ECMA-262 (12th Edition) ECMAScript 2021 Language Specification - page 54

(

, [[WeakMapData]]).

3. Let

entries

be the

.[[WeakMapData]].

4. If

(

key

) is not Object, return

false

{ [[Key]], [[Value]] }

entries

, do

a. If

.[[Key]] is not

empty

and

.[[Key]],

key

) is

true

, then

i. Set

.[[Key]] to

empty

ii. Set

.[[Value]] to

empty

iii. Return

true

6. Return

false

NOTE

The following steps are taken:

1. Let

be the

this

value.

(

, [[WeakMapData]]).

3. Let

entries

be the

.[[WeakMapData]].

4. If

(

key

) is not Object, return

undefined

{ [[Key]], [[Value]] }

entries

, do

a. If

.[[Key]] is not

empty

and

.[[Key]],

key

) is

true

, return

.[[Value]].

6. Return

undefined

The following steps are taken:

1. Let

be the

this

value.

(

, [[WeakMapData]]).

3. Let

entries

be the

.[[WeakMapData]].

4. If

(

key

) is not Object, return

false

{ [[Key]], [[Value]] }

entries

, do

a. If

.[[Key]] is not

empty

and

.[[Key]],

key

) is

true

, return

true

6. Return

false

The following steps are taken:

1. Let

be the

this

value.

(

, [[WeakMapData]]).

3. Let

entries

be the

.[[WeakMapData]].

The value

empty

is used as a specification device to indicate that an entry has been deleted.

Actual implementations may take other actions such as physically removing the entry from
internal data structures.

24.3.3.3 WeakMap.prototype.get (

key

)

24.3.3.4 WeakMap.prototype.has (

key

)

24.3.3.5 WeakMap.prototype.set (

key

value

)

704

4. If

(

key

) is not Object, throw a

TypeError

exception.

{ [[Key]], [[Value]] }

entries

, do

a. If

.[[Key]] is not

empty

and

.[[Key]],

key

) is

true

, then

i. Set

.[[Value]] to

value

ii. Return

6. Let

be the

{ [[Key]]:

key

, [[Value]]:

value

7. Append

as the last element of

entries

8. Return

property is the String value

"WeakMap"

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

false

, [[Enumerable]]:

false

, [[Configurable]]:

true

WeakMap instances are ordinary objects that inherit properties from the WeakMap prototype. WeakMap instances
also have a [[WeakMapData]] internal slot.

WeakSet objects are collections of objects. A distinct object may only occur once as an element of a WeakSet's
collection. A WeakSet may be queried to see if it contains a specific object, but no mechanism is provided for
enumerating the objects it holds. In certain conditions, objects which are not

live

are removed as WeakSet elements, as

described in

9.9.3

An implementation may impose an arbitrarily determined latency between the time an object contained in a WeakSet
becomes inaccessible and the time when the object is removed from the WeakSet. If this latency was observable to
ECMAScript program, it would be a source of indeterminacy that could impact program execution. For that reason,
an ECMAScript implementation must not provide any means to determine if a WeakSet contains a particular object
that does not require the observer to present the observed object.

WeakSet objects must be implemented using either hash tables or other mechanisms that, on average, provide access
times that are sublinear on the number of elements in the collection. The data structure used in this WeakSet objects
specification is only intended to describe the required observable semantics of WeakSet objects. It is not intended to be
a viable implementation model.

NOTE

The WeakSet

%WeakSet%

is the initial value of the

"WeakSet"

property of the

creates and initializes a new WeakSet object when called as a

See the NOTE in

24.3

24.3.3.6 WeakMap.prototype [ @@toStringTag ]

24.3.4 Properties of WeakMap Instances

24.4 WeakSet Objects

24.4.1 The WeakSet Constructor

705

is not intended to be called as a function and will throw an exception when called in that manner.
is designed to be subclassable. It may be used as the value in an

extends

clause of a class definition. Subclass

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

super

call to the WeakSet

to create and initialize the subclass instance with the internal state necessary to support the

WeakSet.prototype

built-in methods.

When the

WeakSet

function is called with optional argument

iterable

, the following steps are taken:

1. If NewTarget is

undefined

, throw a

TypeError

exception.

2. Let

set

OrdinaryCreateFromConstructor

(NewTarget,

"%WeakSet.prototype%"

, « [[WeakSetData]] »).

3. Set

set

.[[WeakSetData]] to a new empty

4. If

iterable

is either

undefined

null

, return

set

5. Let

adder

Get

set

"add"

6. If

IsCallable

adder

) is

false

, throw a

TypeError

exception.

7. Let

iteratorRecord

GetIterator

(

iterable

8. Repeat,

a. Let

next

IteratorStep

(

iteratorRecord

b. If

next

false

, return

set

c. Let

nextValue

be ?

IteratorValue

(

next

d. Let

status

Call

adder

set

, «

nextValue

»).

e. If

status

, return ?

iteratorRecord

status

The WeakSet

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

%Function.prototype%

has the following properties:

The initial value of

WeakSet.prototype

is the

WeakSet prototype object

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

false

, [[Enumerable]]:

false

, [[Configurable]]:

false

The

WeakSet prototype object

%WeakSet.prototype%

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

%Object.prototype%

is an

ordinary object

does not have a [[WeakSetData]] internal slot.

The following steps are taken:

24.4.1.1 WeakSet ( [

iterable

] )

24.4.2 Properties of the WeakSet Constructor

24.4.2.1 WeakSet.prototype

24.4.3 Properties of the WeakSet Prototype Object

24.4.3.1 WeakSet.prototype.add (

value

)

706

1. Let

be the

this

value.

(

, [[WeakSetData]]).

3. If

(

value

) is not Object, throw a

TypeError

exception.

4. Let

entries

be the

.[[WeakSetData]].

5. For each element

entries

, do

a. If

is not

empty

and

value

) is

true

, then

i. Return

6. Append

value

as the last element of

entries

7. Return

The initial value of

WeakSet.prototype.constructor

is the

%WeakSet%

intrinsic object.

The following steps are taken:

1. Let

be the

this

value.

(

, [[WeakSetData]]).

3. If

(

value

) is not Object, return

false

4. Let

entries

be the

.[[WeakSetData]].

5. For each element

entries

, do

a. If

is not

empty

and

value

) is

true

, then

i. Replace the element of

entries

whose value is

with an element whose value is

empty

ii. Return

true

6. Return

false

NOTE

The following steps are taken:

1. Let

be the

this

value.

(

, [[WeakSetData]]).

3. Let

entries

be the

.[[WeakSetData]].

4. If

(

value

) is not Object, return

false

5. For each element

entries

, do

a. If

is not

empty

and

value

) is

true

, return

true

6. Return

false

The descriptions below in this section,

property is the String value

"WeakSet"

The value

empty

is used as a specification device to indicate that an entry has been deleted.

Actual implementations may take other actions such as physically removing the entry from
internal data structures.

24.4.3.2 WeakSet.prototype.constructor

24.4.3.3 WeakSet.prototype.delete (

value

)

24.4.3.4 WeakSet.prototype.has (

value

)

24.4.3.5 WeakSet.prototype [ @@toStringTag ]

707

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

false

, [[Enumerable]]:

false

, [[Configurable]]:

true

WeakSet instances are ordinary objects that inherit properties from the WeakSet prototype. WeakSet instances also
have a [[WeakSetData]] internal slot.

25.4

, and

use the read-modify-write modification function internal data

structure.

read-modify-write modification function

is a mathematical function that is notationally represented as an abstract

closure that takes two Lists of byte values as arguments and returns a

of byte values. These abstract closures

satisfy all of the following properties:

They perform all their algorithm steps atomically.
Their individual algorithm steps are not observable.

NOTE

The abstract operation AllocateArrayBuffer takes arguments

constructor

and

byteLength

(a non-negative

). It is

used to create an ArrayBuffer object. It performs the following steps when called:

1. Let

obj

OrdinaryCreateFromConstructor

(

constructor

"%ArrayBuffer.prototype%"

, « [[ArrayBufferData]],

[[ArrayBufferByteLength]], [[ArrayBufferDetachKey]] »).

2. Let

block

CreateByteDataBlock

byteLength

3. Set

obj

.[[ArrayBufferData]] to

block

4. Set

obj

.[[ArrayBufferByteLength]] to

byteLength

5. Return

obj

To aid verifying that a read-modify-write modification function's algorithm steps constitute a
pure, mathematical function, the following editorial conventions are recommended:

They do not access, directly or transitively via invoked

abstract operations

and abstract

closures, any language or specification values except their parameters and captured
values.
They do not return completion values.

24.4.4 Properties of WeakSet Instances

25 Structured Data

25.1 ArrayBuffer Objects

25.1.1 Notation

25.1.2 Abstract Operations For ArrayBuffer Objects

25.1.2.1 AllocateArrayBuffer (

constructor

byteLength

)

708

The abstract operation IsDetachedBuffer takes argument

arrayBuffer

. It performs the following steps when called:

arrayBuffer

) is Object and it has an [[ArrayBufferData]] internal slot.

2. If

arrayBuffer

.[[ArrayBufferData]] is

null

, return

true

3. Return

false

The abstract operation DetachArrayBuffer takes argument

arrayBuffer

and optional argument

key

. It performs the

following steps when called:

arrayBuffer

) is Object and it has [[ArrayBufferData]], [[ArrayBufferByteLength]], and

[[ArrayBufferDetachKey]] internal slots.

(

arrayBuffer

) is

false

3. If

key

is not present, set

key

undefined

4. If

arrayBuffer

.[[ArrayBufferDetachKey]],

key

) is

false

, throw a

TypeError

exception.

5. Set

arrayBuffer

.[[ArrayBufferData]] to

null

6. Set

arrayBuffer

.[[ArrayBufferByteLength]] to 0.

7. Return

NormalCompletion

(

null

NOTE

The abstract operation CloneArrayBuffer takes arguments

srcBuffer

(an ArrayBuffer object),

srcByteOffset

(a non-

negative

srcLength

cloneConstructor

). It creates a new ArrayBuffer

whose data is a copy of

srcBuffer

's data over the range starting at

srcByteOffset

and continuing for

srcLength

bytes. It

performs the following steps when called:

srcBuffer

) is Object and it has an [[ArrayBufferData]] internal slot.

IsConstructor

(

cloneConstructor

) is

true

3. Let

targetBuffer

be ?

AllocateArrayBuffer

(

cloneConstructor

srcLength

4. If

(

srcBuffer

) is

true

, throw a

TypeError

exception.

5. Let

srcBlock

srcBuffer

.[[ArrayBufferData]].

6. Let

targetBlock

targetBuffer

.[[ArrayBufferData]].

7. Perform

CopyDataBlockBytes

targetBlock

, 0,

srcBlock

srcByteOffset

srcLength

8. Return

targetBuffer

The abstract operation IsUnsignedElementType takes argument

type

. It verifies if the argument

type

is an unsigned

. It performs the following steps when called:

1. If

type

Uint8

Uint8C

Uint16

Uint32

, or

BigUint64

, return

true

Detaching an ArrayBuffer instance disassociates the

Data Block

used as its backing store from the

instance and sets the byte length of the buffer to 0. No operations defined by this specification
use the DetachArrayBuffer abstract operation. However, an ECMAScript

host

or implementation

may define such operations.

25.1.2.2 IsDetachedBuffer (

arrayBuffer

)

25.1.2.3 DetachArrayBuffer (

arrayBuffer

[ ,

key

] )

25.1.2.4 CloneArrayBuffer (

srcBuffer

srcByteOffset

srcLength

cloneConstructor

)

25.1.2.5 IsUnsignedElementType (

type

)

709

2. Return

false

The abstract operation IsUnclampedIntegerElementType takes argument

type

. It verifies if the argument

type

is an

Integer

not including

Uint8C

. It performs the following steps when called:

1. If

type

Int8

Uint8

Int16

Uint16

Int32

, or

Uint32

, return

true

2. Return

false

The abstract operation IsBigIntElementType takes argument

type

. It verifies if the argument

type

is a BigInt

IsUnclampedIntegerElementType

. It performs the following steps when called:

1. If

type

BigUint64

BigInt64

, return

true

2. Return

false

The abstract operation IsNoTearConfiguration takes arguments

type

and

order

. It performs the following steps when

called:

1. If !

type

) is

true

, return

true

2. If !

(

type

) is

true

and

order

is not

Init

Unordered

, return

true

3. Return

false

The abstract operation RawBytesToNumeric takes arguments

type

rawBytes

), and

isLittleEndian

(a Boolean). It performs the following steps when called:

1. Let

elementSize

type

2. If

isLittleEndian

false

, reverse the order of the elements of

rawBytes

3. If

type

Float32

, then

a. Let

value

be the byte elements of

rawBytes

concatenated and interpreted as a little-endian bit string

encoding of an

binary32 value.

b. If

value

is an

binary32 NaN value, return the

NaN

c. Return the

that corresponds to

value

4. If

type

Float64

, then

a. Let

value

be the byte elements of

rawBytes

concatenated and interpreted as a little-endian bit string

encoding of an

binary64 value.

b. If

value

is an

binary64 NaN value, return the

NaN

c. Return the

that corresponds to

value

5. If !

IsUnsignedElementType

(

type

) is

true

, then

a. Let

intValue

be the byte elements of

rawBytes

concatenated and interpreted as a bit string encoding of an

unsigned little-endian binary number.

6. Else,

a. Let

intValue

be the byte elements of

rawBytes

concatenated and interpreted as a bit string encoding of a

25.1.2.6 IsUnclampedIntegerElementType (

type

)

25.1.2.7 IsBigIntElementType (

type

)

25.1.2.8 IsNoTearConfiguration (

type

order

)

25.1.2.9 RawBytesToNumeric (

type

rawBytes

isLittleEndian

)

710

binary little-endian two's complement number of bit length

elementSize

7. If !

(

type

) is

true

, return the BigInt value that corresponds to

intValue

8. Otherwise, return the

that corresponds to

intValue

The abstract operation GetValueFromBuffer takes arguments

arrayBuffer

(an ArrayBuffer or SharedArrayBuffer),

byteIndex

(a non-negative

type

isTypedArray

(a Boolean), and

order

(either

SeqCst

Unordered

) and optional argument

isLittleEndian

(a Boolean). It performs the following steps when called:

arrayBuffer

) is

false

: There are sufficient bytes in

arrayBuffer

starting at

byteIndex

to represent a value of

type

3. Let

block

arrayBuffer

.[[ArrayBufferData]].

4. Let

elementSize

type

5. If

arrayBuffer

) is

true

, then

a. Let

execution

be the [[CandidateExecution]] field of the

surrounding agent

b. Let

eventList

be the [[EventList]] field of the element in

execution

.[[EventsRecords]] whose

[[AgentSignifier]] is

AgentSignifier

().

c. If

isTypedArray

true

and

IsNoTearConfiguration

(

type

order

) is

true

, let

noTear

true

; otherwise let

noTear

false

d. Let

rawValue

be a

of length

elementSize

whose elements are nondeterministically chosen byte values.

e. NOTE: In implementations,

rawValue

is the result of a non-atomic or atomic read instruction on the

underlying hardware. The nondeterminism is a semantic prescription of the

memory model

to describe

observable behaviour of hardware with weak consistency.

f. Let

readEvent

ReadSharedMemory

{ [[Order]]:

order

, [[NoTear]]:

noTear

, [[Block]]:

block

, [[ByteIndex]]:

byteIndex

, [[ElementSize]]:

elementSize

g. Append

readEvent

eventList

h. Append

Chosen Value Record

{ [[Event]]:

readEvent

, [[ChosenValue]]:

rawValue

} to

execution

[[ChosenValues]].

6. Else, let

rawValue

be a

whose elements are bytes from

block

at indices

byteIndex

(inclusive) through

byteIndex

elementSize

(exclusive).

: The number of elements in

rawValue

elementSize

8. If

is not present, set

to the value of the [[LittleEndian]] field of the

9. Return

(

type

rawValue

isLittleEndian

The abstract operation NumericToRawBytes takes arguments

type

value

(a BigInt or a

Number), and

isLittleEndian

(a Boolean). It performs the following steps when called:

1. If

type

Float32

, then

a. Let

rawBytes

be a

whose elements are the 4 bytes that are the result of converting

value

IEEE 754-

2019

binary32 format using roundTiesToEven mode. If

, the bytes are arranged in

false

big endian order. Otherwise, the bytes are arranged in little endian order. If

value

NaN

rawBytes

may

be set to any implementation chosen

binary32 format Not-a-Number encoding. An

implementation must always choose the same encoding for each implementation distinguishable

NaN

value.

25.1.2.10 GetValueFromBuffer (

arrayBuffer

byteIndex

type

isTypedArray

order

[ ,

isLittleEndian

] )

25.1.2.11 NumericToRawBytes (

type

value

isLittleEndian

)

711

2. Else if

type

Float64

, then

a. Let

rawBytes

be a

whose elements are the 8 bytes that are the

binary64 format

encoding of

value

. If

isLittleEndian

false

, the bytes are arranged in big endian order. Otherwise, the

bytes are arranged in little endian order. If

value

NaN

rawBytes

may be set to any implementation

chosen

binary64 format Not-a-Number encoding. An implementation must always

choose the same encoding for each implementation distinguishable

NaN

value.

3. Else,

a. Let

be the Element Size value specified in

type

b. Let

convOp

be the abstract operation named in the Conversion Operation column in

for

Element Type

type

c. Let

intValue

convOp

(

value

)).

d. If

intValue

≥

0, then

i. Let

rawBytes

be a

whose elements are the

-byte binary encoding of

intValue

. If

isLittleEndian

false

, the bytes are ordered in big endian order. Otherwise, the bytes are ordered in little

endian order.

e. Else,

i. Let

rawBytes

be a

whose elements are the

-byte binary two's complement encoding of

intValue

. If

isLittleEndian

false

, the bytes are ordered in big endian order. Otherwise, the bytes

are ordered in little endian order.

4. Return

rawBytes

The abstract operation SetValueInBuffer takes arguments

arrayBuffer

(an ArrayBuffer or SharedArrayBuffer),

byteIndex

(a non-negative

type

value

(a Number or a BigInt),

isTypedArray

(a Boolean), and

order

(one of

SeqCst

Unordered

, or

Init

) and optional argument

isLittleEndian

(a Boolean). It performs the following

steps when called:

arrayBuffer

) is

false

: There are sufficient bytes in

arrayBuffer

starting at

byteIndex

to represent a value of

type

value

) is BigInt if !

(

type

) is

true

; otherwise,

(

value

) is Number.

4. Let

block

arrayBuffer

.[[ArrayBufferData]].

5. Let

elementSize

type

6. If

is not present, set

to the value of the [[LittleEndian]] field of the

surrounding

agent

7. Let

rawBytes

NumericToRawBytes

(

type

value

isLittleEndian

8. If

arrayBuffer

) is

true

, then

a. Let

execution

be the [[CandidateExecution]] field of the

surrounding agent

b. Let

eventList

be the [[EventList]] field of the element in

execution

.[[EventsRecords]] whose

[[AgentSignifier]] is

AgentSignifier

().

c. If

isTypedArray

true

and

IsNoTearConfiguration

(

type

order

) is

true

, let

noTear

true

; otherwise let

noTear

false

d. Append

WriteSharedMemory

{ [[Order]]:

order

, [[NoTear]]:

noTear

, [[Block]]:

block

, [[ByteIndex]]:

byteIndex

, [[ElementSize]]:

elementSize

, [[Payload]]:

rawBytes

} to

eventList

9. Else, store the individual bytes of

rawBytes

into

block

, starting at

block

[

byteIndex

10. Return

NormalCompletion

(

undefined

25.1.2.12 SetValueInBuffer (

arrayBuffer

byteIndex

type

value

isTypedArray

order

[ ,

isLittleEndian

] )

25.1.2.13 GetModifySetValueInBuffer (

arrayBuffer

byteIndex

type

value

[ ,

isLittleEndian

] )

712

The abstract operation GetModifySetValueInBuffer takes arguments

arrayBuffer

(an ArrayBuffer object or a

SharedArrayBuffer object),

byteIndex

(a non-negative

type

read-modify-write modification function

value

(a Number or a

BigInt), and

) and optional argument

isLittleEndian

(a Boolean). It

performs the following steps when called:

arrayBuffer

) is

false

: There are sufficient bytes in

arrayBuffer

starting at

byteIndex

to represent a value of

type

value

) is BigInt if !

(

type

) is

true

; otherwise,

(

value

) is Number.

4. Let

block

arrayBuffer

.[[ArrayBufferData]].

5. Let

elementSize

type

6. If

is not present, set

to the value of the [[LittleEndian]] field of the

surrounding

agent

7. Let

rawBytes

NumericToRawBytes

(

type

value

isLittleEndian

8. If

arrayBuffer

) is

true

, then

a. Let

execution

be the [[CandidateExecution]] field of the

surrounding agent

b. Let

eventList

be the [[EventList]] field of the element in

execution

.[[EventsRecords]] whose

[[AgentSignifier]] is

AgentSignifier

().

c. Let

rawBytesRead

be a

ReadModifyWriteSharedMemory

of length

elementSize

whose elements are nondeterministically chosen byte

values.

d. NOTE: In implementations,

rawBytesRead

is the result of a load-link, of a load-exclusive, or of an

operand of a read-modify-write instruction on the underlying hardware. The nondeterminism is a
semantic prescription of the

memory model

to describe observable behaviour of hardware with weak

consistency.

e. Let

rmwEvent

{ [[Order]]:

SeqCst

, [[NoTear]]:

true

, [[Block]]:

block

[[ByteIndex]]:

byteIndex

, [[ElementSize]]:

elementSize

, [[Payload]]:

rawBytes

, [[ModifyOp]]:

f. Append

rmwEvent

eventList

g. Append

Chosen Value Record

{ [[Event]]:

rmwEvent

, [[ChosenValue]]:

rawBytesRead

} to

execution

[[ChosenValues]].

9. Else,

a. Let

rawBytesRead

be a

of length

elementSize

whose elements are the sequence of

elementSize

bytes

starting with

block

[

byteIndex

b. Let

rawBytesModified

(

rawBytesRead

rawBytes

c. Store the individual bytes of

rawBytesModified

into

block

, starting at

block

[

byteIndex

10. Return

RawBytesToNumeric

(

type

rawBytesRead

isLittleEndian

The ArrayBuffer

%ArrayBuffer%

is the initial value of the

"ArrayBuffer"

property of the

creates and initializes a new ArrayBuffer object when called as a

is not intended to be called as a function and will throw an exception when called in that manner.
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 ArrayBuffer behaviour must include a

super

call to the

ArrayBuffer

to create and initialize subclass instances with the internal state necessary to support

the

ArrayBuffer.prototype

built-in methods.

25.1.3 The ArrayBuffer Constructor

713

When the

ArrayBuffer

function is called with argument

length

, the following steps are taken:

1. If NewTarget is

undefined

, throw a

TypeError

exception.

2. Let

byteLength

(

length

3. Return ?

AllocateArrayBuffer

(NewTarget,

byteLength

The ArrayBuffer

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

%Function.prototype%

has the following properties:

The

isView

function takes one argument

arg

, and performs the following steps:

1. If

ArrayBuffer prototype object

(

arg

) is not Object, return

false

2. If

arg

has a [[ViewedArrayBuffer]] internal slot, return

true

3. Return

false

The initial value of

ArrayBuffer.prototype

is the

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

false

, [[Enumerable]]:

false

, [[Configurable]]:

false

ArrayBuffer[@@species]

is an

whose set accessor function is

undefined

. Its get accessor

function performs the following steps:

1. Return the

this

value.

The value of the

"name"

property of this function is

"get [Symbol.species]"

NOTE

The

ArrayBuffer prototype object

%ArrayBuffer.prototype%

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

%Object.prototype%

is an

ordinary object

does not have an [[ArrayBufferData]] or [[ArrayBufferByteLength]] internal slot.

ArrayBuffer prototype methods normally use their

this

value's

to create a derived

object. However, a subclass

may over-ride that default behaviour by redefining its

@@species

property.

25.1.3.1 ArrayBuffer (

length

)

25.1.4 Properties of the ArrayBuffer Constructor

25.1.4.1 ArrayBuffer.isView (

arg

)

25.1.4.2 ArrayBuffer.prototype

25.1.4.3 get ArrayBuffer [ @@species ]

25.1.5 Properties of the ArrayBuffer Prototype Object

714

ArrayBuffer.prototype.byteLength

is an

whose set accessor function is

undefined

. Its

get accessor function performs the following steps:

1. Let

be the

this

value.

(

, [[ArrayBufferData]]).

3. If

) is

true

, throw a

TypeError

exception.

4. If

(

) is

true

, return

𝔽

5. Let

length

.[[ArrayBufferByteLength]].

6. Return

length

The initial value of

ArrayBuffer.prototype.constructor

%ArrayBuffer%

The following steps are taken:

1. Let

be the

this

value.

(

, [[ArrayBufferData]]).

3. If

) is

true

, throw a

TypeError

exception.

4. If

(

) is

true

, throw a

TypeError

exception.

5. Let

len

.[[ArrayBufferByteLength]].

6. Let

relativeStart

start

7. If

relativeStart

is -

∞

, let

first

be 0.

8. Else if

relativeStart

< 0, let

first

(

len

relativeStart

, 0).

9. Else, let

first

relativeStart

len

10. If

end

undefined

, let

relativeEnd

len

; else let

relativeEnd

(

end

11. If

relativeEnd

is -

∞

, let

final

be 0.

12. Else if

relativeEnd

< 0, let

final

len

relativeEnd

, 0).

13. Else, let

final

(

relativeEnd

len

14. Let

newLen

final

first

, 0).

15. Let

ctor

SpeciesConstructor

(

%ArrayBuffer%

16. Let

new

be ?

Construct

ctor

, «

(

newLen

) »).

17. Perform ?

(

new

, [[ArrayBufferData]]).

18. If

new

) is

true

, throw a

TypeError

exception.

19. If

(

new

) is

true

, throw a

TypeError

exception.

20. If

new

) is

true

, throw a

TypeError

exception.

21. If

new

.[[ArrayBufferByteLength]] <

newLen

, throw a

TypeError

exception.

22. NOTE: Side-effects of the above steps may have detached

23. If

(

) is

true

, throw a

TypeError

exception.

24. Let

fromBuf

.[[ArrayBufferData]].

25. Let

toBuf

new

.[[ArrayBufferData]].

26. Perform

CopyDataBlockBytes

toBuf

, 0,

fromBuf

first

newLen

27. Return

new

25.1.5.1 get ArrayBuffer.prototype.byteLength

25.1.5.2 ArrayBuffer.prototype.constructor

25.1.5.3 ArrayBuffer.prototype.slice (

start

end

)

25.1.5.4 ArrayBuffer.prototype [ @@toStringTag ]

715

ArrayBuffer prototype object

property is the String value

"ArrayBuffer"

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

false

, [[Enumerable]]:

false

, [[Configurable]]:

true

ArrayBuffer instances inherit properties from the

. ArrayBuffer instances each have an

[[ArrayBufferData]] internal slot, an [[ArrayBufferByteLength]] internal slot, and an [[ArrayBufferDetachKey]]
internal slot.

ArrayBuffer instances whose [[ArrayBufferData]] is

null

are considered to be detached and all operators to access or

modify data contained in the ArrayBuffer instance will fail.

ArrayBuffer instances whose [[ArrayBufferDetachKey]] is set to a value other than

undefined

need to have all

DetachArrayBuffer

calls passing that same "detach key" as an argument, otherwise a TypeError will result. This

internal slot is only ever set by certain embedding environments, not by algorithms in this specification.

The abstract operation AllocateSharedArrayBuffer takes arguments

constructor

and

byteLength

(a non-negative

). It is used to create a SharedArrayBuffer object. It performs the following steps when called:

1. Let

obj

OrdinaryCreateFromConstructor

(

constructor

"%SharedArrayBuffer.prototype%"

, «

[[ArrayBufferData]], [[ArrayBufferByteLength]] »).

2. Let

block

CreateSharedByteDataBlock

byteLength

3. Set

obj

.[[ArrayBufferData]] to

block

4. Set

obj

.[[ArrayBufferByteLength]] to

byteLength

5. Return

obj

The abstract operation IsSharedArrayBuffer takes argument

obj

. It tests whether an object is an ArrayBuffer, a

SharedArrayBuffer, or a subtype of either. It performs the following steps when called:

obj

) is Object and it has an [[ArrayBufferData]] internal slot.

2. Let

bufferData

obj

.[[ArrayBufferData]].

3. If

bufferData

null

, return

false

4. If

bufferData

is a

Data Block

, return

false

bufferData

is a

Shared Data Block

6. Return

true

The SharedArrayBuffer

25.1.6 Properties of ArrayBuffer Instances

25.2 SharedArrayBuffer Objects

25.2.1 Abstract Operations for SharedArrayBuffer Objects

25.2.1.1 AllocateSharedArrayBuffer (

constructor

byteLength

)

25.2.1.2 IsSharedArrayBuffer (

obj

)

25.2.2 The SharedArrayBuffer Constructor

716

%SharedArrayBuffer%

is the initial value of the

"SharedArrayBuffer"

property of the

, if that property is present (see

below).
creates and initializes a new SharedArrayBuffer object when called as a

is not intended to be called as a function and will throw an exception when called in that manner.
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 SharedArrayBuffer behaviour must include a

super

call to the

SharedArrayBuffer

to create and initialize subclass instances with the internal state necessary to

support the

SharedArrayBuffer.prototype

built-in methods.

Whenever a

host

does not provide concurrent access to SharedArrayBuffer objects it may omit the

"SharedArrayBuffer"

property of the

NOTE

When the

SharedArrayBuffer

function is called with optional argument

length

, the following steps are taken:

1. If NewTarget is

undefined

, throw a

TypeError

exception.

2. Let

byteLength

AllocateSharedArrayBuffer

(

length

3. Return ?

(NewTarget,

byteLength

The SharedArrayBuffer

SharedArrayBuffer prototype object

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

%Function.prototype%

has the following properties:

The initial value of

SharedArrayBuffer.prototype

is the

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

false

, [[Enumerable]]:

false

, [[Configurable]]:

false

SharedArrayBuffer[@@species]

is an

whose set accessor function is

undefined

. Its get

accessor function performs the following steps:

1. Return the

this

value.

The value of the

"name"

property of this function is

"get [Symbol.species]"

The

SharedArrayBuffer prototype object

Unlike an

ArrayBuffer

, a

SharedArrayBuffer

cannot become detached, and its

internal [[ArrayBufferData]] slot is never

null

25.2.2.1 SharedArrayBuffer ( [

length

] )

25.2.3 Properties of the SharedArrayBuffer Constructor

25.2.3.1 SharedArrayBuffer.prototype

25.2.3.2 get SharedArrayBuffer [ @@species ]

25.2.4 Properties of the SharedArrayBuffer Prototype Object

717

%SharedArrayBuffer.prototype%

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

%Object.prototype%

is an

ordinary object

does not have an [[ArrayBufferData]] or [[ArrayBufferByteLength]] internal slot.

SharedArrayBuffer.prototype.byteLength

is an

whose set accessor function is

undefined

. Its get accessor function performs the following steps:

1. Let

be the

this

value.

(

, [[ArrayBufferData]]).

3. If

) is

false

, throw a

TypeError

exception.

4. Let

length

.[[ArrayBufferByteLength]].

5. Return

length

The initial value of

SharedArrayBuffer.prototype.constructor

%SharedArrayBuffer%

The following steps are taken:

1. Let

be the

this

value.

(

, [[ArrayBufferData]]).

3. If

) is

false

, throw a

TypeError

exception.

4. Let

len

.[[ArrayBufferByteLength]].

5. Let

relativeStart

start

6. If

relativeStart

is -

∞

, let

first

be 0.

7. Else if

relativeStart

< 0, let

first

(

len

relativeStart

, 0).

8. Else, let

first

relativeStart

len

9. If

end

undefined

, let

relativeEnd

len

; else let

relativeEnd

(

end

10. If

relativeEnd

is -

∞

, let

final

be 0.

11. Else if

relativeEnd

< 0, let

final

len

relativeEnd

, 0).

12. Else, let

final

(

relativeEnd

len

13. Let

newLen

final

first

, 0).

14. Let

ctor

SpeciesConstructor

(

%SharedArrayBuffer%

15. Let

new

be ?

Construct

ctor

, «

(

newLen

) »).

16. Perform ?

(

new

, [[ArrayBufferData]]).

17. If

new

) is

false

, throw a

TypeError

exception.

18. If

new

.[[ArrayBufferData]] and

.[[ArrayBufferData]] are the same

Shared Data Block

values, throw a

TypeError

exception.

19. If

new

.[[ArrayBufferByteLength]] <

newLen

, throw a

TypeError

exception.

20. Let

fromBuf

.[[ArrayBufferData]].

21. Let

toBuf

new

.[[ArrayBufferData]].

22. Perform

CopyDataBlockBytes

toBuf

, 0,

fromBuf

first

newLen

23. Return

new

25.2.4.1 get SharedArrayBuffer.prototype.byteLength

25.2.4.2 SharedArrayBuffer.prototype.constructor

25.2.4.3 SharedArrayBuffer.prototype.slice (

start

end

)

718

SharedArrayBuffer prototype object

property is the String value

"SharedArrayBuffer"

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

false

, [[Enumerable]]:

false

, [[Configurable]]:

true

SharedArrayBuffer instances inherit properties from the

. SharedArrayBuffer

instances each have an [[ArrayBufferData]] internal slot and an [[ArrayBufferByteLength]] internal slot.

NOTE

The abstract operation GetViewValue takes arguments

view

requestIndex

isLittleEndian

, and

type

. It is used by

functions on DataView instances to retrieve values from the view's buffer. It performs the following steps when called:

1. Perform ?

(

view

, [[DataView]]).

view

has a [[ViewedArrayBuffer]] internal slot.

3. Let

getIndex

(

requestIndex

4. Set

isLittleEndian

to !

ToBoolean

isLittleEndian

5. Let

buffer

view

.[[ViewedArrayBuffer]].

6. If

(

buffer

) is

true

, throw a

TypeError

exception.

7. Let

viewOffset

view

.[[ByteOffset]].

8. Let

viewSize

view

.[[ByteLength]].

9. Let

elementSize

type

10. If

getIndex

elementSize

viewSize

, throw a

RangeError

exception.

11. Let

bufferIndex

getIndex

viewOffset

12. Return

GetValueFromBuffer

buffer

bufferIndex

type

false

Unordered

isLittleEndian

The abstract operation SetViewValue takes arguments

view

requestIndex

isLittleEndian

type

, and

value

. It is used by

functions on DataView instances to store values into the view's buffer. It performs the following steps when called:

1. Perform ?

(

view

, [[DataView]]).

view

has a [[ViewedArrayBuffer]] internal slot.

3. Let

getIndex

(

requestIndex

4. If !

(

type

) is

true

, let

numberValue

be ?

ToBigInt

(

value

5. Otherwise, let

numberValue

ToNumber

(

value

6. Set

isLittleEndian

to !

ToBoolean