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 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.
NOTE Implementations that add additional capabilities to the set of built-in functions are encouraged to do so by adding new functions rather than adding new parameters to existing functions.
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.
NOTE For example, the Function object that is the initial value of the slice property of the String prototype object is described under the subclause heading “String.prototype.slice (start, end)” which shows the two named arguments start and end; therefore the value of the length property of that Function object is 2.
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.
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.
The value of NaN
is NaN (see 8.5). This property has the attributes {
[[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
The value of Infinity
is +∞ (see 8.5). This property has the
attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
The value of undefined
is undefined (see 8.1). This property has the
attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
When the eval
function is called with one argument x, the following steps are taken:
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 built-in function defined in 15.1.2.1.
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, the number may also
optionally begin with the character pairs 0x
or 0X
.
When the parseInt
function is called, the following steps are taken:
NOTE parseInt
may interpret only a leading portion of string as an
integer value; it ignores any characters that cannot be interpreted as part of the notation of an integer, and no
indication is given that any such characters were ignored.
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:
NOTE parseFloat
may interpret only a leading portion of string
as a
Number value; it ignores any characters that cannot be
interpreted as part of the notation of an decimal literal, and no
indication is given that any such characters were ignored.
Returns true if the argument coerces to NaN, and otherwise returns false.
NOTE A reliable way for ECMAScript code to test if a value X is a NaN is an expression of the form X !== X. The result will be true if and only if X is a NaN.
Returns false if the argument coerces to NaN, +∞, or −∞, and otherwise returns true.
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.
NOTE Many implementations of ECMAScript provide additional functions and methods that manipulate web pages; these functions are beyond the scope of this standard.
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 “:
”, “/
”,
“;
” 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.
;
/
?
:
@
&
=
+
$
,
%
HexDigit HexDigita
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
-
_
.
!
~
*
'
(
)
NOTE The above syntax is based upon RFC 2396 and does not reflect changes introduced by the more recent RFC 3986.
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.
The unescaping and decoding process is described by the abstract operation Decode taking two String arguments string and reservedSet.
NOTE This syntax of Uniform Resource Identifiers is based upon RFC 2396 and does not reflect the more recent RFC 3986 which replaces RFC 2396. A formal description and implementation of UTF-8 is given in RFC 3629.
In UTF-8, characters are encoded using sequences of 1 to 6 octets. The only octet of a “sequence” of one has the higher-order bit set to 0, the remaining 7 bits being used to encode the character value. In a sequence of n octets, n>1, the initial octet has the n higher-order bits set to 1, followed by a bit set to 0. The remaining bits of that octet contain bits from the value of the character to be encoded. The following octets all have the higher-order bit set to 1 and the following bit set to 0, leaving 6 bits in each to contain bits from the character to be encoded. The possible UTF-8 encodings of ECMAScript characters are specified in Table 21.
Code Unit Value | Representation | 1st Octet | 2nd Octet | 3rd Octet | 4th Octet |
---|---|---|---|---|---|
0x0000 - 0x007F |
00000000 0zzzzzzz |
0zzzzzzz | |||
0x0080 - 0x07FF |
00000yyy yyzzzzzz | 110yyyyy | 10zzzzzz | ||
0x0800 - 0xD7FF | xxxxyyyy yyzzzzzz | 1110xxxx | 10yyyyyy | 10zzzzzz | |
0xD800 - 0xDBFF followed by 0xDC00 – 0xDFFF |
110110vv vvwwwwxx followed by 110111yy yyzzzzzz |
11110uuu | 10uuwwww | 10xxyyyy | 10zzzzzz |
0xD800 - 0xDBFF not followed by 0xDC00 – 0xDFFF |
causes URIError | ||||
0xDC00 – 0xDFFF | causes URIError | ||||
0xE000 - 0xFFFF | xxxxyyyy yyzzzzzz | 1110xxxx | 10yyyyyy | 10zzzzzz |
Where
uuuuu = vvvv + 1
to account for the addition of 0x10000 as in Surrogates, section 3.7, of the Unicode Standard.
The range of code unit values 0xD800-0xDFFF is used to encode surrogate pairs; the above transformation combines a UTF-16 surrogate pair into a UTF-32 representation and encodes the resulting 21-bit value in UTF-8. Decoding reconstructs the surrogate pair.
RFC 3629 prohibits the decoding of invalid UTF-8 octet sequences. For example, the invalid sequence C0 80 must not decode into the character U+0000. Implementations of the Decode algorithm are required to throw a URIError when encountering such invalid sequences.
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:
#
”.NOTE The character “#
” is not decoded from escape sequences even
though it is not a reserved URI character.
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:
The encodeURI
function computes a new version of a URI in which each instance of certain characters is
replaced by one, two, three, or four 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:
#
”.NOTE The character “#
” is not encoded to an escape sequence even
though it is not a reserved or unescaped URI character.
The encodeURIComponent
function computes a new version of a URI in which each instance of certain
characters is replaced by one, two, three, or four 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:
See 15.9.2.
See 15.11.6.1.
See 15.11.6.2.
See 15.11.6.3.
See 15.11.6.4.
See 15.11.6.5.
See 15.11.6.6.
See 15.8.
See 15.12.
When Object
is called as a function rather than as a constructor, it performs a type conversion.
When the Object
function is called with no arguments or with one argument value, the following
steps are taken:
When Object
is called as part of a new
expression, it is a constructor that may create an
object.
When the Object
constructor is called with no arguments or with one argument value, the
following steps are taken:
"Object"
.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:
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 }.
When the getPrototypeOf
function is called with argument O, the following steps are taken:
When the getOwnPropertyDescriptor function is called, the following steps are taken:
When the getOwnPropertyNames function is called, the following steps are taken:
new Array ()
where
Array
is the standard built-in constructor with that name.NOTE If O is a String instance, the set of own properties processed in step 4 includes the implicit properties defined in 15.5.5.2 that correspond to character positions within the object's [[PrimitiveValue]] String.
The create function creates a new object with a specified prototype. When the create function is called, the following steps are taken:
Object.defineProperties
with arguments obj and
Properties.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:
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:
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.
When the seal function is called, the following steps are taken:
When the freeze function is called, the following steps are taken:
When the preventExtensions function is called, the following steps are taken:
When the isSealed function is called with argument O, the following steps are taken:
When the isFrozen function is called with argument O, the following steps are taken:
When the isExtensible function is called with argument O, the following steps are taken:
When the keys function is called with argument O, the following steps are taken:
new Array(n)
where
Array
is the standard built-in constructor with that name.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.
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.
The initial value of Object.prototype.constructor
is the standard built-in Object
constructor.
When the toString
method is called, the following steps are taken:
"[object Undefined]"
."[object Null]"
."[object "
, class,
and "]"
.When the toLocaleString method is called, the following steps are taken:
NOTE 1 This function is provided to give all Objects a generic toLocaleString
interface, even though not all may use it. Currently, Array
, Number
, and Date
provide their own locale-sensitive toLocaleString
methods.
NOTE 2 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.
When the valueOf method is called, the following steps are taken:
When the hasOwnProperty
method is called with argument V, the following steps are taken:
NOTE 1 Unlike [[HasProperty]] (8.12.6), this method does not consider objects in the prototype chain.
NOTE 2 The ordering of steps 1 and 2 is chosen to ensure that any exception that would have been thrown by step 1 in previous editions of this specification will continue to be thrown even if the this value is undefined or null.
When the isPrototypeOf
method is called with argument V, the following steps are taken:
NOTE The ordering of steps 1 and 2 is chosen to preserve the behaviour specified by previous editions of this specification for the case where V is not an object and the this value is undefined or null.
When the propertyIsEnumerable
method is called with argument V, the following steps are
taken:
NOTE 1 This method does not consider objects in the prototype chain.
NOTE 2 The ordering of steps 1 and 2 is chosen to ensure that any exception that would have been thrown by step 1 in previous editions of this specification will continue to be thrown even if the this value is undefined or null.
Object instances have no special properties beyond those inherited from the Object prototype object.
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.
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:
When Function
is called as part of a new
expression, it is a constructor: it initialises the
newly created object.
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:
A prototype
property is automatically created for every function, to provide for the possibility that the
function will be used as a constructor.
NOTE It is permissible but not necessary to have one argument for each formal parameter to be specified. For example, all three of the following expressions produce the same result:
new Function("a", "b", "c", "return a+b+c")
new Function("a, b, c", "return a+b+c")
new Function("a,b", "c", "return a+b+c")
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:
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 }.
This is a data property with a value of 1. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
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.
The initial value of Function.prototype.constructor
is the built-in Function
constructor.
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.
When the apply
method is called on an object func with arguments thisArg and
argArray, the following steps are taken:
"length"
.The length
property of the apply
method is 2.
NOTE The thisArg value is passed without modification as the this value. This is a change from Edition 3, where a undefined or null thisArg is replaced with the global object and ToObject is applied to all other values and that result is passed as the this value.
When the call
method is called on an object func with argument thisArg and optional
arguments arg1, arg2 etc, the following steps are taken:
The length
property of the call
method is 1.
NOTE The thisArg value is passed without modification as the this value. This is a change from Edition 3, where a undefined or null thisArg is replaced with the global object and ToObject is applied to all other values and that result is passed as the this value.
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:
"caller"
, PropertyDescriptor
{[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]: false},
and false."arguments"
,
PropertyDescriptor {[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false,
[[Configurable]]: false}, and false.The length
property of the bind
method is 1.
NOTE Function objects created using Function.prototype.bind
do not have a
prototype
property or the [[Code]], [[FormalParameters]], and [[Scope]] internal properties.
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:
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:
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:
In addition to the required internal properties, every function instance has a [[Call]] internal property and in most cases uses 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.
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 }.
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 }.
NOTE Function objects created using Function.prototype.bind
do not have a
prototype
property.
Assume F is a Function object.
When the [[HasInstance]] internal method of F is called with value V, the following steps are taken:
"prototype"
.null
, return false.NOTE Function objects created using Function.prototype.bind
have a different
implementation of [[HasInstance]] defined in 15.3.4.5.3.
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:
"caller"
and v is a strict mode Function object, throw a TypeError
exception.NOTE Function objects created using Function.prototype.bind
use the default
[[Get]] internal method.
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:
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.
When the Array
function is called the following steps are taken:
Array
was used in
a new
expression with the same arguments (15.4.2).When Array
is called as part of a new
expression, it is a constructor: it initialises the newly
created object.
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}.
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}.
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:
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 }.
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:
"Array"
, then return true.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"
.
NOTE The Array prototype object does not have a valueOf
property of its own;
however, it inherits the valueOf
property from the standard built-in Object prototype Object.
The initial value of Array.prototype.constructor
is the standard built-in Array
constructor.
When the toString
method is called, the following steps are taken:
"join"
.NOTE The toString
function is intentionally generic; it does not require that
its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the toString
function can be applied successfully to a host object is implementation-dependent.
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:
"length"
."0"
."toLocaleString"
.1
."toLocaleString"
.NOTE 1 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.
NOTE 2 The toLocaleString
function is intentionally generic; it does not require
that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a
method. Whether the toLocaleString
function can be applied successfully to a host object is
implementation-dependent.
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:
new Array()
where Array
is the
standard built-in constructor with that name."Array"
, then
"length"
.The length
property of the concat
method is 1.
NOTE The concat
function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the concat
function can be applied successfully to a host object is implementation-dependent.
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:
"length"
.","
."0"
.1
.The length
property of the join
method is 1.
NOTE The join
function is intentionally generic; it does not require that its
this value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method.
Whether the join
function can be applied successfully to a host object is implementation-dependent.
The last element of the array is removed from the array and returned.
NOTE The pop
function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the pop
function can be applied successfully to a host object is implementation-dependent.
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:
The length
property of the push
method is 1.
NOTE The push
function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the push
function can be applied successfully to a host object is implementation-dependent.
The elements of the array are rearranged so as to reverse their order. The object is returned as the result of the call.
"length"
.NOTE The reverse
function is intentionally generic; it does not require that its
this value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method.
Whether the reverse
function can be applied successfully to a host object is implementation-dependent.
The first element of the array is removed from the array and returned.
NOTE The shift
function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the shift
function can be applied successfully to a host object is implementation-dependent.
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:
new Array()
where Array
is the
standard built-in constructor with that name.The length
property of the slice
method is 2.
NOTE The slice
function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the slice
function can be applied successfully to a host object is implementation-dependent.
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:
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.
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.
A function comparefn is a consistent comparison function for a set of values S if all of the requirements below are met for all values a, b, and c (possibly the same value) in the set S: The notation a <CF b means comparefn(a,b) < 0; a =CF b means comparefn(a,b) = 0 (of either sign); and a >CF b means comparefn(a,b) > 0.
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)
NOTE The above conditions are necessary and sufficient to ensure that comparefn divides the set S into equivalence classes and that these equivalence classes are totally ordered.
When the SortCompare abstract operation is called with two arguments j and k, the following steps are taken:
NOTE 1 Because non-existent property values always compare greater than undefined property values, and undefined always compares greater than any other value, undefined property values always sort to the end of the result, followed by non-existent property values.
NOTE 2 The sort
function is intentionally generic; it does not require that its
this value be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method.
Whether the sort
function can be applied successfully to a host object is implementation-dependent.
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:
new Array()
where Array
is the
standard built-in constructor with that name.The length
property of the splice
method is 2.
NOTE The splice
function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the splice
function can be applied successfully to a host object is implementation-dependent.
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:
The length
property of the unshift
method is 1.
NOTE The unshift
function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the unshift
function can be applied successfully to a host object is implementation-dependent.
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:
The length
property of the indexOf
method is 1.
NOTE The indexOf
function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the indexOf
function can be applied successfully to a host object is implementation-dependent.
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 minus one (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:
The length
property of the lastIndexOf
method is 1.
NOTE The lastIndexOf
function is intentionally generic; it does not require that
its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the lastIndexOf
function can be applied successfully to a host object is
implementation-dependent.
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:
"length"
.The length
property of the every
method is 1.
NOTE The every
function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the every
function can be applied successfully to a host object is implementation-dependent.
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:
"length"
.The length
property of the some
method is 1.
NOTE The some
function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the some
function can be applied successfully to a host object is implementation-dependent.
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:
"length"
.The length
property of the forEach
method is 1.
NOTE The forEach
function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the forEach
function can be applied successfully to a host object is implementation-dependent.
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:
"length"
.new Array(
len)
where
Array
is the standard built-in constructor with that name and len is the value of
len.The length
property of the map
method is 1.
NOTE The map
function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the map
function can be applied successfully to a host object is implementation-dependent.
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:
"length"
.new Array()
where Array
is the
standard built-in constructor with that name.The length
property of the filter
method is 1.
NOTE The filter
function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the filter
function can be applied successfully to a host object is implementation-dependent.
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 reduce
begins and before
being visited are not visited.
When the reduce
method is called with one or two arguments, the following steps are taken:
"length"
.The length
property of the reduce
method is 1.
NOTE The reduce
function is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the reduce
function can be applied successfully to a host object is implementation-dependent.
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 reduceRight
begins and before being visited are not visited.
When the reduceRight
method is called with one or two arguments, the following steps are taken:
"length"
.The length
property of the reduceRight
method is 1.
NOTE The reduceRight
function is intentionally generic; it does not require that
its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
Whether the reduceRight
function can be applied successfully to a host object is
implementation-dependent.
Array instances inherit properties from the Array prototype object and their [[Class]] internal property value is
"Array"
. Array instances also have the following properties.
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:
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 }.
NOTE Attempting to set the length property of an Array object to a value that is numerically less than or equal to the largest numeric property name of an existing array indexed non-deletable property of the array will result in the length being set to a numeric value that is one greater than that largest numeric property name. See 15.4.5.1.
When String
is called as a function rather than as a constructor, it performs a type conversion.
Returns a String value (not a String object) computed by ToString(value). If value is not supplied, the empty String ""
is returned.
When String
is called as part of a new
expression, it is a constructor: it initialises the
newly created object.
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.
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:
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 }.
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.
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).
The initial value of String.prototype.constructor
is the built-in String
constructor.
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.
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.
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:
NOTE The charAt
function is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a
method.
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:
NOTE The charCodeAt
function is intentionally generic; it does not require that
its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a
method.
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:
The length
property of the concat
method is 1.
NOTE The concat
function is intentionally generic; it does not require that its
this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
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:
0
).-1
.The length
property of the indexOf
method is 1.
NOTE The indexOf
function is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a
method.
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
.The length
property of the lastIndexOf
method is 1.
NOTE The lastIndexOf
function is intentionally generic; it does not require that
its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a
method.
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:
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.
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.
NOTE 1 The localeCompare
method itself is not directly suitable as an argument
to Array.prototype.sort
because the latter requires a function of two arguments.
NOTE 2 This function is intended to rely on whatever language-sensitive comparison functionality is available to the ECMAScript environment from the host environment, and to compare according to the rules of the host environment's current locale. It is strongly recommended that this function treat Strings that are canonically equivalent according to the Unicode standard as identical (in other words, compare the Strings as if they had both been converted to Normalised Form C or D first). It is also recommended that this function not honour Unicode compatibility equivalences or decompositions.
NOTE 3 The second 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.
NOTE 4 The localeCompare
function is intentionally generic; it does not require
that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a
method.
When the match
method is called with argument regexp, the following steps are taken:
"RegExp"
, then let rx be regexp;new
RegExp(
regexp)
where RegExp
is the standard built-in constructor with that
name."global"
.RegExp.prototype.exec
(see
15.10.6.2)"lastIndex"
and 0.new Array()
where Array
is
the standard built-in constructor with that name."lastIndex"
."lastIndex"
and
thisIndex+1."0"
.NOTE The match
function is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a
method.
First set string according to the following steps:
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, "$$1-$1$2")
returns
"$1-$11,$1-$22"
. A $
in newstring that does not match any of the forms below is left
as is.
Characters | Replacement text |
---|---|
$$ |
$ |
$& |
The matched substring. |
$‘ |
The portion of string that precedes the matched substring. |
$’ |
The portion of string that follows the matched substring. |
$n |
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. |
NOTE The replace
function is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a
method.
When the search method is called with argument regexp, the following steps are taken:
"RegExp"
, then let rx be regexp;new
RegExp(
regexp)
where RegExp
is the standard built-in constructor with that
name.lastIndex
and global
properties of regexp are ignored when
performing the search. The lastIndex
property of regexp is left unchanged.NOTE The search
function is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a
method.
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:
The length
property of the slice
method is 2.
NOTE The slice
function is intentionally generic; it does not require that its
this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.
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:
new Array()
where Array
is the
standard built-in constructor with that name."RegExp"
), let R = separator;
otherwise let R = ToString(separator).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):
"RegExp"
), then
The length
property of the split
method is 2.
NOTE 1 The split
method ignores the value of separator.global
for
separators that are RegExp objects.
NOTE 2 The split
function is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a
method.
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:
The length
property of the substring
method is 2.
NOTE The substring
function is intentionally generic; it does not require that
its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a
method.
The following steps are taken:
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).
NOTE 1 The case mapping of some characters may produce multiple characters. In this case the
result String may not be the same length as the source String. Because both toUpperCase
and
toLowerCase
have context-sensitive behaviour, the functions are not symmetrical. In other words,
s.toUpperCase().toLowerCase()
is not necessarily equal to s.toLowerCase()
.
NOTE 2 The toLowerCase
function is intentionally generic; it does not require
that its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a
method.
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.
NOTE 1 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.
NOTE 2 The toLocaleLowerCase
function is intentionally generic; it does not
require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for
use as a method.
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.
NOTE The toUpperCase
function is intentionally generic; it does not require that
its this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a
method.
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.
NOTE 1 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.
NOTE 2 The toLocaleUpperCase
function is intentionally generic; it does not
require that its this value be a String object. Therefore, it can be transferred to other kinds of objects for
use as a method.
The following steps are taken:
NOTE The trim
function is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of objects for use as a
method.
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.
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 }.
String objects use a variation of the [[GetOwnProperty]] internal method used for other native ECMAScript objects (8.12.1). This special internal method provides access to named properties corresponding to the individual characters 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:
When Boolean
is called as a function rather than as a constructor, it performs a type conversion.
Returns a Boolean value (not a Boolean object) computed by ToBoolean(value).
When Boolean
is called as part of a new
expression it is a constructor: it initialises the
newly created object.
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.
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:
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 }.
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).
The initial value of Boolean.prototype.constructor
is the built-in Boolean
constructor.
The following steps are taken:
"Boolean"
, then let b be the value of the [[PrimitiveValue]] internal property of
B."true"
; else return "false"
.The following steps are taken:
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.
When Number
is called as a function rather than as a constructor, it performs a type conversion.
Returns a Number value (not a Number object) computed by ToNumber(value) if value was supplied, else returns +0.
When Number
is called as part of a new
expression it is a constructor: it initialises the newly
created object.
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.
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 properties:
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 }.
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]]: false, [[Configurable]]: false }.
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 }.
The value of Number.NaN
is NaN.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
The value of Number.NEGATIVE_INFINITY is −∞.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
The value of Number.POSITIVE_INFINITY is +∞.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
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.
The initial value of Number.prototype.constructor
is the built-in Number
constructor.
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 generalisation 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.
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
.
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.
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.
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:
0
)."NaN"
.-
"."0"
. Otherwise, let m be the String consisting
of the digits of the decimal representation of n (in order, with no leading zeroes)."."
, and b.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.
NOTE The output of toFixed
may be more precise than toString
for
some values because toString only prints enough significant digits to distinguish the number from adjacent number
values. For example,
(1000000000000000128).toString()
returns "1000000000000000100"
,
while
(1000000000000000128).toFixed(0)
returns "1000000000000000128"
.
Return a String containing this Number value represented in decimal 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:
"NaN"
."-"
."Infinity"
."."
, and b."+".
"0".
"+".
"-"
."e"
, c, and d.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.
NOTE For implementations that provide more accurate conversions than required by the rules above, it is recommended that the following alternative version of step 9.b.i be used as a guideline:
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:
"NaN"
."-"
."Infinity"
."."
, and b."+"
and d = "0"
."+"
."-"
."e"
, c, and
d."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.
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.
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.
NOTE In this specification, the phrase “the Number value for x” has a technical meaning defined in 8.5.
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 }.
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 }.
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 }.
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 }.
NOTE The value of Math.LOG2E
is approximately the reciprocal of the value of
Math.LN2
.
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 }.
NOTE The value of Math.LOG10E
is approximately the reciprocal of the value of
Math.LN10
.
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 }.
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 }.
NOTE The value of Math.SQRT1_2
is approximately the reciprocal of the value of
Math.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 }.
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.
NOTE The behaviour of the functions acos
, asin
, atan
,
atan2
, cos
, exp
, log
, pow
, sin
,
sqrt
, and tan
is not precisely specified here except to require specific results for certain
argument values that represent boundary cases of interest. For other argument values, these functions are intended to
compute approximations to the results of familiar mathematical functions, but some latitude is allowed in the choice of
approximation algorithms. The general intent is that an implementer should be able to use the same mathematical library
for ECMAScript on a given hardware platform that is available to C programmers on that platform.
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).
Returns the absolute value of x; the result has the same magnitude as x but has positive sign.
Returns an implementation-dependent approximation to the arc cosine of x. The result is expressed in radians and ranges from +0 to +π.
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.
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.
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.
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.
The value of Math.ceil(x)
is the same as the value of -Math.floor(-x)
.
Returns an implementation-dependent approximation to the cosine of x. The argument is expressed in radians.
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).
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.
NOTE The value of Math.floor(x)
is the same as the value
of -Math.ceil(-x)
.
Given zero or more arguments, calls ToNumber on each of the arguments and returns the largest of the resulting values.
The length
property of the max
method is 2.
Given zero or more arguments, calls ToNumber on each of the arguments and returns the smallest of the resulting values.
The length
property of the min
method is 2.
Returns an implementation-dependent approximation to the result of raising x to the power y.
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.
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.
NOTE 1 Math.round(3.5)
returns 4, but Math.round(–3.5)
returns –3.
Returns an implementation-dependent approximation to the sine of x. The argument is expressed in radians.
Returns an implementation-dependent approximation to the square root of x.
Returns an implementation-dependent approximation to the tangent of x. The argument is expressed in radians.
A RegExp object contains a regular expression and the associated flags.
NOTE The form and functionality of regular expressions is modelled after the regular expression facility in the Perl 5 programming language.
The RegExp
constructor applies the following grammar to the input pattern String. An error occurs if the
grammar cannot interpret the String as an expansion of Pattern.
|
Disjunction^
$
\
b
\
B
(
?
=
Disjunction )
(
?
!
Disjunction )
?
*
+
?
{
DecimalDigits }
{
DecimalDigits ,
}
{
DecimalDigits ,
DecimalDigits }
.
\
AtomEscape(
Disjunction )
(
?
:
Disjunction )
^
$
\
.
*
+
?
(
)
[
]
{
}
|
c
ControlLetterf
n
r
t
v
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
d
D
s
S
w
W
[
[lookahead ∉ {^
}] ClassRanges ]
[
^
ClassRanges ]
-
ClassAtom ClassRanges-
ClassAtom ClassRanges-
\
or ]
or -
\
ClassEscapeb
A regular expression pattern is converted into an internal procedure using the process described below. An implementation is encouraged to use more efficient algorithms than the ones listed below, as long as the results are the same. The internal procedure is used as the value of a RegExp object's [[Match]] internal property.
The descriptions below use the following variables:
Input is the String being matched by the regular expression pattern. The notation input[n] means the nth character of input, where n can range between 0 (inclusive) and InputLength (exclusive).
InputLength is the number of characters in the Input String.
NcapturingParens is the total number of left capturing parentheses (i.e. the total number
of times the Atom :: (
Disjunction )
production is expanded) in the pattern. A left
capturing parenthesis is any (
pattern character that is matched by the (
terminal of the
Atom :: (
Disjunction )
production.
IgnoreCase is the setting of the RegExp object's ignoreCase
property.
Multiline is the setting of the RegExp object's multiline
property.
Furthermore, the descriptions below use the following internal data structures:
A CharSet is a mathematical set of characters.
A State is an ordered pair (endIndex, captures) where endIndex is an integer and captures is an internal array of NcapturingParens values. States are used to represent partial match states in the regular expression matching algorithms. The endIndex is one plus the index of the last input character matched so far by the pattern, while captures holds the results of capturing parentheses. The nth element of captures is either a String that represents the value obtained by the nth set of capturing parentheses or undefined if the nth set of capturing parentheses hasn't been reached yet. Due to backtracking, many States may be in use at any time during the matching process.
A MatchResult is either a State or the special token failure that indicates that the match failed.
A Continuation procedure is an internal closure (i.e. an internal procedure with some arguments already bound to values) that takes one State argument and returns a MatchResult result. If an internal closure references variables bound in the function that creates the closure, the closure uses the values that these variables had at the time the closure was created. The Continuation attempts to match the remaining portion (specified by the closure's already-bound arguments) of the pattern against the input String, starting at the intermediate state given by its State argument. If the match succeeds, the Continuation returns the final State that it reached; if the match fails, the Continuation returns failure.
A Matcher procedure is an internal closure that takes two arguments -- a State and a Continuation -- and returns a MatchResult result. A Matcher attempts to match a middle subpattern (specified by the closure's already-bound arguments) of the pattern against the input String, starting at the intermediate state given by its State argument. The Continuation argument should be a closure that matches the rest of the pattern. After matching the subpattern of a pattern to obtain a new State, the Matcher then calls Continuation on that new State to test if the rest of the pattern can match as well. If it can, the Matcher returns the State returned by Continuation; if not, the Matcher may try different choices at its choice points, repeatedly calling Continuation until it either succeeds or all possibilities have been exhausted.
An AssertionTester procedure is an internal closure that takes a State argument and returns a Boolean result. The assertion tester tests a specific condition (specified by the closure's already-bound arguments) against the current place in the input String and returns true if the condition matched or false if not.
An EscapeValue is either a character or an integer. An EscapeValue is used to denote the interpretation of a DecimalEscape escape sequence: a character ch means that the escape sequence is interpreted as the character ch, while an integer n means that the escape sequence is interpreted as a backreference to the nth set of capturing parentheses.
The production Pattern :: Disjunction evaluates as follows:
NOTE A Pattern evaluates ("compiles") to an internal procedure value.
RegExp.prototype.exec
can then apply this
procedure to a String and an offset within the String to
determine whether the pattern would match starting at exactly
that offset within the String, and, if it does match, what
the values of the capturing parentheses would be. The
algorithms in 15.10.2 are designed so
that compiling a pattern may throw a SyntaxError exception; on the other hand, once the pattern is successfully
compiled, applying its result internal procedure to find a match in a String cannot throw an exception (except for any
host-defined exceptions that can occur anywhere such as out-of-memory).
The production Disjunction :: Alternative evaluates by evaluating Alternative to obtain a Matcher and returning that Matcher.
The production Disjunction :: Alternative |
Disjunction evaluates as
follows:
NOTE The |
regular expression operator separates two alternatives. The pattern
first tries to match the left Alternative (followed by the sequel of the regular expression); if
it fails, it tries to match the right Disjunction (followed by the sequel of the regular
expression). If the left Alternative, the right Disjunction, and the
sequel all have choice points, all choices in the sequel are tried before moving on to the next choice in the left Alternative. If choices in the left Alternative are exhausted, the right Disjunction is tried instead of the left Alternative. Any capturing
parentheses inside a portion of the pattern skipped by |
produce undefined values instead of
Strings. Thus, for example,
/a|ab/.exec("abc")
returns the result "a"
and not "ab"
. Moreover,
/((a)|(ab))((c)|(bc))/.exec("abc")
returns the array
["abc", "a", "a", undefined, "bc", undefined, "bc"]
and not
["abc", "ab", undefined, "ab", "c", "c", undefined]
The production Alternative :: [empty] evaluates by returning a Matcher that takes two arguments, a State x and a Continuation c, and returns the result of calling c(x).
The production Alternative :: Alternative Term evaluates as follows:
NOTE Consecutive Terms try to simultaneously match consecutive portions of the input String. If the left Alternative, the right Term, and the sequel of the regular expression all have choice points, all choices in the sequel are tried before moving on to the next choice in the right Term, and all choices in the right Term are tried before moving on to the next choice in the left Alternative.
The production Term :: Assertion evaluates by returning an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs the following:
The production Term :: Atom evaluates by evaluating Atom to obtain a Matcher and returning that Matcher.
The production Term :: Atom Quantifier evaluates as follows:
(
Disjunction
)
production is expanded prior to this production's Term plus the total number
of Atom :: (
Disjunction )
productions enclosing this Term.(
Disjunction )
productions enclosed by this
production's Atom.The abstract operation RepeatMatcher takes eight parameters, a Matcher m, an integer min, an integer (or ∞) max, a Boolean greedy, a State x, a Continuation c, an integer parenIndex, and an integer parenCount, and performs the following:
NOTE 1 An Atom followed by a Quantifier is repeated the number of times specified by the Quantifier. A Quantifier can be non-greedy, in which case the Atom pattern is repeated as few times as possible while still matching the sequel, or it can be greedy, in which case the Atom pattern is repeated as many times as possible while still matching the sequel. The Atom pattern is repeated rather than the input String that it matches, so different repetitions of the Atom can match different input substrings.
NOTE 2 If the Atom and the sequel of the regular expression all have choice points, the Atom is first matched as many (or as few, if non-greedy) times as possible. All choices in the sequel are tried before moving on to the next choice in the last repetition of Atom. All choices in the last (nth) repetition of Atom are tried before moving on to the next choice in the next-to-last (n–1)st repetition of Atom; at which point it may turn out that more or fewer repetitions of Atom are now possible; these are exhausted (again, starting with either as few or as many as possible) before moving on to the next choice in the (n-1)st repetition of Atom and so on.
Compare
/a[a-z]{2,4}/.exec("abcdefghi")
which returns "abcde"
with
/a[a-z]{2,4}?/.exec("abcdefghi")
which returns "abc"
.
Consider also
/(aa|aabaac|ba|b|c)*/.exec("aabaac")
which, by the choice point ordering above, returns the array
["aaba", "ba"]
and not any of:
["aabaac", "aabaac"]
["aabaac", "c"]
The above ordering of choice points can be used to write a regular expression that calculates the greatest common divisor of two numbers (represented in unary notation). The following example calculates the gcd of 10 and 15:
"aaaaaaaaaa,aaaaaaaaaaaaaaa".replace(/^(a+)\1*,\1+$/,"$1")
which returns the gcd in unary notation "aaaaa"
.
NOTE 3 Step 4 of the RepeatMatcher clears Atom's captures each time Atom is repeated. We can see its behaviour in the regular expression
/(z)((a+)?(b+)?(c))*/.exec("zaacbbbcac")
which returns the array
["zaacbbbcac", "z", "ac", "a", undefined, "c"]
and not
["zaacbbbcac", "z", "ac", "a", "bbb", "c"]
because each iteration of the outermost *
clears all captured Strings contained in the quantified Atom, which in this case includes capture Strings numbered 2, 3, 4, and 5.
NOTE 4 Step 1 of the RepeatMatcher's d closure states that, once the minimum number of repetitions has been satisfied, any more expansions of Atom that match the empty String are not considered for further repetitions. This prevents the regular expression engine from falling into an infinite loop on patterns such as:
/(a*)*/.exec("b")
or the slightly more complicated:
/(a*)b\1+/.exec("baaaac")
which returns the array
["b", ""]
The production Assertion :: ^
evaluates by returning an internal AssertionTester closure that takes a State argument
x and performs the following:
The production Assertion :: $
evaluates by returning an internal AssertionTester closure that takes a State argument
x and performs the following:
The production Assertion :: \
b
evaluates by returning an internal AssertionTester closure that takes a
State argument x and performs the following:
The production Assertion :: \
B
evaluates by returning an internal AssertionTester closure that takes a
State argument x and performs the following:
The production Assertion :: (
?
=
Disjunction )
evaluates as follows:
The production Assertion :: (
?
!
Disjunction )
evaluates as follows:
The abstract operation IsWordChar takes an integer parameter e and performs the following:
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 |
0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | _ |
The production Quantifier :: QuantifierPrefix evaluates as follows:
The production Quantifier :: QuantifierPrefix ?
evaluates as follows:
The production QuantifierPrefix :: *
evaluates by returning the two results 0 and ∞.
The production QuantifierPrefix :: +
evaluates by returning the two results 1 and ∞.
The production QuantifierPrefix :: ?
evaluates by returning the two results 0 and 1.
The production QuantifierPrefix :: {
DecimalDigits }
evaluates as follows:
The production QuantifierPrefix :: {
DecimalDigits ,
}
evaluates as follows:
The production QuantifierPrefix :: {
DecimalDigits ,
DecimalDigits
}
evaluates as follows:
The production Atom :: PatternCharacter evaluates as follows:
The production Atom :: .
evaluates as follows:
The production Atom :: \
AtomEscape evaluates by evaluating AtomEscape to obtain a Matcher
and returning that Matcher.
The production Atom :: CharacterClass evaluates as follows:
The production Atom :: (
Disjunction )
evaluates as follows:
(
Disjunction )
production is expanded prior to this production's
Atom plus the total number of Atom ::
(
Disjunction )
productions enclosing
this Atom.The production Atom :: (
?
:
Disjunction )
evaluates by evaluating Disjunction to obtain a Matcher and returning that Matcher.
The abstract operation CharacterSetMatcher takes two arguments, a CharSet A and a Boolean flag invert, and performs the following:
The abstract operation Canonicalize takes a character parameter ch and performs the following steps:
String.prototype.toUpperCase
on the one-character String ch.NOTE 1 Parentheses of the form (
Disjunction
)
serve both to group the components of the Disjunction pattern together and to
save the result of the match. The result can be used either in a backreference (\
followed by a nonzero
decimal number), referenced in a replace String, or returned as part of an array from the regular expression matching
internal procedure. To inhibit the capturing behaviour of parentheses, use the form (?:
Disjunction )
instead.
NOTE 2 The form (?=
Disjunction )
specifies
a zero-width positive lookahead. In order for it to succeed, the pattern inside Disjunction must
match at the current position, but the current position is not advanced before matching the sequel. If Disjunction can match at the current position in several ways, only the first one is tried. Unlike
other regular expression operators, there is no backtracking into a (?=
form (this unusual behaviour is
inherited from Perl). This only matters when the Disjunction contains capturing parentheses and
the sequel of the pattern contains backreferences to those captures.
For example,
/(?=(a+))/.exec("baaabac")
matches the empty String immediately after the first b
and therefore returns the array:
["", "aaa"]
To illustrate the lack of backtracking into the lookahead, consider:
/(?=(a+))a*b\1/.exec("baaabac")
This expression returns
["aba", "a"]
and not:
["aaaba", "a"]
NOTE 3 The form (?!
Disjunction )
specifies
a zero-width negative lookahead. In order for it to succeed, the pattern inside Disjunction must
fail to match at the current position. The current position is not advanced before matching the sequel. Disjunction can contain capturing parentheses, but backreferences to them only make sense from within
Disjunction itself. Backreferences to these capturing parentheses from elsewhere in the pattern
always return undefined because the negative lookahead must fail for the pattern to succeed. For example,
/(.*?)a(?!(a+)b\2c)\2(.*)/.exec("baaabaac")
looks for an a
not immediately followed by some positive number n of a
's, a b
,
another n a
's (specified by the first \2
) and a c
. The second \2
is
outside the negative lookahead, so it matches against undefined and therefore always succeeds. The whole
expression returns the array:
["baaabaac", "ba", undefined, "abaac"]
In case-insignificant matches all characters are implicitly
converted to upper case immediately before they are
compared. However, if converting a character to upper case would
expand that character into more than one character (such
as converting "ß"
(\u00DF) into "SS"
),
then
the character is left as-is instead. The character is also left
as-is if it is not an ASCII character but converting it to
upper case would make it into an ASCII character. This prevents
Unicode characters such as \u0131 and \u017F from matching regular expressions such as
/[a‑z]/i
, which are only intended to match ASCII letters. Furthermore, if these conversions were
allowed, then /[^\W]/i
would match each of a
, b
, …, h
, but not
i
or s
.
The production AtomEscape :: DecimalEscape evaluates as follows:
The production AtomEscape :: CharacterEscape evaluates as follows:
The production AtomEscape :: CharacterClassEscape evaluates as follows:
NOTE An escape sequence of the form \
followed by a nonzero decimal number
n matches the result of the nth set of capturing parentheses (see
15.10.2.11). It is an error if the regular expression has fewer than n capturing parentheses. If the
regular expression has n or more capturing parentheses but the nth one is undefined because
it has not captured anything, then the backreference always succeeds.
The production CharacterEscape :: ControlEscape evaluates by returning the character according to Table 23.
ControlEscape | Code Unit | Name | Symbol |
---|---|---|---|
t |
\u0009 |
horizontal tab | <HT> |
n |
\u000A |
line feed (new line) | <LF> |
v |
\u000B |
vertical tab | <VT> |
f |
\u000C |
form feed | <FF> |
r |
\u000D |
carriage return | <CR> |
The production CharacterEscape :: c
ControlLetter evaluates as follows:
The production CharacterEscape :: HexEscapeSequence evaluates by evaluating the CV of the HexEscapeSequence (see 7.8.4) and returning its character result.
The production CharacterEscape :: UnicodeEscapeSequence evaluates by evaluating the CV of the UnicodeEscapeSequence (see 7.8.4) and returning its character result.
The production CharacterEscape :: IdentityEscape evaluates by returning the character represented by IdentityEscape.
The production DecimalEscape :: DecimalIntegerLiteral [lookahead ∉ DecimalDigit] evaluates as follows:
The definition of “the MV of DecimalIntegerLiteral” is in 7.8.3.
NOTE If \
is followed by a decimal number n whose first digit is not
0
, then the escape sequence is considered to be a backreference. It is an error if n is greater
than the total number of left capturing parentheses in the entire regular expression. \0
represents the
<NUL> character and cannot be followed by a decimal digit.
The production CharacterClassEscape :: d
evaluates by returning the ten-element set of characters containing the characters
0
through 9
inclusive.
The production CharacterClassEscape :: D
evaluates by returning the set of all characters not included in the set returned by CharacterClassEscape :: d
.
The production CharacterClassEscape :: s
evaluates by returning the set of characters containing the characters that are on the
right-hand side of the WhiteSpace (7.2) or LineTerminator (7.3) productions.
The production CharacterClassEscape :: S
evaluates by returning the set of all characters not included in the set returned by CharacterClassEscape :: s
.
The production CharacterClassEscape :: w
evaluates by returning the set of characters containing the sixty-three characters:
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 |
0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | _ |
The production CharacterClassEscape :: W
evaluates by returning the set of all characters not included in the set returned by CharacterClassEscape :: w
.
The production CharacterClass :: [
[lookahead ∉ {^
}] ClassRanges ]
evaluates by evaluating ClassRanges
to obtain a CharSet and returning that CharSet and the Boolean false.
The production CharacterClass :: [
^
ClassRanges ]
evaluates
by evaluating ClassRanges to obtain a CharSet and returning that CharSet and the Boolean
true.
The production ClassRanges :: [empty] evaluates by returning the empty CharSet.
The production ClassRanges :: NonemptyClassRanges evaluates by evaluating NonemptyClassRanges to obtain a CharSet and returning that CharSet.
The production NonemptyClassRanges :: ClassAtom evaluates by evaluating ClassAtom to obtain a CharSet and returning that CharSet.
The production NonemptyClassRanges :: ClassAtom NonemptyClassRangesNoDash evaluates as follows:
The production NonemptyClassRanges :: ClassAtom -
ClassAtom ClassRanges evaluates as follows:
The abstract operation CharacterRange takes two CharSet parameters A and B and performs the following:
The production NonemptyClassRangesNoDash :: ClassAtom evaluates by evaluating ClassAtom to obtain a CharSet and returning that CharSet.
The production NonemptyClassRangesNoDash :: ClassAtomNoDash NonemptyClassRangesNoDash evaluates as follows:
The production NonemptyClassRangesNoDash :: ClassAtomNoDash -
ClassAtom ClassRanges evaluates as follows:
NOTE 1 ClassRanges can expand into single ClassAtoms and/or ranges of two ClassAtoms separated by dashes. In the latter
case the ClassRanges includes all characters between the first ClassAtom
and the second ClassAtom, inclusive; an error occurs if either ClassAtom
does not represent a single character (for example, if one is \w
) or if the first ClassAtom's
code unit value is greater than the second ClassAtom's code unit value.
NOTE 2 Even if the pattern ignores case, the case of the two ends of a range is significant
in determining which characters belong to the range. Thus, for example, the pattern /[E-F]/i
matches only
the letters E
, F
, e
, and f
, while the pattern /[E-f]/i
matches all upper and lower-case ASCII letters as well as the symbols [
, \
, ]
,
^
, _
, and `
.
NOTE 3 A -
character can be treated literally or it can denote a range. It is
treated literally if it is the first or last character of ClassRanges, the beginning or end
limit of a range specification, or immediately follows a range specification.
The production ClassAtom :: -
evaluates by returning the CharSet containing the one character -
.
The production ClassAtom :: ClassAtomNoDash evaluates by evaluating ClassAtomNoDash to obtain a CharSet and returning that CharSet.
The production ClassAtomNoDash :: SourceCharacter but not one of \
or ]
or -
evaluates by returning a one-element CharSet containing the character represented by SourceCharacter.
The production ClassAtomNoDash :: \
ClassEscape evaluates by evaluating ClassEscape
to obtain a CharSet and returning that CharSet.
The production ClassEscape :: DecimalEscape evaluates as follows:
The production ClassEscape :: b
evaluates by returning the CharSet containing the one character <BS> (Unicode value
0008).
The production ClassEscape :: CharacterEscape evaluates by evaluating CharacterEscape to obtain a character and returning a one-element CharSet containing that character.
The production ClassEscape :: CharacterClassEscape evaluates by evaluating CharacterClassEscape to obtain a CharSet and returning that CharSet.
NOTE A ClassAtom can use any of the escape sequences that are allowed
in the rest of the regular expression except for \b
, \B
, and backreferences. Inside a CharacterClass, \b
means the backspace character, while \B
and
backreferences raise errors. Using a backreference inside a ClassAtom causes an error.
If pattern is an object R whose [[Class]] internal property is "RegExp"
and
flags is undefined, then return R unchanged. Otherwise call the standard built-in
RegExp
constructor (15.10.4.1) as if by the expression new
RegExp(
pattern,
flags)
and return the object constructed by that
constructor.
When RegExp
is called as part of a new
expression, it is a constructor: it initialises the
newly created object.
If pattern is an object R whose [[Class]] internal property is "RegExp"
and
flags is undefined, then let P be the pattern used to construct R and
let F be the flags used to construct R. If pattern is an object R whose
[[Class]] internal property is "RegExp"
and flags is not undefined, then throw a
TypeError exception. Otherwise, let P be the empty String if pattern is undefined and
ToString(pattern) otherwise, and let
F be the empty String if flags is undefined and ToString(flags) otherwise.
If the characters of P do not have the syntactic form Pattern, then throw a SyntaxError exception. Otherwise let the newly constructed object have a [[Match]] internal property obtained by evaluating ("compiling") the characters of P as a Pattern as described in 15.10.2.
If F contains any character other than "g"
, "i"
, or "m"
, or if it
contains the same character more than once, then throw a SyntaxError exception.
If a SyntaxError exception is not thrown, then:
Let S be a String in the form of a Pattern equivalent to P, in which certain characters are escaped as described below. S may or may not be identical to P or pattern; however, the internal procedure that would result from evaluating S as a Pattern must behave identically to the internal procedure given by the constructed object's [[Match]] internal property.
The characters /
occurring in the pattern shall be escaped in S as necessary to ensure that the
String value formed by concatenating the Strings "/"
, S, "/"
, and F can be
parsed (in an appropriate lexical context) as a RegularExpressionLiteral that behaves identically
to the constructed regular expression. For example, if P is "/"
, then S could be
"\/"
or "\u002F"
, among other possibilities, but not "/"
, because ///
followed by F would be parsed as a SingleLineComment rather than a RegularExpressionLiteral. If P is the empty String, this specification can be met by letting
S be "(?:)"
.
The following properties of the newly constructed object are data properties with the attributes that are specified in 15.10.7. The [[Value]] of each property is set as follows:
The source
property of the newly constructed object is set to S.
The global
property of the newly constructed object is set to a Boolean value that is true if
F contains the character "g"
and false otherwise.
The ignoreCase
property of the newly constructed object is set to a Boolean value that is true if
F contains the character "i"
and false otherwise.
The multiline
property of the newly constructed object is set to a Boolean value that is true if
F contains the character "m"
and false otherwise.
The lastIndex
property of the newly constructed object is set to 0.
The [[Prototype]] internal property of the newly constructed object is set to the standard built-in RegExp prototype object as specified in 15.10.6.
The [[Class]] internal property of the newly constructed object is set to "RegExp"
.
NOTE If pattern is a StringLiteral, the usual escape sequence
substitutions are performed before the String is processed by RegExp. If pattern must contain an escape sequence to be
recognised by RegExp, any backslash \
characters must be escaped within the StringLiteral to prevent them being removed when the contents of the StringLiteral are formed.
The value of the [[Prototype]] internal property of the RegExp constructor is the standard built-in Function prototype object (15.3.4).
Besides the internal properties and the length
property (whose value is 2), the RegExp constructor
has the following properties:
The initial value of RegExp.prototype
is the RegExp prototype object (15.10.6).
This property shall have the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
The value of the [[Prototype]] internal property of the RegExp prototype object is the standard built-in Object prototype
object (15.2.4). The RegExp prototype object is itself a regular expression object; its [[Class]]
is "RegExp". The initial values of the RegExp prototype object’s
data properties (15.10.7) are set as if the object was created by the expression new
RegExp()
where RegExp
is that standard built-in constructor with that name.
The RegExp prototype object does not have a valueOf
property of its own; however, it inherits the
valueOf
property from the Object prototype object.
In the following descriptions of functions that are properties of the RegExp prototype object, the phrase “this
RegExp object” refers to the object that is the this value for the invocation of the function; a
TypeError exception is thrown if the this value is not an object or an object for which the value of the
[[Class]] internal property is not "RegExp"
.
The initial value of RegExp.prototype.constructor
is the standard built-in RegExp
constructor.
Performs a regular expression match of string against the regular expression and returns an Array object containing the results of the match, or null if string did not match.
The String ToString(string) is searched for an occurrence of the regular expression pattern as follows:
lastIndex
".global"
.lastIndex"
, 0, and
true.lastIndex"
, e, and
true.new Array()
where Array
is the
standard built-in constructor with that name.index"
, Property Descriptor {[[Value]]: matchIndex, [[Writable]: true, [[Enumerable]]:
true, [[Configurable]]: true}, and true.input"
, Property Descriptor {[[Value]]: S, [[Writable]: true, [[Enumerable]]:
true, [[Configurable]]: true}, and true.length"
, Property Descriptor {[[Value]]: n + 1}, and true.0"
, Property Descriptor {[[Value]]: matchedSubstr, [[Writable]: true, [[Enumerable]]:
true, [[Configurable]]: true}, and true.The following steps are taken:
RegExp.prototype.exec
(15.10.6.2) algorithm upon this RegExp object using string as the argument.Return the String value formed by concatenating the Strings "/", the
String value of the source property of this RegExp object, and "/"; plus "g" if the global
property is true, "i" if the ignoreCase
property is
true, and "m" if the multiline
property is
true.
NOTE The returned String has the form of a RegularExpressionLiteral that evaluates to another RegExp object with the same behaviour as this object.
RegExp instances inherit properties from the RegExp prototype object and their [[Class]] internal property value is
"RegExp"
. RegExp instances also have a [[Match]] internal property and a length
property.
The value of the [[Match]] internal property is an implementation dependent representation of the Pattern of the RegExp object.
RegExp instances also have the following properties.
The value of the source
property is a String in the form of a Pattern representing
the current regular expression. This property shall have the attributes { [[Writable]]: false, [[Enumerable]]:
false, [[Configurable]]: false }.
The value of the global
property is a Boolean value indicating whether the flags contained the character
“g”
. This property shall have the attributes { [[Writable]]: false, [[Enumerable]]:
false, [[Configurable]]: false }.
The value of the ignoreCase
property is a Boolean value indicating whether the flags contained the
character “i”
. This property shall have the attributes { [[Writable]]: false,
[[Enumerable]]: false, [[Configurable]]: false }.
The value of the multiline
property is a Boolean value indicating whether the flags contained the
character “m”
. This property shall have the attributes { [[Writable]]: false,
[[Enumerable]]: false, [[Configurable]]: false }.
The value of the lastIndex
property specifies the String position at which to start the next match. It is
coerced to an integer when used (see 15.10.6.2). This property shall have the attributes
{ [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.
NOTE Unlike the other standard built-in properties of RegExp instances,
lastIndex
is writable.
Instances of Error objects are thrown as exceptions when runtime errors occur. The Error objects may also serve as base objects for user-defined exception classes.
When Error
is called as a function rather than as a constructor, it creates and initialises a new Error
object. Thus the function call Error(…) is equivalent to
the object creation expression new Error(…) with the same
arguments.
The [[Prototype]] internal property of the newly constructed object is set to the original Error prototype object, the
one that is the initial value of Error.prototype
(15.11.3.1).
The [[Class]] internal property of the newly constructed object is set to "Error"
.
The [[Extensible]] internal property of the newly constructed object is set to true.
If the argument message is not undefined, the message
own property of the newly
constructed object is set to ToString(message).
When Error
is called as part of a new
expression, it is a constructor: it initialises the newly
created object.
The [[Prototype]] internal property of the newly constructed object is set to the original Error prototype object, the
one that is the initial value of Error.prototype
(15.11.3.1).
The [[Class]] internal property of the newly constructed Error object is set to "Error"
.
The [[Extensible]] internal property of the newly constructed object is set to true.
If the argument message is not undefined, the message
own property of the newly
constructed object is set to ToString(message).
The value of the [[Prototype]] internal property of the Error constructor is the Function prototype object (15.3.4).
Besides the internal properties and the length
property (whose value is 1), the Error constructor has
the following property:
The initial value of Error.prototype
is the Error prototype object (15.11.4).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
The Error prototype object is itself an Error object (its [[Class]] is "Error"
).
The value of the [[Prototype]] internal property of the Error prototype object is the standard built-in Object prototype object (15.2.4).
The initial value of Error.prototype.constructor
is the built-in Error
constructor.
The initial value of Error.prototype.name
is "Error".
The initial value of Error.prototype.message
is the empty String.
The following steps are taken:
":"
, a single space character, and msg.Error instances inherit properties from the Error prototype object and their [[Class]] internal property value is
"Error"
. Error instances have no special properties.
One of the NativeError objects below is thrown when a runtime error is detected. All of these objects share the same structure, as described in 15.11.7.
This exception is not currently used within this specification. This object remains for compatibility with previous editions of this specification.
Indicates a numeric value has exceeded the allowable range. See 15.4.2.2, 15.4.5.1, 15.7.4.2, 15.7.4.5, 15.7.4.6, 15.7.4.7, and 15.9.5.43.
Indicate that an invalid reference value has been detected. See 8.7.1, 8.7.2, 10.2.1, 10.2.1.1.4, 10.2.1.2.4, and 11.13.1.
Indicates that a parsing error has occurred. See 11.1.5, 11.3.1, 11.3.2, 11.4.1, 11.4.4, 11.4.5, 11.13.1, 11.13.2, 12.2.1, 12.10.1, 12.14.1, 13.1, 15.1.2.1, 15.3.2.1, 15.10.2.2, 15.10.2.5, 15.10.2.9, 15.10.2.15, 15.10.2.19, 15.10.4.1, and 15.12.2.
Indicates the actual type of an operand is different than the expected type. See 8.6.2, 8.7.2, 8.10.5, 8.12.5, 8.12.7, 8.12.8, 8.12.9, 9.9, 9.10, 10.2.1, 10.2.1.1.3, 10.6, 11.2.2, 11.2.3, 11.4.1, 11.8.6, 11.8.7, 11.3.1, 13.2, 13.2.3, 15, 15.2.3.2, 15.2.3.3, 15.2.3.4, 15.2.3.5, 15.2.3.6, 15.2.3.7, 15.2.3.8, 15.2.3.9, 15.2.3.10, 15.2.3.11, 15.2.3.12, 15.2.3.13, 15.2.3.14, 15.2.4.3, 15.3.4.2, 15.3.4.3, 15.3.4.4, 15.3.4.5, 15.3.4.5.2, 15.3.4.5.3, 15.3.5, 15.3.5.3, 15.3.5.4, 15.4.4.3, 15.4.4.11, 15.4.4.16, 15.4.4.17, 15.4.4.18, 15.4.4.19, 15.4.4.20, 15.4.4.21, 15.4.4.22, 15.4.5.1, 15.5.4.2, 15.5.4.3, 15.6.4.2, 15.6.4.3, 15.7.4, 15.7.4.2, 15.7.4.4, 15.9.5, 15.9.5.44, 15.10.4.1, 15.10.6, 15.11.4.4 and 15.12.3.
Indicates that one of the global URI handling functions was used in a way that is incompatible with its definition. See 15.1.3.
When an ECMAScript implementation detects a runtime error, it throws an instance of one of the NativeError objects
defined in 15.11.6. Each of these objects has the structure described below, differing only in
the name used as the constructor name instead of NativeError, in the name property of the prototype object,
and in the implementation-defined message
property of the prototype object.
For each error object, references to NativeError in the definition should be replaced with the appropriate error object name from 15.11.6.
When a NativeError constructor is called as a function rather than as a constructor, it creates and initialises a new object. A call of the object as a function is equivalent to calling it as a constructor with the same arguments.
The [[Prototype]] internal property of the newly constructed object is set to the prototype object for this error constructor. The [[Class]] internal property of the newly constructed object is set to "Error". The [[Extensible]] internal property of the newly constructed object is set to true.
If the argument message is not undefined, the message
own property of the newly
constructed object is set to ToString(message).
When a NativeError constructor is called as part of a new
expression, it is a constructor: it
initialises the newly created object.
The [[Prototype]] internal property of the newly constructed object is set to the prototype object for this NativeError constructor. The [[Class]] internal property of the newly constructed object is set to "Error". The [[Extensible]] internal property of the newly constructed object is set to true.
If the argument message is not undefined, the message
own property of the newly
constructed object is set to ToString(message).
The value of the [[Prototype]] internal property of a NativeError constructor is the Function prototype object (15.3.4).
Besides the internal properties and the length
property (whose value is 1), each NativeError
constructor has the following property:
The initial value of NativeError.prototype is a NativeError prototype object (15.11.7.7). Each NativeError constructor has a separate prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
Each NativeError prototype object is an Error object (its [[Class]] is "Error").
The value of the [[Prototype]] internal property of each NativeError prototype object is the standard built-in Error prototype object (15.11.4).
The initial value of the constructor
property of the prototype for a given NativeError constructor
is the NativeError constructor function itself (15.11.7).
The initial value of the name
property of the prototype for a given NativeError constructor is the
name of the constructor (the name used instead of NativeError).
The initial value of the message
property of the prototype for a given NativeError constructor is
the empty String.
NOTE The prototypes for the NativeError constructors do not themselves provide a
toString
function, but instances of errors will inherit it from the Error prototype object.
NativeError instances inherit properties from their NativeError prototype object and their [[Class]]
internal property value is "Error"
. NativeError instances have no special properties.
The JSON object is a single object that contains two functions, parse and stringify, that are used to parse and construct JSON texts. The JSON Data Interchange Format is described in RFC 4627 <http://www.ietf.org/rfc/rfc4627.txt>. The JSON interchange format used in this specification is exactly that described by RFC 4627 with two exceptions:
The top level JSONText production of the ECMAScript JSON grammar may consist of any JSONValue rather than being restricted to being a JSONObject or a JSONArray as specified by RFC 4627.
Conforming implementations of JSON.parse and JSON.stringify must support the exact interchange format described in this specification without any deletions or extensions to the format. This differs from RFC 4627 which permits a JSON parser to accept non-JSON forms and extensions.
The value of the [[Prototype]] internal property of the JSON object is the standard built-in Object prototype object (15.2.4). The value of the [[Class]] internal property of the JSON object is "JSON"
. The
value of the [[Extensible]] internal property of the JSON object is set to true.
The JSON object does not have a [[Construct]] internal property; it is not possible to use the JSON object as a constructor
with the new
operator.
The JSON object does not have a [[Call]] internal property; it is not possible to invoke the JSON object as a function.
JSON.stringify produces a String that conforms to the following JSON grammar. JSON.parse accepts a String that conforms to the JSON grammar.
JSON is similar to ECMAScript source text in that it consists of a sequence of characters conforming to the rules of SourceCharacter. The JSON Lexical Grammar defines the tokens that make up a JSON text similar to the manner that the ECMAScript lexical grammar defines the tokens of an ECMAScript source text. The JSON Lexical grammar only recognises the white space character specified by the production JSONWhiteSpace. The JSON lexical grammar shares some productions with the ECMAScript lexical grammar. All nonterminal symbols of the grammar that do not begin with the characters “JSON” are defined by productions of the ECMAScript lexical grammar.
JSONWhiteSpace ::
"
JSONStringCharactersopt "
JSONStringCharacter ::
"
or \ or U+0000 through U+001F\
JSONEscapeSequence"
/
\
b
f
n
r
t
-
opt DecimalIntegerLiteral JSONFractionopt ExponentPartopt.
DecimalDigitsThe JSON Syntactic Grammar defines a valid JSON text in terms of tokens defined by the JSON lexical grammar. The goal symbol of the grammar is JSONText.
{
}
{
JSONMemberList }
:
JSONValue,
JSONMember[
]
[
JSONElementList ]
,
JSONValueThe parse
function parses a JSON text (a
JSON-formatted String) and produces an ECMAScript value. The JSON
format is a restricted form of ECMAScript literal. JSON objects
are realized as ECMAScript objects. JSON arrays are realized
as ECMAScript arrays. JSON strings, numbers, booleans, and null
are realized as ECMAScript Strings, Numbers, Booleans, and
null. JSON uses a more limited set of white space characters than WhiteSpace and allows
Unicode code points U+2028 and U+2029 to directly appear in JSONString literals without using an
escape sequence. The process of parsing is similar to 11.1.4 and 11.1.5
as constrained by the JSON grammar.
The optional reviver parameter is a function that takes two parameters, (key and value). It can filter and transform the results. It is called with each of the key/value pairs produced by the parse, and its return value is used instead of the original value. If it returns what it received, the structure is not modified. If it returns undefined then the property is deleted from the result.
new Object()
, where
Object
is the standard built-in constructor with that name.The abstract operation Walk is a recursive abstract operation that takes two parameters: a holder object and the String name of a property in that object. Walk uses the value of reviver that was originally passed to the above parse function.
"Array"
"length"
.It is not permitted for a conforming implementation of JSON.parse
to extend the JSON grammars. If an
implementation wishes to support a modified or extended JSON
interchange format it must do so by defining a different parse
function.
NOTE In the case where there are duplicate name Strings within an object, lexically preceding values for the same key shall be overwritten.
The stringify
function returns a String in JSON format representing an ECMAScript value. It can take three
parameters. The first parameter is required. The value parameter is an ECMAScript value, which is usually an
object or array, although it can also be a String, Boolean, Number or null. The optional replacer
parameter is either a function that alters the way objects and arrays are stringified, or an array of Strings and Numbers
that acts as a white list for selecting the object properties that will be stringified. The optional space
parameter is a String or Number that allows the result to have white space injected into it to improve human
readability.
These are the steps in stringifying an object:
"Array"
, then
"String"
or "Number"
then
let item be ToString(v).new Object()
, where
Object
is the standard built-in constructor with that name.The abstract operation Str(key, holder) has access
to ReplacerFunction from the invocation of the stringify
method. Its algorithm is as
follows:
"toJSON"
."Number"
then,
"String"
then,
"Boolean"
then,
"null"
."true"
."false"
."null"
."Array"
then
The abstract operation Quote(value) wraps a String value in double quotes and escapes characters within it.
backspace | "b" |
formfeed | "f" |
newline | "n" |
carriage return | "r" |
tab | "t" |
"u"
.The abstract operation JO(value) serializes an object. It has access to the stack, indent, gap, PropertyList, ReplacerFunction, and space of the invocation of the stringify method.
"{}"
."}"
."{"
, the line feed character, indent,
properties, the line feed character, stepback, and "}
".The abstract operation JA(value) serializes an array. It
has access to the stack, indent, gap, and space of the invocation of the
stringify method. The representation of arrays includes only the elements between zero and array.length
– 1 inclusive. Named properties are excluded from the stringification. An
array is stringified as an open left bracket, elements separated by comma, and a closing right bracket.
"length"
."null"
to partial."[]"
."]"
."["
, the line feed character, indent,
properties, the line feed character, stepback, and "]
".NOTE 1 JSON structures are allowed to be nested to any depth, but they must be acyclic. If value is or contains a cyclic structure, then the stringify function must throw a TypeError exception. This is an example of a value that cannot be stringified:
a = [];
a[0] = a;
my_text = JSON.stringify(a); // This must throw an TypeError.
NOTE 2 Symbolic primitive values are rendered as follows:
NOTE 3 String values are wrapped in double quotes. The characters "
and \
are escaped with \
prefixes. Control characters are replaced with escape sequences \u
HHHH, or with the shorter forms, \b
(backspace), \f
(formfeed), \n
(newline), \r
(carriage return), \t
(tab).
NOTE 4 Finite numbers are stringified as if by calling ToString(number). NaN and Infinity regardless of sign are represented as the String null
.
NOTE 5 Values that do not have a JSON representation (such as undefined and functions) do not produce a String. Instead they produce the undefined value. In arrays these values are represented as the String null
. In objects an unrepresentable value causes the property to be excluded from stringification.
NOTE 6 An object is rendered as an opening left brace followed by zero or more properties, separated with commas, closed with a right brace. A property is a quoted String representing the key or property name, a colon, and then the stringified property value. An array is rendered as an opening left bracket followed by zero or more values, separated with commas, closed with a right bracket.