ECMA-262 (12th Edition) ECMAScript 2021 Language Specification - page 30

%Object%

is the initial value of the

"Object"

performs a type conversion when called as a function rather than as a

OrdinaryCreateFromConstructor

is designed to be subclassable. It may be used as the value of an

extends

clause of a class definition.

When the

Object

function is called with optional argument

value

, the following steps are taken:

1. If NewTarget is neither

undefined

nor the active function, then

a. Return ?

(NewTarget,

"%Object.prototype%"

2. If

value

undefined

null

, return !

3. Return !

(

value

The

"length"

property of the

Object

function is

𝔽

The Object

has a [[Prototype]] internal slot whose value is

%Function.prototype%

has a

"length"

property.

has the following additional properties:

The

assign

function is used to copy the values of all of the enumerable own properties from one or more source

objects to a

target

object. When the

assign

function is called, the following steps are taken:

1. Let

be ?

(

target

2. If only one argument was passed, return

3. For each element

nextSource

sources

, do

a. If

nextSource

is neither

undefined

nor

null

, then

i. Let

from

be !

nextSource

ii. Let

keys

be ?

from

.[[OwnPropertyKeys]]().

iii. For each element

nextKey

keys

, do

1. Let

desc

be ?

from

.[[GetOwnProperty]](

nextKey

2. If

desc

is not

undefined

and

desc

.[[Enumerable]] is

true

, then

a. Let

propValue

from

nextKey

b. Perform ?

Set

(

nextKey

propValue

true

4. Return

20.1 Object Objects

20.1.1 The Object Constructor

20.1.1.1 Object ( [

value

] )

20.1.2 Properties of the Object Constructor

20.1.2.1 Object.assign (

target

, ...

sources

)

480

The

"length"

property of the

assign

function is

𝔽

The

create

function creates a new object with a specified prototype. When the

create

function is called, the

following steps are taken:

1. If

(

) is neither Object nor Null, throw a

TypeError

exception.

2. Let

obj

be !

(

3. If

Properties

is not

undefined

, then

a. Return ?

ObjectDefineProperties

(

obj

Properties

4. Return

obj

The

defineProperties

function is used to add own properties and/or update the attributes of existing own

properties of an object. When the

defineProperties

function is called, the following steps are taken:

1. If

(

) is not Object, throw a

TypeError

exception.

ObjectDefineProperties

(

Properties

The abstract operation ObjectDefineProperties takes arguments

and

Properties

. It performs the following steps when

called:

) is Object.

2. Let

props

Properties

3. Let

keys

be ?

props

.[[OwnPropertyKeys]]().

4. Let

descriptors

be a new empty

5. For each element

nextKey

keys

, do

a. Let

propDesc

be ?

props

.[[GetOwnProperty]](

nextKey

b. If

propDesc

is not

undefined

and

propDesc

.[[Enumerable]] is

true

, then

i. Let

descObj

be ?

(

props

nextKey

ii. Let

desc

be ?

ToPropertyDescriptor

(

descObj

iii. Append the pair (a two element

) consisting of

nextKey

and

desc

to the end of

descriptors

6. For each element

pair

descriptors

, do

a. Let

be the first element of

pair

b. Let

desc

be the second element of

pair

c. Perform ?

(

desc

7. Return

The

defineProperty

function is used to add an own property and/or update the attributes of an existing own

property of an object. When the

defineProperty

function is called, the following steps are taken:

1. If

(

) is not Object, throw a

TypeError

exception.

2. Let

key

be ?

(

20.1.2.2 Object.create (

Properties

)

20.1.2.3 Object.defineProperties (

Properties

)

20.1.2.3.1 ObjectDefineProperties (

Properties

)

20.1.2.4 Object.defineProperty (

Attributes

)

481

3. Let

desc

ToPropertyDescriptor

(

Attributes

4. Perform ?

(

key

desc

5. Return

When the

entries

function is called with argument

, the following steps are taken:

1. Let

obj

EnumerableOwnPropertyNames

2. Let

nameList

be ?

(

obj

key+value

3. Return

nameList

When the

freeze

function is called, the following steps are taken:

1. If

(

) is not Object, return

2. Let

status

be ?

SetIntegrityLevel

(

frozen

3. If

status

false

, throw a

TypeError

exception.

4. Return

When the

fromEntries

method is called with argument

iterable

, the following steps are taken:

1. Perform ?

RequireObjectCoercible

(

iterable

2. Let

obj

be !

(

%Object.prototype%

CreateDataPropertyOnObject Functions

obj

is an extensible

ordinary object

with no own properties.

4. Let

stepsDefine

be the algorithm steps defined in

5. Let

lengthDefine

be the number of non-optional parameters of the function definition in

CreateDataPropertyOnObject Functions

6. Let

adder

be !

CreateBuiltinFunction

stepsDefine

lengthDefine

, « »).

7. Return ?

AddEntriesFromIterable

(

obj

iterable

adder

NOTE

A CreateDataPropertyOnObject function is an anonymous built-in function. When a CreateDataPropertyOnObject
function is called with arguments

key

and

value

, the following steps are taken:

1. Let

be the

this

value.

) is Object.

is an extensible

4. Let

propertyKey

CreateDataPropertyOrThrow

(

key

5. Perform !

(

propertyKey

value

6. Return

undefined

The function created for

adder

is never directly accessible to ECMAScript code.

20.1.2.5 Object.entries (

)

20.1.2.6 Object.freeze (

)

20.1.2.7 Object.fromEntries (

iterable

)

20.1.2.7.1 CreateDataPropertyOnObject Functions

482

When the

getOwnPropertyDescriptor

function is called, the following steps are taken:

1. Let

obj

2. Let

key

be ?

(

3. Let

desc

be ?

obj

.[[GetOwnProperty]](

key

4. Return

FromPropertyDescriptor

desc

When the

getOwnPropertyDescriptors

function is called, the following steps are taken:

1. Let

obj

2. Let

ownKeys

be ?

obj

.[[OwnPropertyKeys]]().

3. Let

descriptors

be !

CreateDataPropertyOrThrow

%Object.prototype%

4. For each element

key

ownKeys

, do

a. Let

desc

be ?

obj

.[[GetOwnProperty]](

key

b. Let

descriptor

be !

FromPropertyDescriptor

(

desc

c. If

descriptor

is not

undefined

, perform !

descriptors

key

descriptor

5. Return

descriptors

When the

getOwnPropertyNames

function is called, the following steps are taken:

1. Return ?

GetOwnPropertyKeys

string

When the

getOwnPropertySymbols

function is called with argument

, the following steps are taken:

1. Return ?

GetOwnPropertyKeys

symbol

The abstract operation GetOwnPropertyKeys takes arguments

and

type

(either

string

symbol

). It performs the

following steps when called:

1. Let

obj

2. Let

keys

be ?

obj

.[[OwnPropertyKeys]]().

3. Let

nameList

be a new empty

4. For each element

nextKey

keys

, do

a. If

(

nextKey

) is Symbol and

type

symbol

(

nextKey

) is String and

type

string

, then

i. Append

nextKey

as the last element of

nameList

5. Return

nameList

When the

getPrototypeOf

function is called with argument

, the following steps are taken:

20.1.2.8 Object.getOwnPropertyDescriptor (

)

20.1.2.9 Object.getOwnPropertyDescriptors (

)

20.1.2.10 Object.getOwnPropertyNames (

)

20.1.2.11 Object.getOwnPropertySymbols (

)

20.1.2.11.1 GetOwnPropertyKeys (

type

)

20.1.2.12 Object.getPrototypeOf (

)

483

1. Let

obj

2. Return ?

obj

.[[GetPrototypeOf]]().

When the

function is called with arguments

value1

and

value2

, the following steps are taken:

1. Return

SameValue

value1

value2

When the

isExtensible

function is called with argument

, the following steps are taken:

1. If

(

) is not Object, return

false

IsExtensible

(

When the

isFrozen

function is called with argument

, the following steps are taken:

1. If

(

) is not Object, return

true

TestIntegrityLevel

frozen

When the

isSealed

function is called with argument

, the following steps are taken:

1. If

(

) is not Object, return

true

TestIntegrityLevel

sealed

When the

keys

function is called with argument

, the following steps are taken:

1. Let

obj

EnumerableOwnPropertyNames

2. Let

nameList

be ?

(

obj

key

3. Return

nameList

When the

preventExtensions

function is called, the following steps are taken:

1. If

(

) is not Object, return

2. Let

status

be ?

.[[PreventExtensions]]().

3. If

status

false

, throw a

TypeError

exception.

4. Return

The initial value of

Object.prototype

is the

Object prototype object

20.1.2.13 Object.is (

value1

value2

)

20.1.2.14 Object.isExtensible (

)

20.1.2.15 Object.isFrozen (

)

20.1.2.16 Object.isSealed (

)

20.1.2.17 Object.keys (

)

20.1.2.18 Object.preventExtensions (

)

20.1.2.19 Object.prototype

484

This property has the attributes { [[Writable]]:

false

, [[Enumerable]]:

false

, [[Configurable]]:

false

When the

seal

function is called, the following steps are taken:

1. If

(

) is not Object, return

2. Let

status

be ?

SetIntegrityLevel

(

sealed

3. If

status

false

, throw a

TypeError

exception.

4. Return

When the

setPrototypeOf

function is called with arguments

and

proto

, the following steps are taken:

1. Set

to ?

RequireObjectCoercible

(

2. If

(

proto

) is neither Object nor Null, throw a

TypeError

exception.

3. If

(

) is not Object, return

4. Let

status

be ?

.[[SetPrototypeOf]](

proto

5. If

status

false

, throw a

TypeError

exception.

6. Return

When the

values

function is called with argument

, the following steps are taken:

1. Let

obj

EnumerableOwnPropertyNames

2. Let

nameList

be ?

(

obj

value

3. Return

immutable prototype exotic object

nameList

The

Object prototype object

%Object.prototype%

has an [[Extensible]] internal slot whose value is

true

has the internal methods defined for ordinary objects, except for the [[SetPrototypeOf]] method, which is as
defined in

10.4.7.1

. (Thus, it is an

has a [[Prototype]] internal slot whose value is

null

The initial value of

Object.prototype.constructor

%Object%

When the

hasOwnProperty

method is called with argument

, the following steps are taken:

1. Let

20.1.2.20 Object.seal (

)

20.1.2.21 Object.setPrototypeOf (

proto

)

20.1.2.22 Object.values (

)

20.1.3 Properties of the Object Prototype Object

20.1.3.1 Object.prototype.constructor

20.1.3.2 Object.prototype.hasOwnProperty (

)

485

2. Let

be ?

this

value).

3. Return ?

HasOwnProperty

NOTE

When the

isPrototypeOf

method is called with argument

, the following steps are taken:

1. If

(

) is not Object, return

false

2. Let

be ?

this

value).

3. Repeat,

a. Set

to ?

.[[GetPrototypeOf]]().

b. If

null

, return

false

c. If

SameValue

) is

true

, return

true

NOTE

When the

propertyIsEnumerable

method is called with argument

, the following steps are taken:

1. Let

2. Let

be ?

this

value).

3. Let

desc

be ?

.[[GetOwnProperty]](

4. If

desc

undefined

, return

false

5. Return

desc

.[[Enumerable]].

NOTE 1

NOTE 2

When the

toLocaleString

method is called, the following steps are taken:

1. Let

be the

this

value.

Invoke

(

"toString"

The optional parameters to this function are not used but are intended to correspond to the parameter pattern used by
ECMA-402

toLocaleString

functions. Implementations that do not include ECMA-402 support must not use

The ordering of steps

is chosen to ensure that any exception that would have been

thrown by step

in previous editions of this specification will continue to be thrown even if the

this

value is

undefined

null

The ordering of steps

preserves the behaviour specified by previous editions of this

specification for the case where

is not an object and the

this

value is

undefined

null

This method does not consider objects in the prototype chain.

The ordering of steps

is chosen to ensure that any exception that would have been

thrown by step

in previous editions of this specification will continue to be thrown even if the

this

value is

undefined

null

20.1.3.3 Object.prototype.isPrototypeOf (

)

20.1.3.4 Object.prototype.propertyIsEnumerable (

)

20.1.3.5 Object.prototype.toLocaleString ( [

reserved1

[ ,

reserved2

] ] )

486

those parameter positions for other purposes.

NOTE 1

NOTE 2

When the

toString

method is called, the following steps are taken:

1. If the

this

value is

undefined

, return

"[object Undefined]"

2. If the

this

value is

null

, return

"[object Null]"

3. Let

be !

this

value).

4. Let

isArray

IsArray

(

5. If

isArray

true

, let

builtinTag

"Array"

6. Else if

has a [[ParameterMap]] internal slot, let

builtinTag

"Arguments"

7. Else if

has a [[Call]] internal method, let

builtinTag

"Function"

8. Else if

has an [[ErrorData]] internal slot, let

builtinTag

"Error"

9. Else if

has a [[BooleanData]] internal slot, let

builtinTag

"Boolean"

10. Else if

has a [[NumberData]] internal slot, let

builtinTag

"Number"

11. Else if

has a [[StringData]] internal slot, let

builtinTag

"String"

12. Else if

has a [[DateValue]] internal slot, let

builtinTag

"Date"

13. Else if

has a [[RegExpMatcher]] internal slot, let

builtinTag

"RegExp"

14. Else, let

builtinTag

"Object"

15. Let

tag

be ?

(

@@toStringTag

16. If

(

tag

) is not String, set

tag

builtinTag

17. Return the

"[object "

tag

, and

"]"

NOTE

When the

valueOf

method is called, the following steps are taken:

1. Return ?

(

this

value).

This function provides a generic

toLocaleString

implementation for objects that have no

locale-specific

toString

behaviour.

Array

Number

Date

, and

%TypedArray%

provide

their own locale-sensitive

toLocaleString

methods.

ECMA-402 intentionally does not provide an alternative to this default implementation.

Historically, this function was occasionally used to access the String value of the [[Class]] internal
slot that was used in previous editions of this specification as a nominal type tag for various
built-in objects. The above definition of

toString

preserves compatibility for legacy code that

uses

toString

as a test for those specific kinds of built-in objects. It does not provide a reliable

type testing mechanism for other kinds of built-in or program defined objects. In addition,
programs can use

@@toStringTag

in ways that will invalidate the reliability of such legacy type

tests.

20.1.3.6 Object.prototype.toString ( )

20.1.3.7 Object.prototype.valueOf ( )

20.1.4 Properties of Object Instances

487

Object instances have no special properties beyond those inherited from the

Object prototype object

The Function

creates and initializes a new

%Function%

is the initial value of the

"Function"

property of the

global object

when called as a function rather than as a

. Thus the

function call

Function(…)

is equivalent to the object creation expression

new Function(…)

with the

same arguments.
is designed to be subclassable. It may be used as the value of an

extends

clause of a class definition. Subclass

constructors that intend to inherit the specified Function behaviour must include a

super

call to the Function

to create and initialize a subclass instance with the internal slots necessary for built-in function

behaviour. All ECMAScript syntactic forms for defining function objects create instances of Function. There is
no syntactic means to create instances of Function subclasses except for the built-in GeneratorFunction,
AsyncFunction, and AsyncGeneratorFunction subclasses.

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

When the

Function

function is called with some arguments

, … ,

body

(where

might be 0, that is, there

are no “

” arguments, and where

body

might also not be provided), the following steps are taken:

1. Let

be the

active function object

2. Let

args

be the

argumentsList

that was passed to this function by [[Call]] or [[Construct]].

3. Return ?

CreateDynamicFunction

(

, NewTarget,

normal

args

NOTE

The abstract operation CreateDynamicFunction takes arguments

constructor

newTarget

kind

(either

normal

generator

async

, or

asyncGenerator

), and

args

of ECMAScript language values).

constructor

is the

function that is performing this action.

newTarget

is the

that

new

was initially applied to.

args

is the argument values that were passed to

constructor

. It performs the following steps when called:

has at least two elements.

: The

execution context stack

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"

)

20.2 Function Objects

20.2.1 The Function Constructor

20.2.1.1 Function (

, … ,

body

)

20.2.1.1.1 CreateDynamicFunction (

constructor

newTarget

kind

args

)

488

2. Let

callerContext

be the second to top element of the

execution context stack

3. Let

callerRealm

callerContext

Realm

4. Let

calleeRealm

the current Realm Record

5. Perform ?

HostEnsureCanCompileStrings

(

callerRealm

calleeRealm

6. If

newTarget

undefined

, set

newTarget

constructor

7. If

kind

normal

, then

a. Let

goal

FunctionBody

[~Yield, ~Await]

b. Let

parameterGoal

be the grammar symbol

[~Yield, ~Await]

c. Let

fallbackProto

"%Function.prototype%"

8. Else if

kind

generator

, then

a. Let

goal

GeneratorBody

b. Let

parameterGoal

be the grammar symbol

[+Yield, ~Await]

c. Let

fallbackProto

"%GeneratorFunction.prototype%"

9. Else if

kind

async

, then

a. Let

goal

AsyncFunctionBody

b. Let

parameterGoal

be the grammar symbol

[~Yield, +Await]

c. Let

fallbackProto

"%AsyncFunction.prototype%"

10. Else,

kind

asyncGenerator

b. Let

goal

AsyncGeneratorBody

c. Let

parameterGoal

be the grammar symbol

[+Yield, +Await]

d. Let

fallbackProto

"%AsyncGeneratorFunction.prototype%"

11. Let

argCount

be the number of elements in

args

12. Let

be the empty String.

13. If

argCount

= 0, let

bodyArg

be the empty String.

14. Else if

argCount

= 1, let

bodyArg

args

[0].

15. Else,

argCount

> 1.

b. Let

firstArg

args

[0].

c. Set

to ?

ToString

firstArg

d. Let

be 1.

e. Repeat, while

argCount

- 1,

i. Let

nextArg

args

[

ii. Let

nextArgString

ToString

(

nextArg

iii. Set

to the

","

(a comma), and

nextArgString

iv. Set

+ 1.

f. Let

bodyArg

args

[

16. Let

bodyString

be the

of 0x000A (LINE FEED), ?

ToString

(

bodyArg

), and 0x000A (LINE

FEED).

17. Let

prefix

be the prefix associated with

kind

Table 50

18. Let

sourceString

be the

prefix

" anonymous("

, 0x000A (LINE FEED),

") {"

bodyString

and

"}"

19. Let

sourceText

be !

StringToCodePoints

(

sourceString

20. Perform the following substeps in an

implementation-defined

order, possibly interleaving parsing and error

detection:

a. Let

parameters

ParseText

StringToCodePoints

(

parameterGoal

b. If

parameters

is a

of errors, throw a

SyntaxError

exception.

489

c. Let

body

ParseText

StringToCodePoints

(

bodyString

goal

d. If

body

is a

FunctionBodyContainsUseStrict

of errors, throw a

SyntaxError

exception.

e. Let

strict

body

f. If

strict

true

, apply the

early error

rules for

UniqueFormalParameters

parameters

g. If

strict

true

IsSimpleParameterList

parameters

false

, throw a

SyntaxError

exception.

h. If any element of the

BoundNames

parameters

also occurs in the

LexicallyDeclaredNames

body

throw a

SyntaxError

exception.

i. If

body

SuperCall

true

, throw a

SyntaxError

exception.

j. If

parameters

SuperCall

true

, throw a

SyntaxError

exception.

k. If

body

SuperProperty

true

, throw a

SyntaxError

exception.

l. If

parameters

SuperProperty

true

, throw a

SyntaxError

exception.

m. If

kind

generator

asyncGenerator

, then

i. If

parameters

YieldExpression

true

, throw a

SyntaxError

exception.

n. If

kind

async

asyncGenerator

, then

i. If

parameters

AwaitExpression

true

, throw a

SyntaxError

exception.

o. If

strict

true

, then

i. If

BoundNames

parameters

contains any duplicate elements, throw a

SyntaxError

exception.

21. Let

proto

GetPrototypeFromConstructor

(

newTarget

fallbackProto

22. Let

realmF

the current Realm Record

23. Let

scope

realmF

.[[GlobalEnv]].

24. Let

be !

OrdinaryFunctionCreate

proto

sourceText

parameters

body

non-lexical-this

scope

25. Perform

SetFunctionName

"anonymous"

26. If

kind

generator

, then

a. Let

prototype

be !

%GeneratorFunction.prototype.prototype%

(

b. Perform

(

"prototype"

, PropertyDescriptor { [[Value]]:

prototype

, [[Writable]]:

true

, [[Enumerable]]:

false

, [[Configurable]]:

false

}).

27. Else if

kind

asyncGenerator

, then

a. Let

prototype

be !

%AsyncGeneratorFunction.prototype.prototype%

(

b. Perform

(

"prototype"

, PropertyDescriptor { [[Value]]:

prototype

, [[Writable]]:

true

, [[Enumerable]]:

false

, [[Configurable]]:

false

}).

28. Else if

kind

normal

, perform

MakeConstructor

to provide for the possibility that the function will be used as a

29. NOTE: Functions whose

kind

async

are not constructible and do not have a [[Construct]] internal method or

"prototype"

property.

30. Return

NOTE

Table 50: Dynamic Function SourceText Prefixes

Kind

Prefix

normal

"function"

generator

"function*"

async

"async function"

asyncGenerator

"async function*"

CreateDynamicFunction defines a

"prototype"

property on any function it creates whose

kind

not

async

490

has a [[Prototype]] internal slot whose value is

%Function.prototype%

has the following properties:

This is a

data property

with a value of 1. This property has the attributes { [[Writable]]:

false

, [[Enumerable]]:

false

[[Configurable]]:

true

The value of

Function.prototype

is the

Function prototype object

This property has the attributes { [[Writable]]:

false

, [[Enumerable]]:

false

, [[Configurable]]:

false

The

Function prototype object

%Function.prototype%

is itself a built-in

accepts any arguments and returns

undefined

when invoked.

does not have a [[Construct]] internal method; it cannot be used as a

with the

new

operator.

has a [[Prototype]] internal slot whose value is

%Object.prototype%

does not have a

"prototype"

property.

has a

"length"

property whose value is

𝔽

has a

"name"

property whose value is the empty String.

NOTE

When the

apply

method is called with arguments

thisArg

and

argArray

, the following steps are taken:

1. Let

func

be the

this

value.

2. If

func

) is

false

, throw a

TypeError

exception.

3. If

argArray

undefined

null

, then

a. Perform

PrepareForTailCall

().

b. Return ?

Call

(

func

thisArg

4. Let

argList

be ?

CreateListFromArrayLike

(

argArray

5. Perform

PrepareForTailCall

().

6. Return ?

Call

(

func

thisArg

argList

The Function prototype object is specified to be a

to ensure compatibility with

ECMAScript code that was created prior to the ECMAScript 2015 specification.

20.2.2 Properties of the Function Constructor

20.2.2.1 Function.length

20.2.2.2 Function.prototype

20.2.3 Properties of the Function Prototype Object

20.2.3.1 Function.prototype.apply (

thisArg

argArray

)

491

NOTE 1

NOTE 2

When the

bind

method is called with argument

thisArg

and zero or more

args

, it performs the following steps:

1. Let

Target

be the

this

value.

2. If

Target

) is

false

, throw a

TypeError

exception.

3. Let

BoundFunctionCreate

Target

thisArg

args

4. Let

be 0.

5. Let

targetHasLength

HasOwnProperty

(

Target

"length"

6. If

targetHasLength

true

, then

a. Let

targetLen

Target

"length"

b. If

(

targetLen

) is Number, then

i. If

targetLen

∞

𝔽

, set

to +

∞

ii. Else if

targetLen

∞

𝔽

, set

to 0.

iii. Else,

1. Let

targetLenAsInt

be !

ToIntegerOrInfinity

(

targetLen

targetLenAsInt

is finite.

3. Let

argCount

be the number of elements in

args

4. Set

max

(

targetLenAsInt

argCount

, 0).

7. Perform !

SetFunctionLength

8. Let

targetName

be ?

(

Target

"name"

9. If

(

targetName

) is not String, set

targetName

to the empty String.

10. Perform

SetFunctionName

bound function exotic object

targetName

"bound"

11. Return

NOTE 1

NOTE 2

When the

call

method is called with argument

thisArg

and zero or more

args

, the following steps are taken:

1. Let

func

be the

this

value.

The

thisArg

value is passed without modification as the

this

value. This is a change from Edition

3, where an

undefined

null

thisArg

to all other values and that result is passed as the

this

value. Even though the

thisArg

is passed

without modification, non-strict functions still perform these transformations upon entry to the
function.

func

is an arrow function or a

then the

thisArg

will be ignored by

the function [[Call]] in step

Function objects created using

Function.prototype.bind

are exotic objects. They also do

not have a

"prototype"

property.

Target

is an arrow function or a

bound function exotic object

then the

thisArg

passed to this

method will not be used by subsequent calls to

20.2.3.2 Function.prototype.bind (

thisArg

, ...

args

)

20.2.3.3 Function.prototype.call (

thisArg

, ...

args

)

492

2. If

func

) is

false

, throw a

TypeError

exception.

3. Perform

PrepareForTailCall

().

4. Return ?

Call

(

func

thisArg

args

NOTE 1

NOTE 2

The initial value of

Function.prototype.constructor

%Function%

When the

toString

method is called, the following steps are taken:

1. Let

func

be the

this

value.

2. If

Unicode code points and !

(

func

) is Object and

func

has a [[SourceText]] internal slot and

func

.[[SourceText]] is a sequence of

HostHasSourceTextAvailable

func

) is

true

, then

a. Return !

CodePointsToString

func

.[[SourceText]]).

3. If

func

is a

built-in function object

, return an

implementation-defined

String source code representation of

func

The representation must have the syntax of a

NativeFunction

. Additionally, if

func

has an [[InitialName]]

internal slot and

func

.[[InitialName]] is a String, the portion of the returned String that would be matched by

NativeFunctionAccessor

opt

PropertyName

must be the value of

func

.[[InitialName]].

4. If

(

func

) is Object and

func

) is

true

, return an

implementation-defined

String source code

representation of

func

. The representation must have the syntax of a

NativeFunction

5. Throw a

TypeError

exception.

NativeFunction

function

NativeFunctionAccessor

opt

PropertyName

[~Yield, ~Await]

opt

(

[~Yield, ~Await]

)

{

[

native

code

]

}

NativeFunctionAccessor

get

set

When the

@@hasInstance

method of an object

is called with value

, the following steps are taken:

1. Let

be the

this

value.

OrdinaryHasInstance

bound function exotic object

The

thisArg

value is passed without modification as the

this

value. This is a change from Edition

3, where an

undefined

null

thisArg

to all other values and that result is passed as the

this

value. Even though the

thisArg

is passed

without modification, non-strict functions still perform these transformations upon entry to the
function.

func

is an arrow function or a

then the

thisArg

will be ignored by

the function [[Call]] in step

20.2.3.4 Function.prototype.constructor

20.2.3.5 Function.prototype.toString ( )

20.2.3.6 Function.prototype [ @@hasInstance ] (

)

493

The value of the

"name"

property of this function is

"[Symbol.hasInstance]"

This property has the attributes { [[Writable]]:

false

, [[Enumerable]]:

false

, [[Configurable]]:

false

NOTE

This property is non-writable and non-configurable to prevent tampering that could be used to globally expose the
target function of a bound function.

Every Function instance is an ECMAScript

and has the internal slots listed in

Table 29

. Function objects

created using the

Function.prototype.bind

method (

20.2.3.2

) have the internal slots listed in

Table 30

Function instances have the following properties:

The value of the

"length"

property is an

integral Number

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]]:

true

The value of the

"name"

property is a String that is descriptive of the function. The name has no semantic significance

but is typically a variable or

property name

that is used to refer to the function at its point of definition in ECMAScript

code. This property has the attributes { [[Writable]]:

false

, [[Enumerable]]:

false

, [[Configurable]]:

true

Anonymous functions objects that do not have a contextual name associated with them by this specification use the
empty String as the value of the

"name"

property.

Function instances that can be used as a

have a

"prototype"

property. Whenever such a Function instance

is created another

ordinary object

is also created and is the initial value of the function's

"prototype"

property. Unless

otherwise specified, the value of the

"prototype"

property is used to initialize the [[Prototype]] internal slot of the

object created when that function is invoked as a

This is the default implementation of

@@hasInstance

that most functions inherit.

@@hasInstance

is called by the

instanceof

operator to determine whether a value is an

instance of a specific

. An expression such as

evaluates as

function can control which objects are recognized as its instances by

instanceof

by exposing a different

@@hasInstance

method on the function.

instanceof

F[@@hasInstance](v)

20.2.4 Function Instances

20.2.4.1 length

20.2.4.2 name

20.2.4.3 prototype

494

This property has the attributes { [[Writable]]:

true

, [[Enumerable]]:

false

, [[Configurable]]:

false

NOTE

The

host-defined

abstract operation HostHasSourceTextAvailable takes argument

func

). It allows

host

environments to prevent the source text from being provided for

func

An implementation of HostHasSourceTextAvailable must complete normally in all cases. This operation must be
deterministic with respect to its parameters. Each time it is called with a specific

func

as its argument, it must return

the same completion record. The default implementation of HostHasSourceTextAvailable is to unconditionally return
a normal completion with a value of

true

The Boolean