ECMA-262 (12th Edition) ECMAScript 2021 Language Specification - page 22

8.  Set 

obj

.[[BoundTargetFunction]] to 

targetFunction

.

9.  Set 

obj

.[[BoundThis]] to 

boundThis

.

10.  Set 

obj

.[[BoundArguments]] to 

boundArgs

.

11.  Return 

obj

.

An Array object is an 

exotic object

 that gives special treatment to 

array index

 property keys (see 

6.1.7

). A property

whose 

property name

 is an 

array index

 is also called an 

element

. Every Array object has a non-configurable 

"length"

property whose value is always a non-negative 

integral Number

 whose 

mathematical value

 is less than 2

32

. The value

of the 

"length"

 property is numerically greater than the name of every own property whose name is an 

array index

;

whenever an own property of an Array object is created or changed, other properties are adjusted as necessary to
maintain this invariant. Specifically, whenever an own property is added whose name is an 

array index

, the value of

the 

"length"

 property is changed, if necessary, to be one more than the numeric value of that 

array index

; and

whenever the value of the 

"length"

 property is changed, every own property whose name is an 

array index

 whose

value is not smaller than the new length is 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.

NOTE

An object is an 

Array exotic object

 (or simply, an Array object) if its [[DefineOwnProperty]] internal method uses the

following implementation, and its other essential internal methods use the definitions found in 

10.1

. These methods

are installed in 

ArrayCreate

.

The [[DefineOwnProperty]] internal method of an 

Array exotic object

 

A

 takes arguments 

P

 (a property key) and 

Desc

(a 

Property Descriptor

). It performs the following steps when called:

1. 

Assert

: 

IsPropertyKey

(

P

) is 

true

.

2.  If 

P

 is 

"length"

, then

a.  Return ? 

ArraySetLength

(

A

, 

Desc

).

3.  Else if 

P

 is an 

array index

, then

a.  Let 

oldLenDesc

 be 

OrdinaryGetOwnProperty

(

A

, 

"length"

).

b. 

Assert

: ! 

IsDataDescriptor

(

oldLenDesc

) is 

true

.

c. 

Assert

: 

oldLenDesc

.[[Configurable]] is 

false

.

d.  Let 

oldLen

 be 

oldLenDesc

.[[Value]].

e. 

Assert

: 

oldLen

 is a non-negative 

integral Number

.

f.  Let 

index

 be ! 

ToUint32

(

P

).

g.  If 

index

 

≥

 

oldLen

 and 

oldLenDesc

.[[Writable]] is 

false

, return 

false

.

h.  Let 

succeeded

 be ! 

OrdinaryDefineOwnProperty

(

A

, 

P

, 

Desc

).

i.  If 

succeeded

 is 

false

, return 

false

.

j.  If 

index

 

≥

 

oldLen

, then

i.  Set 

oldLenDesc

.[[Value]] to 

index

 + 

1

𝔽

.

ii.  Let 

succeeded

 be 

OrdinaryDefineOwnProperty

(

A

, 

"length"

, 

oldLenDesc

).

iii. 

Assert

: 

succeeded

 is 

true

.

A String 

property name

 

P

 is an 

array index

 if and only if 

ToString

(

ToUint32

(

P

)) equals 

P

 and

ToUint32

(

P

) is not the same value as 

(2

32

 - 1).

10.4.2  Array Exotic Objects

10.4.2.1  [[DefineOwnProperty]] ( 

P

, 

Desc

 )

243

k.  Return 

true

.

4.  Return 

OrdinaryDefineOwnProperty

(

A

, 

P

, 

Desc

).

The abstract operation ArrayCreate takes argument 

length

 (a non-negative 

integer

) and optional argument 

proto

. It is

used to specify the creation of new Array exotic objects. It performs the following steps when called:

1.  If 

length

 > 2

32

 - 1, throw a 

RangeError

 exception.

2.  If 

proto

 is not present, set 

proto

 to 

%Array.prototype%

.

3.  Let 

A

 be ! 

MakeBasicObject

(« [[Prototype]], [[Extensible]] »).

4.  Set 

A

.[[Prototype]] to 

proto

.

5.  Set 

A

.[[DefineOwnProperty]] as specified in 

10.4.2.1

.

6.  Perform ! 

OrdinaryDefineOwnProperty

(

A

, 

"length"

, PropertyDescriptor { [[Value]]: 

(

length

), [[Writable]]: 

true

,

[[Enumerable]]: 

false

, [[Configurable]]: 

false

 }).

7.  Return 

A

.

The abstract operation ArraySpeciesCreate takes arguments 

originalArray

 and 

length

 (a non-negative 

integer

). It is

used to specify the creation of a new Array object using a 

constructor

 function that is derived from 

originalArray

. It

performs the following steps when called:

1.  Let 

isArray

 be ? 

IsArray

(

originalArray

).

2.  If 

isArray

 is 

false

, return ? 

ArrayCreate

(

length

).

3.  Let 

C

 be ? 

Get

(

originalArray

, 

"constructor"

).

4.  If 

IsConstructor

(

C

) is 

true

, then

a.  Let 

thisRealm

 be 

the current Realm Record

.

b.  Let 

realmC

 be ? 

GetFunctionRealm

(

C

).

c.  If 

thisRealm

 and 

realmC

 are not the same 

Realm Record

, then

i.  If 

SameValue

(

C

, 

realmC

.[[Intrinsics]].[[

%Array%

]]) is 

true

, set 

C

 to 

undefined

.

5.  If 

Type

(

C

) is Object, then

a.  Set 

C

 to ? 

Get

(

C

, 

@@species

).

b.  If 

C

 is 

null

, set 

C

 to 

undefined

.

6.  If 

C

 is 

undefined

, return ? 

ArrayCreate

(

length

).

7.  If 

IsConstructor

(

C

) is 

false

, throw a 

TypeError

 exception.

8.  Return ? 

Construct

(

C

length

) »).

NOTE

The abstract operation ArraySetLength takes arguments 

A

 (an Array object) and 

Desc

 (a 

Property Descriptor

). It

performs the following steps when called:

If 

originalArray

 was created using the standard built-in Array 

constructor

 for a 

realm

 that is not

the 

realm

 of the 

running execution context

, then a new Array is created using the 

realm

 of the

running execution context

. This maintains compatibility with Web browsers that have historically

had that behaviour for the 

Array.prototype

Array.prototype

 methods that now are defined using

ArraySpeciesCreate.

10.4.2.2  ArrayCreate ( 

length

 [ , 

proto

 ] )

10.4.2.3  ArraySpeciesCreate ( 

originalArray

, 

length

 )

10.4.2.4  ArraySetLength ( 

A

, 

Desc

 )

244

1.  If 

Desc

.[[Value]] is absent, then

a.  Return 

OrdinaryDefineOwnProperty

(

A

, 

"length"

, 

Desc

).

2.  Let 

newLenDesc

 be a copy of 

Desc

.

3.  Let 

newLen

 be ? 

ToUint32

(

Desc

.[[Value]]).

4.  Let 

numberLen

 be ? 

ToNumber

(

Desc

.[[Value]]).

5.  If 

newLen

 is not the same value as 

numberLen

, throw a 

RangeError

 exception.

6.  Set 

newLenDesc

.[[Value]] to 

newLen

.

7.  Let 

oldLenDesc

 be 

OrdinaryGetOwnProperty

(

A

, 

"length"

).

8. 

Assert

: ! 

IsDataDescriptor

(

oldLenDesc

) is 

true

.

9. 

Assert

: 

oldLenDesc

.[[Configurable]] is 

false

.

10.  Let 

oldLen

 be 

oldLenDesc

.[[Value]].

11.  If 

newLen

 

≥

 

oldLen

, then

a.  Return 

OrdinaryDefineOwnProperty

(

A

, 

"length"

, 

newLenDesc

).

12.  If 

oldLenDesc

.[[Writable]] is 

false

, return 

false

.

13.  If 

newLenDesc

.[[Writable]] is absent or has the value 

true

, let 

newWritable

 be 

true

.

14.  Else,

a.  NOTE: Setting the [[Writable]] attribute to 

false

 is deferred in case any elements cannot be deleted.

b.  Let 

newWritable

 be 

false

.

c.  Set 

newLenDesc

.[[Writable]] to 

true

.

15.  Let 

succeeded

 be ! 

OrdinaryDefineOwnProperty

(

A

, 

"length"

, 

newLenDesc

).

16.  If 

succeeded

 is 

false

, return 

false

.

17.  For each own property key 

P

 of 

A

 that is an 

array index

, whose numeric value is greater than or equal to

newLen

, in descending numeric index order, do

a.  Let 

deleteSucceeded

 be ! 

A

.[[Delete]](

P

).

b.  If 

deleteSucceeded

 is 

false

, then

i.  Set 

newLenDesc

.[[Value]] to ! 

ToUint32

(

P

) + 

1

𝔽

.

ii.  If 

newWritable

 is 

false

, set 

newLenDesc

.[[Writable]] to 

false

.

iii.  Perform ! 

OrdinaryDefineOwnProperty

(

A

, 

"length"

, 

newLenDesc

).

iv.  Return 

false

.

18.  If 

newWritable

 is 

false

, then

a.  Let 

succeeded

 be ! 

OrdinaryDefineOwnProperty

(

A

, 

"length"

, PropertyDescriptor { [[Writable]]: 

false

 }).

b. 

Assert

: 

succeeded

 is 

true

.

19.  Return 

true

.

NOTE

A String object is an 

exotic object

 that encapsulates a String value and exposes virtual 

integer

-indexed data properties

corresponding to the individual code unit elements of the String value. String exotic objects always have a 

data

property

 named 

"length"

 whose value is the number of code unit elements in the encapsulated String value. Both the

code unit data properties and the 

"length"

 property are non-writable and non-configurable.

An object is a 

String exotic object

 (or simply, a String object) if its [[GetOwnProperty]], [[DefineOwnProperty]], and

[[OwnPropertyKeys]] internal methods use the following implementations, and its other essential internal methods

In steps 

3

 and 

4

, if 

Desc

.[[Value]] is an object then its 

valueOf

valueOf

 method is called twice. This is

legacy behaviour that was specified with this effect starting with the 2

nd

 Edition of this

specification.

10.4.3  String Exotic Objects

245

use the definitions found in 

10.1

. These methods are installed in 

StringCreate

.

String exotic objects have the same internal slots as ordinary objects. They also have a [[StringData]] internal slot.

The [[GetOwnProperty]] internal method of a 

String exotic object

 

S

 takes argument 

P

 (a property key). It performs the

following steps when called:

1. 

Assert

: 

IsPropertyKey

(

P

) is 

true

.

2.  Let 

desc

 be 

OrdinaryGetOwnProperty

(

S

, 

P

).

3.  If 

desc

 is not 

undefined

, return 

desc

.

4.  Return ! 

StringGetOwnProperty

(

S

, 

P

).

The [[DefineOwnProperty]] internal method of a 

String exotic object

 

S

 takes arguments 

P

 (a property key) and 

Desc

 (a

Property Descriptor

). It performs the following steps when called:

1. 

Assert

: 

IsPropertyKey

(

P

) is 

true

.

2.  Let 

stringDesc

 be ! 

StringGetOwnProperty

(

S

, 

P

).

3.  If 

stringDesc

 is not 

undefined

, then

a.  Let 

extensible

 be 

S

.[[Extensible]].

b.  Return ! 

IsCompatiblePropertyDescriptor

(

extensible

, 

Desc

, 

stringDesc

).

4.  Return ! 

OrdinaryDefineOwnProperty

(

S

, 

P

, 

Desc

).

The [[OwnPropertyKeys]] internal method of a 

String exotic object

 

O

 takes no arguments. It performs the following

steps when called:

1.  Let 

keys

 be a new empty 

List

.

2.  Let 

str

 be 

O

.[[StringData]].

3. 

Assert

: 

Type

(

str

) is String.

4.  Let 

len

 be the length of 

str

.

5.  For each 

integer

 

i

 starting with 0 such that 

i

 < 

len

, in ascending order, do

a.  Add ! 

ToString

(

(

i

)) as the last element of 

keys

.

6.  For each own property key 

P

 of 

O

 such that 

P

 is an 

array index

 and ! 

ToIntegerOrInfinity

(

P

) 

≥

 

len

, in ascending

numeric index order, do

a.  Add 

P

 as the last element of 

keys

.

7.  For each own property key 

P

 of 

O

 such that 

Type

(

P

) is String and 

P

 is not an 

array index

, in ascending

chronological order of property creation, do

a.  Add 

P

 as the last element of 

keys

.

8.  For each own property key 

P

 of 

O

 such that 

Type

(

P

) is Symbol, in ascending chronological order of property

creation, do

a.  Add 

P

 as the last element of 

keys

.

9.  Return 

keys

.

10.4.3.1  [[GetOwnProperty]] ( 

P

 )

10.4.3.2  [[DefineOwnProperty]] ( 

P

, 

Desc

 )

10.4.3.3  [[OwnPropertyKeys]] ( )

10.4.3.4  StringCreate ( 

value

, 

prototype

 )

246

The abstract operation StringCreate takes arguments 

value

 (a String) and 

prototype

. It is used to specify the creation of

new String exotic objects. It performs the following steps when called:

1.  Let 

S

 be ! 

MakeBasicObject

(« [[Prototype]], [[Extensible]], [[StringData]] »).

2.  Set 

S

.[[Prototype]] to 

prototype

.

3.  Set 

S

.[[StringData]] to 

value

.

4.  Set 

S

.[[GetOwnProperty]] as specified in 

10.4.3.1

.

5.  Set 

S

.[[DefineOwnProperty]] as specified in 

10.4.3.2

.

6.  Set 

S

.[[OwnPropertyKeys]] as specified in 

10.4.3.3

.

7.  Let 

length

 be the number of code unit elements in 

value

.

8.  Perform ! 

DefinePropertyOrThrow

(

S

, 

"length"

, PropertyDescriptor { [[Value]]: 

length

), [[Writable]]: 

false

,

[[Enumerable]]: 

false

, [[Configurable]]: 

false

 }).

9.  Return 

S

.

The abstract operation StringGetOwnProperty takes arguments 

S

 and 

P

. It performs the following steps when called:

1. 

Assert

: 

S

 is an Object that has a [[StringData]] internal slot.

2. 

Assert

: 

IsPropertyKey

(

P

) is 

true

.

3.  If 

Type

(

P

) is not String, return 

undefined

.

4.  Let 

index

 be ! 

CanonicalNumericIndexString

(

P

).

5.  If 

index

 is 

undefined

, return 

undefined

.

6.  If 

IsIntegralNumber

(

index

) is 

false

, return 

undefined

.

7.  If 

index

 is 

-0

𝔽

, return 

undefined

.

8.  Let 

str

 be 

S

.[[StringData]].

9. 

Assert

: 

Type

(

str

) is String.

10.  Let 

len

 be the length of 

str

.

index

) < 0 or 

len

 

≤

 

(

index

), return 

undefined

.

12.  Let 

resultStr

 be the String value of length 1, containing one code unit from 

str

, specifically the code unit at index

(

index

).

13.  Return the PropertyDescriptor { [[Value]]: 

resultStr

, [[Writable]]: 

false

, [[Enumerable]]: 

true

, [[Configurable]]:

false

 }.

Most ECMAScript functions make an arguments object available to their code. Depending upon the characteristics of
the function definition, its arguments object is either an 

ordinary object

 or an 

arguments exotic object

. An 

arguments

exotic object

 is an 

exotic object

 whose 

array index

 properties map to the formal parameters bindings of an invocation

of its associated ECMAScript function.

An object is an 

arguments exotic object

 if its internal methods use the following implementations, with the ones not

specified here using those found in 

10.1

. These methods are installed in 

CreateMappedArgumentsObject

.

NOTE 1

Arguments exotic objects have the same internal slots as ordinary objects. They also have a [[ParameterMap]] internal

While 

CreateUnmappedArgumentsObject

 is grouped into this clause, it creates an 

ordinary

object

, not an 

arguments exotic object

.

10.4.3.5  StringGetOwnProperty ( 

S

, 

P

 )

10.4.4  Arguments Exotic Objects

247

slot. Ordinary arguments objects also have a [[ParameterMap]] internal slot whose value is always undefined. For
ordinary argument objects the [[ParameterMap]] internal slot is only used by 

Object.prototype.toString

Object.prototype.toString

(

20.1.3.6

) to identify them as such.

NOTE 2

NOTE 3

NOTE 4

NOTE 5

The [[GetOwnProperty]] internal method of an 

arguments exotic object

 

args

 takes argument 

P

 (a property key). It

performs the following steps when called:

1.  Let 

desc

 be 

OrdinaryGetOwnProperty

(

args

, 

P

).

2.  If 

desc

 is 

undefined

, return 

desc

.

3.  Let 

map

 be 

args

.[[ParameterMap]].

4.  Let 

isMapped

 be ! 

HasOwnProperty

(

map

, 

P

).

5.  If 

isMapped

 is 

true

, then

a.  Set 

desc

.[[Value]] to 

Get

(

map

, 

P

).

6.  Return 

desc

.

The 

integer

-indexed data properties of an 

arguments exotic object

 whose numeric name values

are less than the number of formal parameters of the corresponding 

function object

 initially share

their values with the corresponding argument bindings in the function's 

execution context

. This

means that changing the property changes the corresponding value of the argument binding and
vice-versa. This correspondence is broken if such a property is deleted and then redefined or if
the property is changed into an 

accessor property

. If the arguments object is an 

ordinary object

,

the values of its properties are simply a copy of the arguments passed to the function and there is
no dynamic linkage between the property values and the formal parameter values.

The ParameterMap object and its property values are used as a device for specifying the
arguments object correspondence to argument bindings. The ParameterMap object and the
objects that are the values of its properties are not directly observable from ECMAScript code. An
ECMAScript implementation does not need to actually create or use such objects to implement
the specified semantics.

Ordinary arguments objects define a non-configurable 

accessor property

 named 

"callee"

 which

throws a 

TypeError

 exception on access. The 

"callee"

 property has a more specific meaning for

arguments exotic objects, which are created only for some class of non-strict functions. The
definition of this property in the ordinary variant exists to ensure that it is not defined in any
other manner by conforming ECMAScript implementations.

ECMAScript implementations of arguments exotic objects have historically contained an 

accessor

property

 named 

"caller"

. Prior to ECMAScript 2017, this specification included the definition of a

throwing 

"caller"

 property on ordinary arguments objects. Since implementations do not contain

this extension any longer, ECMAScript 2017 dropped the requirement for a throwing 

"caller"

accessor.

10.4.4.1  [[GetOwnProperty]] ( 

P

 )

10.4.4.2  [[DefineOwnProperty]] ( 

P

, 

Desc

 )

248

The [[DefineOwnProperty]] internal method of an 

arguments exotic object

 

args

 takes arguments 

P

 (a property key)

and 

Desc

 (a 

Property Descriptor

). It performs the following steps when called:

1.  Let 

map

 be 

args

.[[ParameterMap]].

2.  Let 

isMapped

 be 

HasOwnProperty

(

map

, 

P

).

3.  Let 

newArgDesc

 be 

Desc

.

4.  If 

isMapped

 is 

true

 and 

IsDataDescriptor

(

Desc

) is 

true

, then

a.  If 

Desc

.[[Value]] is not present and 

Desc

.[[Writable]] is present and its value is 

false

, then

i.  Set 

newArgDesc

 to a copy of 

Desc

.

ii.  Set 

newArgDesc

.[[Value]] to 

Get

(

map

, 

P

).

5.  Let 

allowed

 be ? 

OrdinaryDefineOwnProperty

(

args

, 

P

, 

newArgDesc

).

6.  If 

allowed

 is 

false

, return 

false

.

7.  If 

isMapped

 is 

true

, then

a.  If 

IsAccessorDescriptor

(

Desc

) is 

true

, then

i.  Call 

map

.[[Delete]](

P

).

b.  Else,

i.  If 

Desc

.[[Value]] is present, then

1.  Let 

setStatus

 be 

Set

(

map

, 

P

, 

Desc

.[[Value]], 

false

).

2. 

Assert

: 

setStatus

 is 

true

 because formal parameters mapped by argument objects are

always writable.

ii.  If 

Desc

.[[Writable]] is present and its value is 

false

, then

1.  Call 

map

.[[Delete]](

P

).

8.  Return 

true

.

The [[Get]] internal method of an 

arguments exotic object

 

args

 takes arguments 

P

 (a property key) and 

Receiver

 (an

ECMAScript language value

). It performs the following steps when called:

1.  Let 

map

 be 

args

.[[ParameterMap]].

2.  Let 

isMapped

 be ! 

HasOwnProperty

(

map

, 

P

).

3.  If 

isMapped

 is 

false

, then

a.  Return ? 

OrdinaryGet

(

args

, 

P

, 

Receiver

).

4.  Else,

a. 

Assert

: 

map

 contains a formal parameter mapping for 

P

.

b.  Return 

Get

(

map

, 

P

).

The [[Set]] internal method of an 

arguments exotic object

 

args

 takes arguments 

P

 (a property key), 

V

 (an 

ECMAScript

language value

), and 

Receiver

 (an 

ECMAScript language value

). It performs the following steps when called:

1.  If 

SameValue

(

args

, 

Receiver

) is 

false

, then

a.  Let 

isMapped

 be 

false

.

2.  Else,

a.  Let 

map

 be 

args

.[[ParameterMap]].

b.  Let 

isMapped

 be ! 

HasOwnProperty

(

map

, 

P

).

3.  If 

isMapped

 is 

true

, then

a.  Let 

setStatus

 be 

Set

(

map

, 

P

, 

V

, 

false

).

10.4.4.3  [[Get]] ( 

P

, 

Receiver

 )

10.4.4.4  [[Set]] ( 

P

, 

V

, 

Receiver

 )

249

b. 

Assert

: 

setStatus

 is 

true

 because formal parameters mapped by argument objects are always writable.

4.  Return ? 

OrdinarySet

(

args

, 

P

, 

V

, 

Receiver

).

The [[Delete]] internal method of an 

arguments exotic object

 

args

 takes argument 

P

 (a property key). It performs the

following steps when called:

1.  Let 

map

 be 

args

.[[ParameterMap]].

2.  Let 

isMapped

 be ! 

HasOwnProperty

(

map

, 

P

).

3.  Let 

result

 be ? 

OrdinaryDelete

(

args

, 

P

).

4.  If 

result

 is 

true

 and 

isMapped

 is 

true

, then

a.  Call 

map

.[[Delete]](

P

).

5.  Return 

result

.

The abstract operation CreateUnmappedArgumentsObject takes argument 

argumentsList

. It performs the following

steps when called:

1.  Let 

len

 be the number of elements in 

argumentsList

.

2.  Let 

obj

 be ! 

OrdinaryObjectCreate

(

%Object.prototype%

, « [[ParameterMap]] »).

3.  Set 

obj

.[[ParameterMap]] to 

undefined

.

4.  Perform 

DefinePropertyOrThrow

(

obj

, 

"length"

(

len

), [[Writable]]: 

true

,

[[Enumerable]]: 

false

, [[Configurable]]: 

true

 }).

5.  Let 

index

 be 0.

6.  Repeat, while 

index

 < 

len

,

a.  Let 

val

 be 

argumentsList

[

index

].

b.  Perform ! 

CreateDataPropertyOrThrow

(

obj

, ! 

ToString

(

(

index

)), 

val

).

c.  Set 

index

 to 

index

 + 1.

7.  Perform ! 

DefinePropertyOrThrow

(

obj

, 

@@iterator

, PropertyDescriptor { [[Value]]: %Array.prototype.values%,

[[Writable]]: 

true

, [[Enumerable]]: 

false

, [[Configurable]]: 

true

 }).

8.  Perform ! 

DefinePropertyOrThrow

(

obj

, 

"callee"

, PropertyDescriptor { [[Get]]: 

%ThrowTypeError%

, [[Set]]:

%ThrowTypeError%

, [[Enumerable]]: 

false

, [[Configurable]]: 

false

 }).

9.  Return 

obj

.

The abstract operation CreateMappedArgumentsObject takes arguments 

func

 (an Object), 

formals

 (a 

Parse Node

),

argumentsList

 (a 

List

), and 

env

 (an 

Environment Record

). It performs the following steps when called:

1. 

Assert

: 

formals

 does not contain a rest parameter, any binding patterns, or any initializers. It may contain

duplicate identifiers.

2.  Let 

len

 be the number of elements in 

argumentsList

.

3.  Let 

obj

 be ! 

MakeBasicObject

(« [[Prototype]], [[Extensible]], [[ParameterMap]] »).

4.  Set 

obj

.[[GetOwnProperty]] as specified in 

10.4.4.1

.

5.  Set 

obj

.[[DefineOwnProperty]] as specified in 

10.4.4.2

.

6.  Set 

obj

.[[Get]] as specified in 

10.4.4.3

.

7.  Set 

obj

.[[Set]] as specified in 

10.4.4.4

.

10.4.4.5  [[Delete]] ( 

P

 )

10.4.4.6  CreateUnmappedArgumentsObject ( 

argumentsList

 )

10.4.4.7  CreateMappedArgumentsObject ( 

func

, 

formals

, 

argumentsList

, 

env

 )

250

8.  Set 

obj

.[[Delete]] as specified in 

10.4.4.5

.

9.  Set 

obj

.[[Prototype]] to 

%Object.prototype%

.

10.  Let 

map

 be ! 

OrdinaryObjectCreate

(

null

).

11.  Set 

obj

.[[ParameterMap]] to 

map

.

12.  Let 

parameterNames

 be the 

BoundNames

 of 

formals

.

13.  Let 

numberOfParameters

 be the number of elements in 

parameterNames

.

14.  Let 

index

 be 0.

15.  Repeat, while 

index

 < 

len

,

a.  Let 

val

 be 

argumentsList

[

index

].

b.  Perform ! 

CreateDataPropertyOrThrow

(

obj

, ! 

ToString

(

(

index

)), 

val

).

c.  Set 

index

 to 

index

 + 1.

16.  Perform ! 

DefinePropertyOrThrow

(

obj

, 

"length"

len

), [[Writable]]: 

true

,

[[Enumerable]]: 

false

, [[Configurable]]: 

true

 }).

17.  Let 

mappedNames

 be a new empty 

List

.

18.  Let 

index

 be 

numberOfParameters

 - 1.

19.  Repeat, while 

index

 

≥

 0,

a.  Let 

name

 be 

parameterNames

[

index

].

b.  If 

name

 is not an element of 

mappedNames

, then

i.  Add 

name

 as an element of the list 

mappedNames

.

ii.  If 

index

 < 

len

, then

1.  Let 

g

 be 

MakeArgGetter

(

name

, 

env

).

2.  Let 

p

 be 

MakeArgSetter

(

name

, 

env

).

3.  Perform 

map

.[[DefineOwnProperty]](! 

ToString

(

index

)), PropertyDescriptor { [[Set]]: 

p

,

[[Get]]: 

g

, [[Enumerable]]: 

false

, [[Configurable]]: 

true

 }).

c.  Set 

index

 to 

index

 - 1.

20.  Perform ! 

DefinePropertyOrThrow

(

obj

, 

@@iterator

, PropertyDescriptor { [[Value]]: %Array.prototype.values%,

[[Writable]]: 

true

, [[Enumerable]]: 

false

, [[Configurable]]: 

true

 }).

21.  Perform ! 

DefinePropertyOrThrow

(

obj

, 

"callee"

, PropertyDescriptor { [[Value]]: 

func

, [[Writable]]: 

true

,

[[Enumerable]]: 

false

, [[Configurable]]: 

true

 }).

22.  Return 

obj

.

The abstract operation MakeArgGetter takes arguments 

name

 (a String) and 

env

 (an 

Environment Record

). It creates a

built-in 

function object

 that when executed returns the value bound for 

name

 in 

env

. It performs the following steps

when called:

1.  Let 

steps

 be the steps of an ArgGetter function as specified below.

2.  Let 

length

 be the number of non-optional parameters of an ArgGetter function as specified below.

3.  Let 

getter

 be ! 

CreateBuiltinFunction

(

steps

, 

length

, 

""

, « [[Name]], [[Env]] »).

4.  Set 

getter

.[[Name]] to 

name

.

5.  Set 

getter

.[[Env]] to 

env

.

6.  Return 

getter

.

An ArgGetter function is an anonymous built-in function with [[Name]] and [[Env]] internal slots. When an ArgGetter
function that expects no arguments is called it performs the following steps:

1.  Let 

f

 be the 

active function object

.

2.  Let 

name

 be 

f

.[[Name]].

10.4.4.7.1  MakeArgGetter ( 

name

, 

env

 )

251

3.  Let 

env

 be 

f

.[[Env]].

4.  Return 

env

.GetBindingValue(

name

, 

false

).

NOTE

The abstract operation MakeArgSetter takes arguments 

name

 (a String) and 

env

 (an 

Environment Record

). It creates a

built-in 

function object

 that when executed sets the value bound for 

name

 in 

env

. It performs the following steps when

called:

1.  Let 

steps

 be the steps of an ArgSetter function as specified below.

2.  Let 

length

 be the number of non-optional parameters of an ArgSetter function as specified below.

3.  Let 

setter

 be ! 

CreateBuiltinFunction

(

steps

, 

length

, 

""

, « [[Name]], [[Env]] »).

4.  Set 

setter

.[[Name]] to 

name

.

5.  Set 

setter

.[[Env]] to 

env

.

6.  Return 

setter

.

An ArgSetter function is an anonymous built-in function with [[Name]] and [[Env]] internal slots. When an ArgSetter
function is called with argument 

value

 it performs the following steps:

1.  Let 

f

 be the 

active function object

.

2.  Let 

name

 be 

f

.[[Name]].

3.  Let 

env

 be 

f

.[[Env]].

4.  Return 

env

.SetMutableBinding(

name

, 

value

, 

false

).

NOTE

An 

Integer-Indexed exotic object

 is an 

exotic object

 that performs special handling of 

integer index

 property keys.

Integer-Indexed exotic objects

 have the same internal slots as ordinary objects and additionally

[[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], [[ContentType]], and [[TypedArrayName]] internal slots.

An object is an 

Integer-Indexed exotic object

 if its [[GetOwnProperty]], [[HasProperty]], [[DefineOwnProperty]], [[Get]],

[[Set]], [[Delete]], and [[OwnPropertyKeys]] internal methods use the definitions in this section, and its other essential
internal methods use the definitions found in 

10.1

. These methods are installed by 

IntegerIndexedObjectCreate

.

The [[GetOwnProperty]] internal method of an 

Integer-Indexed exotic object

 

O

 takes argument 

P

 (a property key). It

performs the following steps when called:

1. 

Assert

: 

IsPropertyKey

(

P

) is 

true

.

2. 

Assert

: 

O

 is an 

Integer-Indexed exotic object

.

3.  If 

Type

(

P

) is String, then

a.  Let 

numericIndex

 be ! 

CanonicalNumericIndexString

(

P

).

b.  If 

numericIndex

 is not 

undefined

, then

ArgGetter functions are never directly accessible to ECMAScript code.

ArgSetter functions are never directly accessible to ECMAScript code.

10.4.4.7.2  MakeArgSetter ( 

name

, 

env

 )

10.4.5  Integer-Indexed Exotic Objects

10.4.5.1  [[GetOwnProperty]] ( 

P

 )

252

i.  Let 

value

 be ! 

IntegerIndexedElementGet

(

O

, 

numericIndex

).

ii.  If 

value

 is 

undefined

, return 

undefined

.

iii.  Return the PropertyDescriptor { [[Value]]: 

value

, [[Writable]]: 

true

, [[Enumerable]]: 

true

,

[[Configurable]]: 

true

 }.

4.  Return 

OrdinaryGetOwnProperty

(

O

, 

P

).

The [[HasProperty]] internal method of an 

Integer-Indexed exotic object

 

O

 takes argument 

P

 (a property key). It

performs the following steps when called:

1. 

Assert

: 

IsPropertyKey

(

P

) is 

true

.

2. 

Assert

: 

O

 is an 

Integer-Indexed exotic object

.

3.  If 

Type

(

P

) is String, then

a.  Let 

numericIndex

 be ! 

CanonicalNumericIndexString

(

P

).

b.  If 

numericIndex

 is not 

undefined

, return ! 

IsValidIntegerIndex

(

O

, 

numericIndex

).

4.  Return ? 

OrdinaryHasProperty

(

O

, 

P

).

The [[DefineOwnProperty]] internal method of an 

Integer-Indexed exotic object

 

O

 takes arguments 

P

 (a property key)

and 

Desc

 (a 

Property Descriptor

). It performs the following steps when called:

1. 

Assert

: 

IsPropertyKey

(

P

) is 

true

.

2. 

Assert

: 

O

 is an 

Integer-Indexed exotic object

.

3.  If 

Type

(

P

) is String, then

a.  Let 

numericIndex

 be ! 

CanonicalNumericIndexString

(

P

).

b.  If 

numericIndex

 is not 

undefined

, then

i.  If ! 

IsValidIntegerIndex

(

O

, 

numericIndex

) is 

false

, return 

false

.

ii.  If 

Desc

 has a [[Configurable]] field and if 

Desc

.[[Configurable]] is 

false

, return 

false

.

iii.  If 

Desc

 has an [[Enumerable]] field and if 

Desc

.[[Enumerable]] is 

false

, return 

false

.

iv.  If ! 

IsAccessorDescriptor

(

Desc

) is 

true

, return 

false

.

v.  If 

Desc

 has a [[Writable]] field and if 

Desc

.[[Writable]] is 

false

, return 

false

.

vi.  If 

Desc

 has a [[Value]] field, perform ? 

IntegerIndexedElementSet

(

O

, 

numericIndex

, 

Desc

.[[Value]]).

vii.  Return 

true

.

4.  Return ! 

OrdinaryDefineOwnProperty

(

O

, 

P

, 

Desc

).

The [[Get]] internal method of an 

Integer-Indexed exotic object

 

O

 takes arguments 

P

 (a property key) and 

Receiver

 (an

ECMAScript language value

). It performs the following steps when called:

1. 

Assert

: 

IsPropertyKey

(

P

) is 

true

.

2.  If 

Type

(

P

) is String, then

a.  Let 

numericIndex

 be ! 

CanonicalNumericIndexString

(

P

).

b.  If 

numericIndex

 is not 

undefined

, then

i.  Return ! 

IntegerIndexedElementGet

(

O

, 

numericIndex

).

3.  Return ? 

OrdinaryGet

(

O

, 

P

, 

Receiver

).

10.4.5.2  [[HasProperty]] ( 

P

 )

10.4.5.3  [[DefineOwnProperty]] ( 

P

, 

Desc

 )

10.4.5.4  [[Get]] ( 

P

, 

Receiver

 )

253

The [[Set]] internal method of an 

Integer-Indexed exotic object

 

O

 takes arguments 

P

 (a property key), 

V

 (an

ECMAScript language value

), and 

Receiver

 (an 

ECMAScript language value

). It performs the following steps when

called:

1. 

Assert

: 

IsPropertyKey

(

P

) is 

true

.

2.  If 

Type

(

P

) is String, then

a.  Let 

numericIndex

 be ! 

CanonicalNumericIndexString

(

P

).

b.  If 

numericIndex

 is not 

undefined

, then

i.  Perform ? 

IntegerIndexedElementSet

(

O

, 

numericIndex

, 

V

).

ii.  Return 

true

.

3.  Return ? 

OrdinarySet

(

O

, 

P

, 

V

, 

Receiver

).

The [[Delete]] internal method of an 

Integer-Indexed exotic object

 

O

 takes arguments 

P

 (a property key). It performs

the following steps when called:

1. 

Assert

: 

IsPropertyKey

(

P

) is 

true

.

2. 

Assert

: 

O

 is an 

Integer-Indexed exotic object

.

3.  If 

Type

(

P

) is String, then

a.  Let 

numericIndex

 be ! 

CanonicalNumericIndexString

(

P

).

b.  If 

numericIndex

 is not 

undefined

, then

i.  If ! 

IsValidIntegerIndex

(

O

, 

numericIndex

) is 

false

, return 

true

; else return 

false

.

4.  Return ? 

OrdinaryDelete

(

O

, 

P

).

The [[OwnPropertyKeys]] internal method of an 

Integer-Indexed exotic object

 

O

 takes no arguments. It performs the

following steps when called:

1.  Let 

keys

 be a new empty 

List

.

2. 

Assert

: 

O

 is an 

Integer-Indexed exotic object

.

3.  If 

IsDetachedBuffer

(

O

.[[ViewedArrayBuffer]]) is 

false

, then

a.  For each 

integer

 

i

 starting with 0 such that 

i

 < 

O

.[[ArrayLength]], in ascending order, do

i.  Add ! 

ToString

(

(

i

)) as the last element of 

keys

.

4.  For each own property key 

P

 of 

O

 such that 

Type

(

P

) is String and 

P

 is not an 

integer index

, in ascending

chronological order of property creation, do

a.  Add 

P

 as the last element of 

keys

.

5.  For each own property key 

P

 of 

O

 such that 

Type

(

P

) is Symbol, in ascending chronological order of property

creation, do

a.  Add 

P

 as the last element of 

keys

.

6.  Return 

keys

.

The abstract operation IntegerIndexedObjectCreate takes argument 

prototype

. It is used to specify the creation of new

Integer-Indexed exotic objects

. It performs the following steps when called:

10.4.5.5  [[Set]] ( 

P

, 

V

, 

Receiver

 )

10.4.5.6  [[Delete]] ( 

P

 )

10.4.5.7  [[OwnPropertyKeys]] ( )

10.4.5.8  IntegerIndexedObjectCreate ( 

prototype

 )

254

1.  Let 

internalSlotsList

 be « [[Prototype]], [[Extensible]], [[ViewedArrayBuffer]], [[TypedArrayName]],

[[ContentType]], [[ByteLength]], [[ByteOffset]], [[ArrayLength]] ».

2.  Let 

A

 be ! 

MakeBasicObject

(

internalSlotsList

).

3.  Set 

A

.[[GetOwnProperty]] as specified in 

10.4.5.1

.

4.  Set 

A

.[[HasProperty]] as specified in 

10.4.5.2

.

5.  Set 

A

.[[DefineOwnProperty]] as specified in 

10.4.5.3

.

6.  Set 

A

.[[Get]] as specified in 

10.4.5.4

.

7.  Set 

A

.[[Set]] as specified in 

10.4.5.5

.

8.  Set 

A

.[[Delete]] as specified in 

10.4.5.6

.

9.  Set 

A

.[[OwnPropertyKeys]] as specified in 

10.4.5.7

.

10.  Set 

A

.[[Prototype]] to 

prototype

.

11.  Return 

A

.

The abstract operation IsValidIntegerIndex takes arguments 

O

 and 

index

 (a Number). It performs the following steps

when called:

1. 

Assert

: 

O

 is an 

Integer-Indexed exotic object

.

2.  If 

IsDetachedBuffer

(

O

.[[ViewedArrayBuffer]]) is 

true

, return 

false

.

3.  If ! 

IsIntegralNumber

(

index

) is 

false

, return 

false

.

4.  If 

index

 is 

-0

𝔽

, return 

false

.

index

index

) 

≥

 

O

.[[ArrayLength]], return 

false

.

6.  Return 

true

.

The abstract operation IntegerIndexedElementGet takes arguments 

O

 and 

index

 (a Number). It performs the following

steps when called:

1. 

Assert

: 

O

 is an 

Integer-Indexed exotic object

.

2.  If ! 

IsValidIntegerIndex

(

O

, 

index

) is 

false

, return 

undefined

.

3.  Let 

offset

 be 

O

.[[ByteOffset]].

4.  Let 

arrayTypeName

 be the String value of 

O

.[[TypedArrayName]].

5.  Let 

elementSize

 be the Element Size value specified in 

Table 60

 for 

arrayTypeName

.

6.  Let 

indexedPosition

 be (

(

index

) 

×

 

elementSize

) + 

offset

.

7.  Let 

elementType

 be the Element Type value in 

Table 60

 for 

arrayTypeName

.

8.  Return 

GetValueFromBuffer

(

O

.[[ViewedArrayBuffer]], 

indexedPosition

, 

elementType

, 

true

, 

Unordered

).

The abstract operation IntegerIndexedElementSet takes arguments 

O

, 

index

 (a Number), and 

value

. It performs the

following steps when called:

1. 

Assert

: 

O

 is an 

Integer-Indexed exotic object

.

2.  If 

O

.[[ContentType]] is 

BigInt

, let 

numValue

 be ? 

ToBigInt

(

value

).

3.  Otherwise, let 

numValue

 be ? 

ToNumber

(

value

).

4.  If ! 

IsValidIntegerIndex

(

O

, 

index

) is 

true

, then

a.  Let 

offset

 be 

O

.[[ByteOffset]].

10.4.5.9  IsValidIntegerIndex ( 

O

, 

index

 )

10.4.5.10  IntegerIndexedElementGet ( 

O

, 

index

 )

10.4.5.11  IntegerIndexedElementSet ( 

O

, 

index

, 

value

 )

255

b.  Let 

arrayTypeName

 be the String value of 

O

.[[TypedArrayName]].

c.  Let 

elementSize

 be the Element Size value specified in 

Table 60

 for 

arrayTypeName

.

d.  Let 

indexedPosition

 be (

(

index

) 

×

 

elementSize

) + 

offset

.

e.  Let 

elementType

 be the Element Type value in 

Table 60

 for 

arrayTypeName

.

f.  Perform 

SetValueInBuffer

(

O

.[[ViewedArrayBuffer]], 

indexedPosition

, 

elementType

, 

numValue

, 

true

,

Unordered

).

5.  Return 

NormalCompletion

(

undefined

).

NOTE

A 

module namespace exotic object

 is an 

exotic object

 that exposes the bindings exported from an ECMAScript 

Module

(See 

16.2.3

). There is a one-to-one correspondence between the String-keyed own properties of a 

module namespace

exotic object

 and the binding names exported by the 

Module

. The exported bindings include any bindings that are

indirectly exported using 

export *

export *

 export items. Each String-valued own property key is the 

StringValue

 of the

corresponding exported binding name. These are the only String-keyed properties of a 

module namespace exotic

object

. Each such property has the attributes { [[Writable]]: 

true

, [[Enumerable]]: 

true

, [[Configurable]]: 

false

 }. Module

namespace exotic objects are not extensible.

An object is a 

module namespace exotic object

 if its [[SetPrototypeOf]], [[IsExtensible]], [[PreventExtensions]],

[[GetOwnProperty]], [[DefineOwnProperty]], [[HasProperty]], [[Get]], [[Set]], [[Delete]], and [[OwnPropertyKeys]]
internal methods use the definitions in this section, and its other essential internal methods use the definitions found
in 

10.1

. These methods are installed by 

ModuleNamespaceCreate

.

Module namespace exotic objects have the internal slots defined in 

Table 31

.

Table 31: Internal Slots of Module Namespace Exotic Objects

Internal

Slot

Type

Description

[[Module]]

Module
Record

The 

Module Record

 whose exports this namespace exposes.

[[Exports]]

List

 of

String

A 

List

 whose elements are the String values of the exported names exposed as own

properties of this object. The list is ordered as if an Array of those String values had been
sorted using %Array.prototype.sort% using 

undefined

 as 

comparefn

.

[[Prototype]] Null

This slot always contains the value 

null

 (see 

10.4.6.1

).

Module namespace exotic objects provide alternative definitions for all of the internal methods except
[[GetPrototypeOf]], which behaves as defined in 

10.1.1

.

The [[SetPrototypeOf]] internal method of a 

module namespace exotic object

 

O

 takes argument 

V

 (an Object or 

null

).

This operation always appears to succeed, but it has no effect when attempting to write past the
end of a TypedArray or to a TypedArray which is backed by a detached ArrayBuffer.

10.4.6  Module Namespace Exotic Objects

10.4.6.1  [[SetPrototypeOf]] ( 

V

 )

256

It performs the following steps when called:

1.  Return ? 

SetImmutablePrototype

(

O

, 

V

).

The [[IsExtensible]] internal method of a 

module namespace exotic object

 takes no arguments. It performs the

following steps when called:

1.  Return 

false

.

The [[PreventExtensions]] internal method of a 

module namespace exotic object

 takes no arguments. It performs the

following steps when called:

1.  Return 

true

.

The [[GetOwnProperty]] internal method of a 

module namespace exotic object

 

O

 takes argument 

P

 (a property key). It

performs the following steps when called:

1.  If 

Type

(

P

) is Symbol, return 

OrdinaryGetOwnProperty

(

O

, 

P

).

2.  Let 

exports

 be 

O

.[[Exports]].

3.  If 

P

 is not an element of 

exports

, return 

undefined

.

4.  Let 

value

 be ? 

O

.[[Get]](

P

, 

O

).

5.  Return PropertyDescriptor { [[Value]]: 

value

, [[Writable]]: 

true

, [[Enumerable]]: 

true

, [[Configurable]]: 

false

 }.

The [[DefineOwnProperty]] internal method of a 

module namespace exotic object

 

O

 takes arguments 

P

 (a property

key) and 

Desc

 (a 

Property Descriptor

). It performs the following steps when called:

1.  If 

Type

(

P

) is Symbol, return 

OrdinaryDefineOwnProperty

(

O

, 

P

, 

Desc

).

2.  Let 

current

 be ? 

O

.[[GetOwnProperty]](

P

).

3.  If 

current

 is 

undefined

, return 

false

.

4.  If 

Desc

.[[Configurable]] is present and has value 

true

, return 

false

.

5.  If 

Desc

.[[Enumerable]] is present and has value 

false

, return 

false

.

6.  If ! 

IsAccessorDescriptor

(

Desc

) is 

true

, return 

false

.

7.  If 

Desc

.[[Writable]] is present and has value 

false

, return 

false

.

8.  If 

Desc

.[[Value]] is present, return 

SameValue

(

Desc

.[[Value]], 

current

.[[Value]]).

9.  Return 

true

.

The [[HasProperty]] internal method of a 

module namespace exotic object

 

O

 takes argument 

P

 (a property key). It

performs the following steps when called:

1.  If 

Type

(

P

) is Symbol, return 

OrdinaryHasProperty

(

O

, 

P

).

10.4.6.2  [[IsExtensible]] ( )

10.4.6.3  [[PreventExtensions]] ( )

10.4.6.4  [[GetOwnProperty]] ( 

P

 )

10.4.6.5  [[DefineOwnProperty]] ( 

P

, 

Desc

 )

10.4.6.6  [[HasProperty]] ( 

P

 )

257

2.  Let 

exports

 be 

O

.[[Exports]].

3.  If 

P

 is an element of 

exports

, return 

true

.

4.  Return 

false

.

The [[Get]] internal method of a 

module namespace exotic object

 

O

 takes arguments 

P

 (a property key) and 

Receiver

(an 

ECMAScript language value

). It performs the following steps when called:

1. 

Assert

: 

IsPropertyKey

(

P

) is 

true

.

2.  If 

Type

(

P

) is Symbol, then

a.  Return ? 

OrdinaryGet

(

O

, 

P

, 

Receiver

).

3.  Let 

exports

 be 

O

.[[Exports]].

4.  If 

P

 is not an element of 

exports

, return 

undefined

.

5.  Let 

m

 be 

O

.[[Module]].

6.  Let 

binding

 be ! 

m

.ResolveExport(

P

).

7. 

Assert

: 

binding

 is a 

ResolvedBinding Record

.

8.  Let 

targetModule

 be 

binding

.[[Module]].

9. 

Assert

: 

targetModule

 is not 

undefined

.

10.  If 

binding

.[[BindingName]] is 

"*namespace*"

, then

a.  Return ? 

GetModuleNamespace

(

targetModule

).

11.  Let 

targetEnv

 be 

targetModule

.[[Environment]].

12.  If 

targetEnv

 is 

undefined

, throw a 

ReferenceError

 exception.

13.  Return ? 

targetEnv

.GetBindingValue(

binding

.[[BindingName]], 

true

).

NOTE

The [[Set]] internal method of a 

module namespace exotic object

 takes arguments 

P

 (a property key), 

V

 (an

ECMAScript language value

), and 

Receiver

 (an 

ECMAScript language value

). It performs the following steps when

called:

1.  Return 

false

.

The [[Delete]] internal method of a 

module namespace exotic object

 

O

 takes argument 

P

 (a property key). It performs

the following steps when called:

1. 

Assert

: 

IsPropertyKey

(

P

) is 

true

.

2.  If 

Type

(

P

) is Symbol, then

a.  Return ? 

OrdinaryDelete

(

O

, 

P

).

3.  Let 

exports

 be 

O

.[[Exports]].

4.  If 

P

 is an element of 

exports

, return 

false

.

ResolveExport is side-effect free. Each time this operation is called with a specific 

exportName

,

resolveSet

 pair as arguments it must return the same result. An implementation might choose to

pre-compute or cache the ResolveExport results for the [[Exports]] of each 

module namespace

exotic object

.

10.4.6.7  [[Get]] ( 

P

, 

Receiver

 )

10.4.6.8  [[Set]] ( 

P

, 

V

, 

Receiver

 )

10.4.6.9  [[Delete]] ( 

P

 )

258