 1, the result is +0.

15.8.2.3   asin (x)

Returns an implementation-dependent approximation to the arc sine of x. The result is expressed in radians and ranges from −π/2 to +π/2.

• If x is NaN, the result is NaN.
• If x is greater than 1, the result is NaN.
• If x is less than –1, the result is NaN.
• If x is +0, the result is +0.
• If x is −0, the result is −0.

15.8.2.4   atan (x)

Returns an implementation-dependent approximation to the arc tangent of x. The result is expressed in radians and ranges from −π/2 to +π/2.

• If x is NaN, the result is NaN.
• If x is +0, the result is +0.
• If x is −0, the result is −0.
• If x is +∞, the result is an implementation-dependent approximation to +π/2.
• If x is −∞, the result is an implementation-dependent approximation to −π/2.

15.8.2.5   atan2 (y, x)

Returns an implementation-dependent approximation to the arc tangent of the quotient y/x of the arguments y and x, where the signs of y and x are used to determine the quadrant of the result. Note that it is intentional and traditional for the two-argument arc tangent function that the argument named y be first and the argument named x be second. The result is expressed in radians and ranges from −π to +π.

• If either x or y is NaN, the result is NaN.
• If y>0 and x is +0, the result is an implementation-dependent approximation to +π/2.
• If y>0 and x is −0, the result is an implementation-dependent approximation to +π/2.
• If y is +0 and x>0, the result is +0.
• If y is +0 and x is +0, the result is +0.
• If y is +0 and x is −0, the result is an implementation-dependent approximation to +π.
• If y is +0 and x<0, the result is an implementation-dependent approximation to +π.
• If y is −0 and x>0, the result is −0.
• If y is −0 and x is +0, the result is −0.
• If y is −0 and x is −0, the result is an implementation-dependent approximation to −π.
• If y is −0 and x<0, the result is an implementation-dependent approximation to −π.
• If y<0 and x is +0, the result is an implementation-dependent approximation to −π/2.
• If y<0 and x is −0, the result is an implementation-dependent approximation to −π/2.

• If   y > 0 and y is finite and x is +∞, the result is +0.
• If   y > 0 and y is finite and x is −∞, the result if an implementation-dependent approximation to +π.
• If   y < 0 and y is finite and x is +∞, the result is −0.
• If   y <0 and y is finite and x is −∞, the result is an implementation-dependent approximation to −π.
• If y is +∞ and x is finite, the result is an implementation-dependent approximation to +π/2.
• If y is −∞ and x is finite, the result is an implementation-dependent approximation to −π/2.
• If y is +∞ and x is +∞, the result is an implementation-dependent approximation to +π/4.
• If y is +∞ and x is −∞, the result is an implementation-dependent approximation to +3π/4.
• If y is −∞ and x is +∞, the result is an implementation-dependent approximation to −π/4.
• If y is −∞ and x is −∞, the result is an implementation-dependent approximation to −3π/4.

15.8.2.6   ceil (x)

Returns the smallest (closest to −∞) Number value that is not less than x and is equal to a mathematical integer. If x is already an integer, the result is x.

• If x is NaN, the result is NaN.
• If x is +0, the result is +0.
• If x is −0, the result is −0.
• If x is +∞, the result is +∞.
• If x is −∞, the result is −∞.
• If x is less than 0 but greater than -1, the result is −0.

The value of Math.ceil(x) is the same as the value of -Math.floor(-x).

15.8.2.7   cos (x)

Returns an implementation-dependent approximation to the cosine of x. The argument is expressed in radians.

• If x is NaN, the result is NaN.
• If x is +0, the result is 1.
• If x is −0, the result is 1.
• If x is +∞, the result is NaN.
• If x is −∞, the result is NaN.

15.8.2.8   exp (x)

Returns an implementation-dependent approximation to the exponential function of x (e raised to the power of x, where e is the base of the natural logarithms).

• If x is NaN, the result is NaN.
• If x is +0, the result is 1.
• If x is −0, the result is 1.
• If x is +∞, the result is +∞.
• If x is −∞, the result is +0.

15.8.2.9   floor (x)

Returns the greatest (closest to +∞) Number value that is not greater than x and is equal to a mathematical integer. If x is already an integer, the result is x.

• If x is NaN, the result is NaN.
• If x is +0, the result is +0.
• If x is −0, the result is −0.
• If x is +∞, the result is +∞.
• If x is −∞, the result is −∞.

• If x is greater than 0 but less than 1, the result is +0.

NOTE The value of Math.floor(x) is the same as the value of -Math.ceil(-x).

15.8.2.10   log (x)

• Returns an implementation-dependent approximation to the natural logarithm of x.
• If x is NaN, the result is NaN.
• If x is less than 0, the result is NaN.
• If x is +0 or −0, the result is −∞.
• If x is 1, the result is +0.
• If x is +∞, the result is +∞.

15.8.2.11   max ( [ value1 [ , value2 [ , … ] ] ] )

Given zero or more arguments, calls ToNumber on each of the arguments and returns the largest of the resulting values.

• If no arguments are given, the result is −∞.
• If any value is NaN, the result is NaN.
• The comparison of values to determine the largest value is done as in 11.8.5 except that +0 is considered to be larger than −0.

The length property of the max method is 2.

15.8.2.12   min ( [ value1 [ , value2 [ , … ] ] ] )

Given zero or more arguments, calls ToNumber on each of the arguments and returns the smallest of the resulting values.

• If no arguments are given, the result is +∞.
• If any value is NaN, the result is NaN.
• The comparison of values to determine the smallest value is done as in 11.8.5 except that +0 is considered to be larger than −0.

The length property of the min method is 2.

15.8.2.13   pow (x, y)

Returns an implementation-dependent approximation to the result of raising x to the power y.

• If y is NaN, the result is NaN.
• If y is +0, the result is 1, even if x is NaN.
• If y is −0, the result is 1, even if x is NaN.
• If x is NaN and y is nonzero, the result is NaN.
• If abs(x)>1 and y is +∞, the result is +∞.
• If abs(x)>1 and y is −∞, the result is +0.
• If abs(x)==1 and y is +∞, the result is NaN.
• If abs(x)==1 and y is −∞, the result is NaN.
• If abs(x)<1 and y is +∞, the result is +0.
• If abs(x)<1 and y is −∞, the result is +∞.
• If x is +∞ and y>0, the result is +∞.
• If x is +∞ and y<0, the result is +0.
• If x is −∞ and y>0 and y is an odd integer, the result is −∞.
• If x is −∞ and y>0 and y is not an odd integer, the result is +∞.
• If x is −∞ and y<0 and y is an odd integer, the result is −0.

• If x is −∞ and y<0 and y is not an odd integer, the result is +0.
• If x is +0 and y>0, the result is +0.
• If x is +0 and y<0, the result is +∞.
• If x is −0 and y>0 and y is an odd integer, the result is −0.
• If x is −0 and y>0 and y is not an odd integer, the result is +0.
• If x is −0 and y<0 and y is an odd integer, the result is −∞.
• If x is −0 and y<0 and y is not an odd integer, the result is +∞.
• If x<0 and x is finite and y is finite and y is not an integer, the result is NaN.

15.8.2.14   random ( )

Returns a Number value with positive sign, greater than or equal to 0 but less than 1, chosen randomly or pseudo randomly with approximately uniform distribution over that range, using an implementation-dependent algorithm or strategy. This function takes no arguments.

15.8.2.15   round (x)

Returns the Number value that is closest to x and is equal to a mathematical integer. If two integer Number values are equally close to x, then the result is the Number value that is closer to +∞. If x is already an integer, the result is x.

• If x is NaN, the result is NaN.
• If x is +0, the result is +0.
• If x is −0, the result is −0.
• If x is +∞, the result is +∞.
• If x is −∞, the result is −∞.
• If x is greater than 0 but less than 0.5, the result is +0.
• If x is less than 0 but greater than or equal to -0.5, the result is −0.

15.8.2.16   sin (x)

Returns an implementation-dependent approximation to the sine of x. The argument is expressed in radians.

• If x is NaN, the result is NaN.
• If x is +0, the result is +0.
• If x is −0, the result is −0.
• If x is +∞ or −∞, the result is NaN.

15.8.2.17   sqrt (x)

Returns an implementation-dependent approximation to the square root of x.

• If x is NaN, the result is NaN.
• If x is less than 0, the result is NaN.
• If x is +0, the result is +0.
• If x is −0, the result is −0.
• If x is +∞, the result is +∞.

15.8.2.18   tan (x)

Returns an implementation-dependent approximation to the tangent of x. The argument is expressed in radians.

• If x is NaN, the result is NaN.
• If x is +0, the result is +0.
• If x is −0, the result is −0.
• If x is +∞ or −∞, the result is NaN.

15.9   Date Objects

15.9.1   Overview of Date Objects and Definitions of Abstract Operators

The following functions are abstract operations that operate on time values (defined in 15.9.1.1). Note that, in every case, if any argument to one of these functions is NaN, the result will be NaN.

15.9.1.1   Time Values and Time Range

A Date object contains a Number indicating a particular instant in time to within a millisecond. Such a Number is called a time value. A time value may also be NaN, indicating that the Date object does not represent a specific instant of time.

Time is measured in ECMAScript in milliseconds since 01 January, 1970 UTC. In time values leap seconds are ignored. It is assumed that there are exactly 86,400,000 milliseconds per day. ECMAScript Number values can represent all integers from –9,007,199,254,740,991 to 9,007,199,254,740,991; this range suffices to measure times to millisecond precision for any instant that is within approximately 285,616 years, either forward or backward, from 01 January, 1970 UTC.

The actual range of times supported by ECMAScript Date objects is slightly smaller: exactly –100,000,000 days to 100,000,000 days measured relative to midnight at the beginning of 01 January, 1970 UTC. This gives a range of 8,640,000,000,000,000 milliseconds to either side of 01 January, 1970 UTC.

The exact moment of midnight at the beginning of 01 January, 1970 UTC is represented by the value +0.

15.9.1.2   Day Number and Time within Day

A given time value t belongs to day number


where the number of milliseconds per day is


The remainder is called the time within the day:

15.9.1.3   Year Number

ECMAScript uses an extrapolated Gregorian system to map a day number to a year number and to determine the month and date within that year. In this system, leap years are precisely those which are (divisible by 4) and ((not divisible by 100) or (divisible by 400)). The number of days in year number y is therefore defined by

All non-leap years have 365 days with the usual number of days per month and leap years have an extra day in February. The day number of the first day of year y is given by:

DayFromYear(y)

= 365 × (y−1970) + floor((y−1969)/4) − floor((y−1901)/100) + floor((y−1601)/400)

The time value of the start of a year is:

TimeFromYear(y)

= msPerDay × DayFromYear(y)

A time value determines a year by:

YearFromTime(t)

= the largest integer y (closest to positive infinity) such that TimeFromYear(y) ≤ t

The leap-year function is 1 for a time within a leap year and otherwise is zero:

InLeapYear(t)	= 0 if DaysInYear(YearFromTime(t)) = 365
	= 1 if DaysInYear(YearFromTime(t)) = 366

15.9.1.4   Month Number

Months are identified by an integer in the range 0 to 11, inclusive. The mapping MonthFromTime(t) from a time value t to a month number is defined by:

MonthFromTime(t)	= 0	if	0	≤ DayWithinYear(t) < 31
	= 1	if	31	≤ DayWithinYear (t) < 59 + InLeapYear(t)
	= 2	if	59 + InLeapYear(t)	≤ DayWithinYear (t) < 90 + InLeapYear(t)
	= 3	if	90 + InLeapYear(t)	≤ DayWithinYear (t) < 120 + InLeapYear(t)
	= 4	if	120 + InLeapYear(t)	≤ DayWithinYear (t) < 151 + InLeapYear(t)
	= 5	if	151 + InLeapYear(t)	≤ DayWithinYear (t) < 181 + InLeapYear(t)
	= 6	if	181 + InLeapYear(t)	≤ DayWithinYear (t) < 212 + InLeapYear(t)
	= 7	if	212 + InLeapYear(t)	≤ DayWithinYear (t) < 243 + InLeapYear(t)
	= 8	if	243 + InLeapYear(t)	≤ DayWithinYear (t) < 273 + InLeapYear(t)
	= 9	if	273 + InLeapYear(t)	≤ DayWithinYear (t) < 304 + InLeapYear(t)
	= 10	if	304 + InLeapYear(t)	≤ DayWithinYear (t) < 334 + InLeapYear(t)
	= 11	if	334 + InLeapYear(t)	≤ DayWithinYear (t) < 365 + InLeapYear(t)

where

A month value of 0 specifies January; 1 specifies February; 2 specifies March; 3 specifies April; 4 specifies May; 5 specifies June; 6 specifies July; 7 specifies August; 8 specifies September; 9 specifies October; 10 specifies November; and 11 specifies December. Note that MonthFromTime(0) = 0, corresponding to Thursday, 01 January, 1970.

15.9.1.5   Date Number

A date number is identified by an integer in the range 1 through 31, inclusive. The mapping DateFromTime(t) from a time value t to a month number is defined by:

DateFromTime(t)	= DayWithinYear(t) + 1	if MonthFromTime(t) = 0
	= DayWithinYear(t) − 30	if MonthFromTime(t) = 1
	= DayWithinYear(t) − 58 − InLeapYear(t)	if MonthFromTime(t) = 2
	= DayWithinYear(t) − 89 − InLeapYear(t)	if MonthFromTime(t) = 3
	= DayWithinYear(t) − 119 − InLeapYear(t)	if MonthFromTime(t) = 4
	= DayWithinYear(t) − 150 − InLeapYear(t)	if MonthFromTime(t) = 5
	= DayWithinYear(t) − 180 − InLeapYear(t)	if MonthFromTime(t) = 6
	= DayWithinYear(t) − 211 − InLeapYear(t)	if MonthFromTime(t) = 7
	= DayWithinYear(t) − 242 − InLeapYear(t)	if MonthFromTime(t) = 8
	= DayWithinYear(t) − 272 − InLeapYear(t)	if MonthFromTime(t) = 9
	= DayWithinYear(t) − 303 − InLeapYear(t)	if MonthFromTime(t) = 10
	= DayWithinYear(t) − 333 − InLeapYear(t)	if MonthFromTime(t) = 11

15.9.1.6   Week Day

The weekday for a particular time value t is defined as
A weekday value of 0 specifies Sunday; 1 specifies Monday; 2 specifies Tuesday; 3 specifies Wednesday; 4 specifies Thursday; 5 specifies Friday; and 6 specifies Saturday. Note that WeekDay(0) = 4, corresponding to Thursday, 01 January, 1970.

15.9.1.7   Local Time Zone Adjustment

An implementation of ECMAScript is expected to determine the local time zone adjustment. The local time zone adjustment is a value LocalTZA measured in milliseconds which when added to UTC represents the local standard time. Daylight saving time is not reflected by LocalTZA. The value LocalTZA does not vary with time but depends only on the geographic location.

15.9.1.8   Daylight Saving Time Adjustment

An implementation of ECMAScript is expected to determine the daylight saving time algorithm. The algorithm to determine the daylight saving time adjustment DaylightSavingTA(t), measured in milliseconds, must depend only on four things:

(1) the time since the beginning of the year (2) whether t is in a leap year (3) the week day of the beginning of the year and (4) the geographic location.

The implementation of ECMAScript should not try to determine whether the exact time was subject to daylight saving time, but just whether daylight saving time would have been in effect if the current daylight saving time algorithm had been used at the time. This avoids complications such as taking into account the years that the locale observed daylight saving time year round.

If the host environment provides functionality for determining daylight saving time, the implementation of ECMAScript is free to map the year in question to an equivalent year (same leap-year-ness and same starting week day for the year) for which the host environment provides daylight saving time information. The only restriction is that all equivalent years should produce the same result.

15.9.1.9   Local Time

Conversion from UTC to local time is defined by Conversion from local time to UTC is defined by Note that UTC(LocalTime(t)) is not necessarily always equal to t.

15.9.1.10   Hours, Minutes, Second, and Milliseconds

The following functions are useful in decomposing time values:

MinFromTime(t)	= floor(t / msPerMinute) modulo MinutesPerHour
SecFromTime(t	= floor(t / msPerSecond) modulo SecondsPerMinute
msFromTime(t)	= t modulo msPerSecond

where

HoursPerDay	= 24
MinutesPerHour	= 60
SecondsPerMinute	= 60
msPerSecond	= 1000
msPerMinute	= 60000 = msPerSecond × SecondsPerMinute
msPerHour	= 3600000 = msPerMinute × MinutesPerHour

15.9.1.11   MakeTime (hour, min, sec, ms)

The operator MakeTime calculates a number of milliseconds from its four arguments, which must be ECMAScript Number values. This operator functions as follows:

1. If hour is not finite or min is not finite or sec is not finite or ms is not finite, return NaN.
2. Let h be ToInteger(hour).
3. Let m be ToInteger(min).
4. Let s be ToInteger(sec).
5. Let milli be ToInteger(ms).
6. Let t be h * msPerHour + m * msPerMinute + s * msPerSecond + milli, performing the arithmetic according to IEEE 754 rules (that is, as if using the ECMAScript operators * and +).
7. Return t.

15.9.1.12   MakeDay (year, month, date)

The operator MakeDay calculates a number of days from its three arguments, which must be ECMAScript Number values. This operator functions as follows:

1. If year is not finite or month is not finite or date is not finite, return NaN.
2. Let y be ToInteger(year).
3. Let m be ToInteger(month).
4. Let dt be ToInteger(date).
5. Let ym be y + floor(m /12).
6. Let mn be m modulo 12.
7. Find a value t such that YearFromTime(t) == ym and MonthFromTime(t) == mn) and DateFromTime(t) == 1; but if this is not possible (because some argument is out of range), return NaN.
8. Return Day(t) + dt − 1.

15.9.1.13   MakeDate (day, time)

The operator MakeDate calculates a number of milliseconds from its two arguments, which must be ECMAScript Number values. This operator functions as follows:

1. If day is not finite or time is not finite, return NaN.
2. Return day × msPerDay + time.

15.9.1.14   TimeClip (time)

The operator TimeClip calculates a number of milliseconds from its argument, which must be an ECMAScript Number value. This operator functions as follows:

1. If time is not finite, return NaN.

2. If abs(time) > 8.64 x 10¹⁵, return NaN.
3. Return an implementation-dependent choice of either ToInteger(time) or ToInteger(time) + (+0). (Adding a positive zero converts −0 to +0.)

15.9.1.15   Date Time String Format

ECMAScript defines a string interchange format for date-times based upon a simplification of the ISO 8601 Extended Format. The format is as follows: YYYY-MM-DDTHH:mm:ss.sssZ

Where the fields are as follows:

YYYY	is the decimal digits of the year in the Gregorian calendar.
-	“:” (hyphen) appears literally twice in the string.
MM	is the month of the year from 01 (January) to 12 (December).
DD	is the day of the month from 01 to 31.
T	“T” appears literally in the string, to indicate the beginning of the time element.
HH	is the number of complete hours that have passed since midnight as two decimal digits.
:	“:” (colon) appears literally twice in the string.
mm	is the number of complete minutes since the start of the hour as two decimal digits.
ss	is the number of complete seconds since the start of the minute as two decimal digits.
.	“.” (dot) appears literally in the string.
sss	is the number of complete milliseconds since the start of the second as three decimal digits.
	Both the “.” and the milliseconds field may be omitted.
Z	is the time zone offset specified as “Z” (for UTC) or either “+” or “-” followed by a time
	expression hh:mm

This format includes date-only forms:

YYYY
YYYY-MM
YYYY-MM-DD

It also includes time-only forms with an optional time zone offset appended:

THH:mm
THH:mm:ss
THH:mm:ss.sss

Also included are “date-times” which may be any combination of the above.

All numbers must be base 10.

Illegal values (out-of-bounds as well as syntax errors) in a format string means that the format string is not a valid instance of this format.

15.9.1.15.1   Extended years

ECMAScript requires the ability to specify 6 digit years (extended years); approximately 285,616 years, either forward or backward, from 01 January, 1970 UTC. To represent years before 0 or after 9999, ISO 8601 permits the expansion of the year representation, but only by prior agreement between the sender and the receiver. In the simplified ECMAScript format such an expanded year representation shall have 2 extra year digits and is always prefixed with a + or – sign. The year 0 is considered positive and hence prefixed with a + sign.

15.9.2   The Date Constructor Called as a Function

When Date is called as a function rather than as a constructor, it returns a String representing the current time (UTC).

NOTE The function call Date(…) is not equivalent to the object creation expression new Date(…) with the same arguments.

15.9.2.1   Date ( [ year [, month [, date [, hours [, minutes [, seconds [, ms ] ] ] ] ] ] ] )

All of the arguments are optional; any arguments supplied are accepted but are completely ignored. A String is created and returned as if by the expression (new Date()).toString() where Date is the standard built-in constructor with that name and toString is the standard built-in method Date.prototype.toString.

15.9.3   The Date Constructor

When Date is called as part of a new expression, it is a constructor: it initializes the newly created object.

15.9.3.1   new Date (year, month [, date [, hours [, minutes [, seconds [, ms ] ] ] ] ] )

When Date is called with two to seven arguments, it computes the date from year, month, and (optionally) date, hours, minutes, seconds and ms.

The [[Prototype]] internal property of the newly constructed object is set to the original Date prototype object, the one that is the initial value of Date.prototype (15.9.4.1).

The [[Class]] internal property of the newly constructed object is set to "Date".

The [[Extensible]] internal property of the newly constructed object is set to true.

The [[PrimitiveValue]] internal property of the newly constructed object is set as follows:

1. Let y be ToNumber(year).
2. Let m be ToNumber(month).
3. If date is supplied then let dt be ToNumber(date); else let dt be 1.
4. If hours is supplied then let h be ToNumber(hours); else let h be 0.
5. If minutes is supplied then let min be ToNumber(minutes); else let min be 0.
6. If seconds is supplied then let s be ToNumber(seconds); else let s be 0.
7. If ms is supplied then let milli be ToNumber(ms); else let milli be 0.
8. If y is not NaN and 0 ≤ ToInteger(y) ≤ 99, then let yr be 1900+ToInteger(y); otherwise, let yr be y.
9. Let finalDate be MakeDate(MakeDay(yr, m, dt), MakeTime(h, min, s, milli)).
10. Set the [[PrimitiveValue]] internal property of the newly constructed object to TimeClip(UTC(finalDate)).

15.9.3.2   new Date (value)

The [[Prototype]] internal property of the newly constructed object is set to the original Date prototype object, the one that is the initial value of Date.prototype (15.9.4.1).

The [[Class]] internal property of the newly constructed object is set to "Date".

The [[Extensible]] internal property of the newly constructed object is set to true.

The [[PrimitiveValue]] internal property of the newly constructed object is set as follows:

1. Let v be ToPrimitive(value).
2. If Type(v) is String, then
   1. Parse v as a date, in exactly the same manner as for the parse method (15.9.4.2); let V be the time value for this date.
3. Else, let V be ToNumber(v).
4. Set the [[PrimitiveValue]] internal property of the newly constructed object to TimeClip(V) and return.

15.9.3.3   new Date ( )

The [[Prototype]] internal property of the newly constructed object is set to the original Date prototype object, the one that is the initial value of Date.prototype (15.9.4.1).

The [[Class]] internal property of the newly constructed object is set to "Date".

The [[Extensible]] internal property of the newly constructed object is set to true.

The [[PrimitiveValue]] internal property of the newly constructed object is set to the time value (UTC) identifying the current time.

15.9.4   Properties of the Date Constructor

The value of the [[Prototype]] internal property of the Date constructor is the Function prototype object (15.3.4). Besides the internal properties and the length property (whose value is 7), the Date constructor has the following properties:

15.9.4.1   Date.prototype

The initial value of Date.prototype is the built-in Date prototype object (15.9.5).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

15.9.4.2   Date.parse (string)

The parse function applies the ToString operator to its argument and interprets the resulting String as a date and time; it returns a Number, the UTC time value corresponding to the date and time. The String may be interpreted as a local time, a UTC time, or a time in some other time zone, depending on the contents of the String. The function first attempts to parse the format of the String according to the rules called out in Date Time String Format (15.9.1.15). If the String does not conform to that format the function may fall back to any implementation-specific heuristics or implementation-specific date formats. Unrecognizable Strings or dates containing illegal element values in the format String shall cause Date.parse to return NaN.

If x is any Date object whose milliseconds amount is zero within a particular implementation of ECMAScript, then all of the following expressions should produce the same numeric value in that implementation, if all the properties referenced have their initial values: However, the expression

is not required to produce the same Number value as the preceding three expressions and, in general, the value produced by Date.parse is implementation-dependent when given any String value that does not conform to the Date Time String Format (15.9.1.15) and that could not be produced in that implementation by the toString or toUTCString method.

15.9.4.3   Date.UTC (year, month [, date [, hours [, minutes [, seconds [, ms ] ] ] ] ] )

When the UTC function is called with fewer than two arguments, the behaviour is implementation-dependent.
When the UTC function is called with two to seven arguments, it computes the date from year, month and (optionally) date, hours, minutes, seconds and ms. The following steps are taken:

1. Let y be ToNumber(year).
2. Let m be ToNumber(month).
3. If date is supplied then let dt be ToNumber(date); else let dt be 1.
4. If hours is supplied then let h be ToNumber(hours); else let h be 0.
5. If minutes is supplied then let min be ToNumber(minutes); else let min be 0.
6. If seconds is supplied then let s be ToNumber(seconds); else let s be 0.
7. If ms is supplied then let milli be ToNumber(ms); else let milli be 0.
8. If y is not NaN and 0 ≤ ToInteger(y) ≤ 99, then let yr be 1900+ToInteger(y); otherwise, let yr be y.
9. Return TimeClip(MakeDate(MakeDay(yr, m, dt), MakeTime(h, min, s, milli))).

The length property of the UTC function is 7.

15.9.4.4   Date.now ( )

The now function return a Number value that is the time value designating the UTC date and time of the occurrence of the call to now.

15.9.5   Properties of the Date Prototype Object

The Date prototype object is itself a Date object (its [[Class]] is "Date") whose [[PrimitiveValue]] is NaN. The value of the [[Prototype]] internal property of the Date prototype object is the standard built-in Object prototype object (15.2.4).

In following descriptions of functions that are properties of the Date prototype object, the phrase “this Date object” refers to the object that is the this value for the invocation of the function. Unless explicitly noted otherwise, none of these functions are generic; a TypeError exception is thrown if the this value is not an object for which the value of the [[Class]] internal property is "Date". Also, the phrase “this time value” refers to the Number value for the time represented by this Date object, that is, the value of the [[PrimitiveValue]] internal property of this Date object.

15.9.5.1   Date.prototype.constructor

The initial value of Date.prototype.constructor is the built-in Date constructor.

15.9.5.2   Date.prototype.toString ( )

This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent the Date in the current time zone in a convenient, human-readable form.

15.9.5.3   Date.prototype.toDateString ( )

This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent the “date” portion of the Date in the current time zone in a convenient, human-readable form.

15.9.5.4   Date.prototype.toTimeString ( )

This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent the “time” portion of the Date in the current time zone in a convenient, human-readable form.

15.9.5.5   Date.prototype.toLocaleString ( )

This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of the host environment’s current locale. NOTE The first parameter to this function is likely to be used in a future version of this standard; it is recommended that implementations do not use this parameter position for anything else.

15.9.5.6   Date.prototype.toLocaleDateString ( )

This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent the “date” portion of the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of the host environment’s current locale.

15.9.5.7   Date.prototype.toLocaleTimeString ( )

This function returns a String value. The contents of the String are implementation-dependent, but are intended to represent the “time” portion of the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of the host environment’s current locale.

15.9.5.8   Date.prototype.valueOf ( )

The valueOf function returns a Number, which is this time value.

15.9.5.9   Date.prototype.getTime ( )

1. Return this time value.

15.9.5.10   Date.prototype.getFullYear ( )

1. Let t be this time value.
2. If t is NaN, return NaN.
3. Return YearFromTime(LocalTime(t)).

15.9.5.11   Date.prototype.getUTCFullYear ( )

1. Let t be this time value.
2. If t is NaN, return NaN.

3. Return YearFromTime(t).

15.9.5.12   Date.prototype.getMonth ( )

1. Let t be this time value.
2. If t is NaN, return NaN.
3. Return MonthFromTime(LocalTime(t)).

15.9.5.13   Date.prototype.getUTCMonth ( )

1. Let t be this time value.
2. If t is NaN, return NaN.
3. Return MonthFromTime(t).

15.9.5.14   Date.prototype.getDate ( )

1. Let t be this time value.
2. If t is NaN, return NaN.
3. Return DateFromTime(LocalTime(t)).

15.9.5.15   Date.prototype.getUTCDate ( )

1. Let t be this time value.
2. If t is NaN, return NaN.
3. Return DateFromTime(t).

15.9.5.16   Date.prototype.getDay ( )

1. Let t be this time value.
2. If t is NaN, return NaN.
3. Return WeekDay(LocalTime(t)).

15.9.5.17   Date.prototype.getUTCDay ( )

1. Let t be this time value.
2. If t is NaN, return NaN.
3. Return WeekDay(t).

15.9.5.18   Date.prototype.getHours ( )

1. Let t be this time value.
2. If t is NaN, return NaN.
3. Return HourFromTime(LocalTime(t)).

15.9.5.19   Date.prototype.getUTCHours ( )

1. Let t be this time value.
2. If t is NaN, return NaN.
3. Return HourFromTime(t).

15.9.5.20   Date.prototype.getMinutes ( )

1. Let t be this time value.
2. If t is NaN, return NaN.
3. Return MinFromTime(LocalTime(t)).

15.9.5.21   Date.prototype.getUTCMinutes ( )

1. Let t be this time value.
2. If t is NaN, return NaN.
3. Return MinFromTime(t).

15.9.5.22   Date.prototype.getSeconds ( )

1. Let t be this time value.
2. If t is NaN, return NaN.
3. Return SecFromTime(LocalTime(t)).

15.9.5.23   Date.prototype.getUTCSeconds ( )

1. Let t be this time value.
2. If t is NaN, return NaN.
3. Return SecFromTime(t).

15.9.5.24   Date.prototype.getMilliseconds ( )

1. Let t be this time value.
2. If t is NaN, return NaN.
3. Return msFromTime(LocalTime(t)).

15.9.5.25   Date.prototype.getUTCMilliseconds ( )

1. Let t be this time value.
2. If t is NaN, return NaN.
3. Return msFromTime(t).

15.9.5.26   Date.prototype.getTimezoneOffset ( )

Returns the difference between local time and UTC time in minutes.

1. Let t be this time value.
2. If t is NaN, return NaN.
3. Return (t − LocalTime(t)) / msPerMinute.

15.9.5.27   Date.prototype.setTime (time)

1. Let v be TimeClip(ToNumber(time)).
2. Set the [[PrimitiveValue]] internal property of this Date object to v.
3. Return v.

15.9.5.28   Date.prototype.setMilliseconds (ms)

1. Let t be the result of LocalTime(this time value).
2. Let time be MakeTime(HourFromTime(t), MinFromTime(t), SecFromTime(t), ToNumber(ms)).
3. Let u be TimeClip(UTC(MakeDate(Day(t), time))).
4. Set the [[PrimitiveValue]] internal property of this Date object to u.
5. Return u.

15.9.5.29   Date.prototype.setUTCMilliseconds (ms)

1. Let t be this time value.
2. Let time be MakeTime(HourFromTime(t), MinFromTime(t), SecFromTime(t), ToNumber(ms)).
3. Let v be TimeClip(MakeDate(Day(t), time)).
4. Set the [[PrimitiveValue]] internal property of this Date object to v.

5. Return v.

15.9.5.30   Date.prototype.setSeconds (sec [, ms ] )

If ms is not specified, this behaves as if ms were specified with the value getMilliseconds().

1. Let t be the result of LocalTime(this time value).
2. Let s be ToNumber(sec).
3. If ms is not specified, then let milli be msFromTime(t); otherwise, let milli be ToNumber(ms).
4. Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), MinFromTime(t), s, milli)).
5. Let u be TimeClip(UTC(date)).
6. Set the [[PrimitiveValue]] internal property of this Date object to u.
7. Return u.

The length property of the setSeconds method is 2.

15.9.5.31   Date.prototype.setUTCSeconds (sec [, ms ] )

If ms is not specified, this behaves as if ms were specified with the value getUTCMilliseconds().

1. Let t be this time value.
2. Let s be ToNumber(sec).
3. If ms is not specified, then let milli be msFromTime(t); otherwise, let milli be ToNumber(ms).
4. Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), MinFromTime(t), s, milli)).
5. Let v be TimeClip(date).
6. Set the [[PrimitiveValue]] internal property of this Date object to v.
7. Return v.

The length property of the setUTCSeconds method is 2.

15.9.5.32   Date.prototype.setMinutes (min [, sec [, ms ] ] )

If sec is not specified, this behaves as if sec were specified with the value getSeconds().

If ms is not specified, this behaves as if ms were specified with the value getMilliseconds().

1. Let t be the result of LocalTime(this time value).
2. Let m be ToNumber(min).
3. If sec is not specified, then let s be SecFromTime(t); otherwise, let s be ToNumber(sec).
4. If ms is not specified, then let milli be msFromTime(t); otherwise, let milli be ToNumber(ms).
5. Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), m, s, milli)).
6. Let u be TimeClip(UTC(date)).
7. Set the [[PrimitiveValue]] internal property of this Date object to u.
8. Return u.

The length property of the setMinutes method is 3.

15.9.5.33   Date.prototype.setUTCMinutes (min [, sec [, ms ] ] )

If sec is not specified, this behaves as if sec were specified with the value getUTCSeconds().

If ms is not specified, this function behaves as if ms were specified with the value return by
getUTCMilliseconds().

1. Let t be this time value.
2. Let m be ToNumber(min).
3. If sec is not specified, then let s be SecFromTime(t); otherwise, let s be ToNumber(sec).
4. If ms is not specified, then let milli be msFromTime(t); otherwise, let milli be ToNumber(ms).

5. Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), m, s, milli)).
6. Let v be TimeClip(date).
7. Set the [[PrimitiveValue]] internal property of this Date object to v.
8. Return v.

The length property of the setUTCMinutes method is 3.

15.9.5.34   Date.prototype.setHours (hour [, min [, sec [, ms ] ] ] )

If min is not specified, this behaves as if min were specified with the value getMinutes().

If sec is not specified, this behaves as if sec were specified with the value getSeconds().

If ms is not specified, this behaves as if ms were specified with the value getMilliseconds().

1. Let t be the result of LocalTime(this time value).
2. Let h be ToNumber(hour).
3. If min is not specified, then let m be MinFromTime(t); otherwise, let m be ToNumber(min).
4. If If sec is not specified, then let s be SecFromTime(t); otherwise, let s be ToNumber(sec).
5. If ms is not specified, then let milli be msFromTime(t); otherwise, let milli be ToNumber(ms).
6. Let date be MakeDate(Day(t), MakeTime(h, m, s, milli)).
7. Let u be TimeClip(UTC(date)).
8. Set the [[PrimitiveValue]] internal property of this Date object to u.
9. Return u.

The length property of the setHours method is 4.

15.9.5.35   Date.prototype.setUTCHours (hour [, min [, sec [, ms ] ] ] )

If min is not specified, this behaves as if min were specified with the value getUTCMinutes().

If sec is not specified, this behaves as if sec were specified with the value getUTCSeconds().

If ms is not specified, this behaves as if ms were specified with the value getUTCMilliseconds().

1. Let t be this time value.
2. Let h be ToNumber(hour).
3. If min is not specified, then let m be MinFromTime(t); otherwise, let m be ToNumber(min).
4. If sec is not specified, then let s be SecFromTime(t); otherwise, let s be ToNumber(sec).
5. If ms is not specified, then let milli be msFromTime(t); otherwise, let milli be ToNumber(ms).
6. Let newDate be MakeDate(Day(t), MakeTime(h, m, s, milli)).
7. Let v be TimeClip(newDate).
8. Set the [[PrimitiveValue]] internal property of this Date object to v.
9. Return v.

The length property of the setUTCHours method is 4.

15.9.5.36   Date.prototype.setDate (date)

1. Let t be the result of LocalTime(this time value).
2. Let dt be ToNumber(date).
3. Let newDate be MakeDate(MakeDay(YearFromTime(t), MonthFromTime(t), dt), TimeWithinDay(t)).
4. Let u be TimeClip(UTC(newDate)).
5. Set the [[PrimitiveValue]] internal property of this Date object to u.
6. Return u.