ECMA-262 (12th Edition) ECMAScript 2021 Language Specification - page 33

this

value).

2. Let

be ?

(

, 0).

3. Let

be 0.

4. Prepend

items

5. For each element

items

, do

a. Let

spreadable

be ?

IsConcatSpreadable

b. If

spreadable

true

, then

i. Let

be 0.

ii. Let

len

be ?

(

iii. If

len

> 2

- 1, throw a

TypeError

exception.

iv. Repeat, while

len

1. Let

be !

)).

2. Let

exists

(

3. If

exists

true

, then

a. Let

subElement

be ?

(

b. Perform ?

(

, !

)),

subElement

4. Set

+ 1.

5. Set

+ 1.

c. Else,

i. NOTE:

is added as a single item rather than spread.

ii. If

≥

- 1, throw a

TypeError

exception.

iii. Perform ?

(

, !

)),

iv. Set

+ 1.

6. Perform ?

(

"length"

(

true

7. Return

The

"length"

property of the

concat

method is

𝔽

The Array prototype object is specified to be an

Array exotic object

to ensure compatibility with

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

23.1.3.1 Array.prototype.concat ( ...

items

)

637

NOTE 1

NOTE 2

The abstract operation IsConcatSpreadable takes argument

. It performs the following steps when called:

1. If

Type

(

) is not Object, return

false

2. Let

spreadable

be ?

(

@@isConcatSpreadable

3. If

spreadable

is not

undefined

, return !

spreadable

4. Return ?

IsArray

(

The initial value of

Array.prototype.constructor

%Array%

The

copyWithin

method takes up to three arguments

target

start

and

end

NOTE 1

The following steps are taken:

1. Let

be ?

this

value).

2. Let

len

be ?

(

3. Let

relativeTarget

be ?

(

target

4. If

relativeTarget

is -

∞

, let

be 0.

5. Else if

relativeTarget

< 0, let

(

len

relativeTarget

, 0).

6. Else, let

(

relativeTarget

len

7. Let

relativeStart

start

8. If

relativeStart

is -

∞

, let

from

be 0.

9. Else if

relativeStart

< 0, let

from

len

relativeStart

, 0).

10. Else, let

from

(

relativeStart

len

11. If

end

undefined

, let

relativeEnd

len

; else let

relativeEnd

be ?

(

end

12. If

relativeEnd

is -

∞

, let

final

be 0.

13. Else if

relativeEnd

< 0, let

final

len

relativeEnd

, 0).

14. Else, let

final

(

relativeEnd

len

15. Let

count

(

final

from

len

16. If

from

and

from

count

, then

a. Let

direction

be -1.

b. Set

from

count

- 1.

The explicit setting of the

"length"

property in step

is necessary to ensure that its value is

correct in situations where the trailing elements of the result Array are not present.

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.

The

end

argument is optional with the length of the

this

value as its default value. If

target

negative, it is treated as

length

target

where

length

is the length of the array. If

start

is negative, it

is treated as

length

start

. If

end

is negative, it is treated as

length

end

23.1.3.1.1 IsConcatSpreadable (

)

23.1.3.2 Array.prototype.constructor

23.1.3.3 Array.prototype.copyWithin (

target

start

[ ,

end

] )

638

c. Set

count

- 1.

17. Else,

a. Let

direction

be 1.

18. Repeat, while

count

> 0,

a. Let

fromKey

be !

from

)).

b. Let

toKey

be !

)).

c. Let

fromPresent

(

fromKey

d. If

fromPresent

true

, then

i. Let

fromVal

fromKey

ii. Perform ?

(

toKey

fromVal

true

e. Else,

fromPresent

false

ii. Perform ?

DeletePropertyOrThrow

toKey

f. Set

from

direction

g. Set

direction

h. Set

count

- 1.

19. Return

NOTE 2

The following steps are taken:

1. Let

be ?

this

value).

2. Return

CreateArrayIterator

key+value

The

copyWithin

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.

23.1.3.4 Array.prototype.entries ( )

23.1.3.5 Array.prototype.every (

callbackfn

[ ,

thisArg

] )

639

NOTE 1

When the

every

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

1. Let

be ?

this

value).

2. Let

len

be ?

(

3. If

callbackfn

) is

false

, throw a

TypeError

exception.

4. Let

be 0.

5. Repeat, while

len

a. Let

be !

)).

b. Let

kPresent

(

c. If

kPresent

true

, then

i. Let

kValue

be ?

(

ii. Let

testResult

(

callbackfn

thisArg

, «

kValue

»)).

iii. If

testResult

false

, return

false

d. Set

+ 1.

6. Return

true

NOTE 2

The

fill

method takes up to three arguments

value

start

and

end

callbackfn

should be a function that accepts three arguments and returns a value that is coercible

to a Boolean value.

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

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.

23.1.3.6 Array.prototype.fill (

value

[ ,

start

[ ,

end

] ] )

640

NOTE 1

The following steps are taken:

1. Let

be ?

this

value).

2. Let

len

be ?

(

3. Let

relativeStart

start

4. If

relativeStart

is -

∞

, let

be 0.

5. Else if

relativeStart

< 0, let

len

relativeStart

, 0).

6. Else, let

(

relativeStart

len

7. If

end

undefined

, let

relativeEnd

len

; else let

relativeEnd

(

end

8. If

relativeEnd

is -

∞

, let

final

be 0.

9. Else if

relativeEnd

< 0, let

final

len

relativeEnd

, 0).

10. Else, let

final

(

relativeEnd

len

11. Repeat, while

final

a. Let

be !

)).

b. Perform ?

(

value

true

c. Set

+ 1.

12. Return

NOTE 2

NOTE 1

The

start

and

end

arguments are optional with default values of 0 and the length of the

this

value.

start

is negative, it is treated as

length

start

where

length

is the length of the array. If

end

negative, it is treated as

length

end

The

fill

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.

callbackfn

should be a function that accepts three arguments and returns a value that is coercible

to a Boolean value.

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

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.

23.1.3.7 Array.prototype.filter (

callbackfn

[ ,

thisArg

] )

641

When the

filter

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

1. Let

be ?

this

value).

2. Let

len

be ?

(

3. If

callbackfn

) is

false

, throw a

TypeError

exception.

4. Let

be ?

(

, 0).

5. Let

be 0.

6. Let

be 0.

7. Repeat, while

len

a. Let

be !

)).

b. Let

kPresent

(

c. If

kPresent

true

, then

i. Let

kValue

be ?

(

ii. Let

selected

(

callbackfn

thisArg

, «

kValue

(

»)).

iii. If

selected

true

, then

1. Perform ?

(

, !

)),

kValue

2. Set

+ 1.

d. Set

+ 1.

8. Return

NOTE 2

The

find

method is called with one or two arguments,

predicate

and

thisArg

NOTE 1

When the

find

method is called, the following steps are taken:

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.

predicate

should be a function that accepts three arguments and returns a value that is coercible to

a Boolean value.

find

calls

predicate

once for each element of the array, in ascending order, until

it finds one where

predicate

returns

true

. If such an element is found,

find

immediately returns

that element value. Otherwise,

find

returns

undefined

If a

thisArg

parameter is provided, it will be used as the

this

value for each invocation of

predicate

If it is not provided,

undefined

is used instead.

predicate

is called with three arguments: the value of the element, the index of the element, and

the object being traversed.

find

does not directly mutate the object on which it is called but the object may be mutated by

the calls to

predicate

The range of elements processed by

find

is set before the first call to

predicate

. Elements that are

appended to the array after the call to

find

begins will not be visited by

predicate

. If existing

elements of the array are changed, their value as passed to

predicate

will be the value at the time

that

find

visits them; elements that are deleted after the call to

find

begins and before being

visited are not visited.

23.1.3.8 Array.prototype.find (

predicate

[ ,

thisArg

] )

642

1. Let

be ?

this

value).

2. Let

len

be ?

(

3. If

predicate

) is

false

, throw a

TypeError

exception.

4. Let

be 0.

5. Repeat, while

len

a. Let

be !

)).

b. Let

kValue

be ?

(

c. Let

testResult

(

predicate

thisArg

, «

kValue

»)).

d. If

testResult

true

, return

kValue

e. Set

+ 1.

6. Return

undefined

NOTE 2

NOTE 1

When the

findIndex

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

1. Let

be ?

this

value).

2. Let

len

be ?

(

3. If

predicate

) is

false

, throw a

TypeError

exception.

4. Let

be 0.

5. Repeat, while

len

a. Let

be !

)).

b. Let

kValue

be ?

(

c. Let

testResult

(

predicate

thisArg

, «

kValue

»)).

d. If

testResult

true

, return

(

The

find

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.

predicate

should be a function that accepts three arguments and returns a value that is coercible to

a Boolean value.

findIndex

calls

predicate

once for each element of the array, in ascending

order, until it finds one where

predicate

returns

true

. If such an element is found,

findIndex

immediately returns the index of that element value. Otherwise,

findIndex

returns -1.

If a

thisArg

parameter is provided, it will be used as the

this

value for each invocation of

predicate

If it is not provided,

undefined

is used instead.

predicate

is called with three arguments: the value of the element, the index of the element, and

the object being traversed.

findIndex

does not directly mutate the object on which it is called but the object may be

mutated by the calls to

predicate

The range of elements processed by

findIndex

is set before the first call to

predicate

. Elements

that are appended to the array after the call to

findIndex

begins will not be visited by

predicate

. If existing elements of the array are changed, their value as passed to

predicate

will be

the value at the time that

findIndex

visits them; elements that are deleted after the call to

findIndex

begins and before being visited are not visited.

23.1.3.9 Array.prototype.findIndex (

predicate

[ ,

thisArg

] )

643

e. Set

+ 1.

6. Return

-1

𝔽

NOTE 2

When the

flat

method is called with zero or one arguments, the following steps are taken:

1. Let

be ?

this

value).

2. Let

sourceLen

3. Let

depthNum

be 1.

4. If

depth

is not

undefined

, then

a. Set

depthNum

depth

b. If

depthNum

< 0, set

depthNum

to 0.

5. Let

be ?

(

, 0).

6. Perform ?

FlattenIntoArray

(

sourceLen

, 0,

depthNum

7. Return

The abstract operation FlattenIntoArray takes arguments

target

source

sourceLen

(a non-negative

integer

start

(a non-

negative

integer

), and

depth

(a non-negative

integer

or +

∞

) and optional arguments

mapperFunction

and

thisArg

. It

performs the following steps when called:

Type

target

) is Object.

Type

source

) is Object.

: If

mapperFunction

is present, then !

(

mapperFunction

) is

true

thisArg

is present, and

depth

is 1.

4. Let

targetIndex

start

5. Let

sourceIndex

𝔽

6. Repeat, while

(

sourceIndex

) <

sourceLen

a. Let

be !

(

sourceIndex

b. Let

exists

(

source

c. If

exists

true

, then

i. Let

element

source

ii. If

mapperFunction

is present, then

1. Set

element

(

mapperFunction

thisArg

, «

element

sourceIndex

source

»).

iii. Let

shouldFlatten

false

iv. If

depth

> 0, then

1. Set

shouldFlatten

to ?

IsArray

(

element

v. If

shouldFlatten

true

, then

1. If

depth

is +

∞

, let

newDepth

be +

∞

2. Else, let

newDepth

depth

- 1.

3. Let

elementLen

be ?

(

element

4. Set

targetIndex

FlattenIntoArray

(

target

element

elementLen

targetIndex

newDepth

vi. Else,

1. If

targetIndex

≥

- 1, throw a

TypeError

exception.

The

findIndex

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.

23.1.3.10 Array.prototype.flat ( [

depth

] )

23.1.3.10.1 FlattenIntoArray (

target

source

sourceLen

start

depth

[ ,

mapperFunction

thisArg

] )

644

2. Perform ?

(

target

, !

targetIndex

)),

element

3. Set

targetIndex

+ 1.

d. Set

sourceIndex

𝔽

7. Return

targetIndex

When the

flatMap

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

1. Let

be ?

this

value).

2. Let

sourceLen

3. If !

mapperFunction

) is

false

, throw a

TypeError

exception.

4. Let

be ?

(

, 0).

5. Perform ?

FlattenIntoArray

(

sourceLen

, 0, 1,

mapperFunction

thisArg

6. Return

NOTE 1

When the

forEach

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

1. Let

be ?

this

value).

2. Let

len

be ?

(

3. If

callbackfn

) is

false

, throw a

TypeError

exception.

4. Let

be 0.

5. Repeat, while

len

a. Let

be !

)).

b. Let

kPresent

(

c. If

kPresent

true

, then

i. Let

kValue

be ?

(

ii. Perform ?

(

callbackfn

thisArg

, «

kValue

»).

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

callbackfn

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.

23.1.3.11 Array.prototype.flatMap (

mapperFunction

[ ,

thisArg

] )

23.1.3.12 Array.prototype.forEach (

callbackfn

[ ,

thisArg

] )

645

d. Set

+ 1.

6. Return

undefined

NOTE 2

NOTE 1

When the

includes

method is called, the following steps are taken:

1. Let

be ?

this

value).

2. Let

len

be ?

(

3. If

len

is 0, return

false

4. Let

fromIndex

: If

fromIndex

undefined

, then

is 0.

6. If

is +

∞

, return

false

7. Else if

is -

∞

, set

to 0.

8. If

≥

0, then

a. Let

9. Else,

a. Let

len

b. If

< 0, set

to 0.

10. Repeat, while

len

a. Let

elementK

be ?

(

))).

b. If

SameValueZero

searchElement

elementK

) is

true

, return

true

c. Set

+ 1.

11. Return

false

NOTE 2

NOTE 3

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.

includes

compares

searchElement

to the elements of the array, in ascending order, using the

SameValueZero

algorithm, and if found at any position, returns

true

; otherwise,

false

is returned.

The optional second argument

fromIndex

defaults to

𝔽

(i.e. the whole array is searched). If it is

greater than or equal to the length of the array,

false

is returned, i.e. the array will not be

searched. If it is less than

𝔽

, it is used as the offset from the end of the array to compute

fromIndex

. If the computed index is less than

𝔽

, the whole array will be searched.

The

includes

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.

The

includes

method intentionally differs from the similar

indexOf

method in two ways.

First, it uses the

SameValueZero

algorithm, instead of

, allowing it to

detect

NaN

array elements. Second, it does not skip missing array elements, instead treating

them as

undefined

23.1.3.13 Array.prototype.includes (

searchElement

[ ,

fromIndex

] )

23.1.3.14 Array.prototype.indexOf (

searchElement

[ ,

fromIndex

] )

646

NOTE 1

When the

indexOf

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

1. Let

be ?

this

value).

2. Let

len

be ?

(

3. If

len

is 0, return

-1

𝔽

4. Let

fromIndex

: If

fromIndex

undefined

, then

is 0.

6. If

is +

∞

, return

-1

𝔽

7. Else if

is -

∞

, set

to 0.

8. If

≥

0, then

a. Let

9. Else,

a. Let

len

b. If

< 0, set

to 0.

10. Repeat, while

len

a. Let

kPresent

(

))).

b. If

kPresent

true

, then

i. Let

elementK

be ?

(

))).

ii. Let

same

be the result of performing

searchElement

===

elementK

iii. If

same

true

, return

(

c. Set

+ 1.

11. Return

-1

𝔽

NOTE 2

NOTE 1

The

join

method takes one argument,

separator

, and performs the following steps:

1. Let

be ?

this

value).

2. Let

len

be ?

(

3. If

separator

undefined

, let

sep

be the single-element String

","

indexOf

compares

searchElement

to the elements of the array, in ascending order, using the

algorithm, and if found at one or more indices, returns the smallest

such index; otherwise,

-1

𝔽

is returned.

The optional second argument

fromIndex

defaults to

𝔽

(i.e. the whole array is searched). If it is

greater than or equal to the length of the array,

-1

𝔽

is returned, i.e. the array will not be searched.

If it is less than

𝔽

, it is used as the offset from the end of the array to compute

fromIndex

. If the

computed index is less than

𝔽

, the whole array will be searched.

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.

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.

23.1.3.15 Array.prototype.join (

separator

)

647

4. Else, let

sep

be ?

(

separator

5. Let

be the empty String.

6. Let

be 0.

7. Repeat, while

len

a. If

> 0, set

to the

string-concatenation

and

sep

b. Let

element

(

))).

c. If

element

undefined

null

, let

next

be the empty String; otherwise, let

next

be ?

element

d. Set

to the

string-concatenation

and

next

e. Set

+ 1.

8. Return

NOTE 2

The following steps are taken:

1. Let

be ?

this

value).

2. Return

CreateArrayIterator

key

NOTE 1

When the

lastIndexOf

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

1. Let

be ?

this

value).

2. Let

len

be ?

(

3. If

len

is 0, return

-1

𝔽

4. If

fromIndex

is present, let

fromIndex

); else let

len

- 1.

5. If

is -

∞

, return

-1

𝔽

6. If

≥

0, then

a. Let

(

len

- 1).

7. Else,

a. Let

len

8. Repeat, while

≥

a. Let

kPresent

(

))).

b. If

kPresent

true

, then

i. Let

elementK

be ?

(

))).

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.

lastIndexOf

compares

searchElement

to the elements of the array in descending order using

the

algorithm, and if found at one or more indices, returns the largest

such index; otherwise,

-1

𝔽

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 less than

𝔽

, it is used as the offset from the end of the array to compute

fromIndex

. If the computed index is less than

𝔽

-1

𝔽

is returned.

23.1.3.16 Array.prototype.keys ( )

23.1.3.17 Array.prototype.lastIndexOf (

searchElement

[ ,

fromIndex

] )

648

ii. Let

same

be the result of performing

searchElement

===

elementK

iii. If

same

true

, return

(

c. Set

- 1.

9. Return

-1

𝔽

NOTE 2

NOTE 1

When the

map

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

1. Let

be ?

this

value).

2. Let

len

be ?

(

3. If

callbackfn

) is

false

, throw a

TypeError

exception.

4. Let

be ?

(

len

5. Let

be 0.

6. Repeat, while

len

a. Let

be !

)).

b. Let

kPresent

(

c. If

kPresent

true

, then

i. Let

kValue

be ?

(

ii. Let

mappedValue

(

callbackfn

thisArg

, «

kValue

»).

iii. Perform ?

(

mappedValue

d. Set

+ 1.

7. Return

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.

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

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.

23.1.3.18 Array.prototype.map (

callbackfn

[ ,

thisArg

] )

649

NOTE 2

NOTE 1

When the

pop

method is called, the following steps are taken:

1. Let

be ?

this

value).

2. Let

len

be ?

(

3. If

len

= 0, then

a. Perform ?

(

"length"

𝔽

true

b. Return

undefined

4. Else,

len

> 0.

b. Let

newLen

len

- 1).

c. Let

index

be !

(

newLen

d. Let

element

index

e. Perform ?

DeletePropertyOrThrow

index

f. Perform ?

(

"length"

newLen

true

g. Return

element

NOTE 2

NOTE 1

When the

push

method is called with zero or more arguments, the following steps are taken:

1. Let

be ?

this

value).

2. Let

len

be ?

(

3. Let

argCount

be the number of elements in

items

4. If

len

argCount

> 2

- 1, throw a

TypeError

exception.

5. For each element

items

, do

a. Perform ?

(

, !

len

)),

true

b. Set

len

+ 1.

6. Perform ?

(

"length"

len

true

7. Return

len

The

"length"

property of the

push

method is

𝔽

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.

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

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.

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.

23.1.3.19 Array.prototype.pop ( )

23.1.3.20 Array.prototype.push ( ...

items

)

650

NOTE 2

NOTE 1

When the

reduce

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

1. Let

be ?

this

value).

2. Let

len

be ?

(

3. If

callbackfn

) is

false

, throw a

TypeError

exception.

4. If

len

= 0 and

initialValue

is not present, throw a

TypeError

exception.

5. Let

be 0.

6. Let

accumulator

undefined

7. If

initialValue

is present, then

a. Set

accumulator

initialValue

8. Else,

a. Let

kPresent

false

b. Repeat, while

kPresent

false

and

len

i. Let

be !

)).

ii. Set

kPresent

(

iii. If

kPresent

true

, then

1. Set

accumulator

iv. Set

+ 1.

c. If

kPresent

false

, throw a

TypeError

exception.

9. Repeat, while

len

a. Let

be !

)).

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.

callbackfn

should be a function that takes four arguments.

reduce

calls the callback, as a

function, once for each element after the first element present in the array, in ascending order.

callbackfn

is called with four arguments: the

previousValue

(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 supplied in the call to

reduce

, then

previousValue

will be equal

initialValue

and

currentValue

will be equal to the first value in the array. If no

initialValue

was

supplied, 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.

23.1.3.21 Array.prototype.reduce (

callbackfn

[ ,

initialValue

] )

651

b. Let

kPresent

(

c. If

kPresent

true

, then

i. Let

kValue

be ?

(

ii. Set

accumulator

(

callbackfn

undefined

, «

accumulator

kValue

»).

d. Set

+ 1.

10. Return

accumulator

NOTE 2

NOTE 1

When the

reduceRight

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

1. Let

be ?

this

value).

2. Let

len

be ?

(

3. If

callbackfn

) is

false

, throw a

TypeError

exception.

4. If

len

is 0 and

initialValue

is not present, throw a

TypeError

exception.

5. Let

len

- 1.

6. Let

accumulator

undefined

7. If

initialValue

is present, then

a. Set

accumulator

initialValue

8. Else,

a. Let

kPresent

false

b. Repeat, while

kPresent

false

and

≥

i. Let

be !