ECMAScript® 2024 Language Specification

Draft ECMA-262 / February 15, 2024

25.1 ArrayBuffer Objects

25.1.1 Notation

The descriptions below in this section, 25.4, and 29 use the read-modify-write modification function internal data structure.

A 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 List 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

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 Records.

25.1.2 Fixed-length and Resizable ArrayBuffer Objects

A fixed-length ArrayBuffer is an ArrayBuffer whose byte length cannot change after creation.

A resizable ArrayBuffer is an ArrayBuffer whose byte length may change after creation via calls to ArrayBuffer.prototype.resize ( newLength ).

The kind of ArrayBuffer object that is created depends on the arguments passed to ArrayBuffer ( length [ , options ] ).

25.1.3 Abstract Operations For ArrayBuffer Objects

25.1.3.1 AllocateArrayBuffer ( constructor, byteLength [ , maxByteLength ] )

The abstract operation AllocateArrayBuffer takes arguments constructor (a constructor) and byteLength (a non-negative integer) and optional argument maxByteLength (a non-negative integer or empty) and returns either a normal completion containing an ArrayBuffer or a throw completion. It is used to create an ArrayBuffer. It performs the following steps when called:

  1. Let slots be « [[ArrayBufferData]], [[ArrayBufferByteLength]], [[ArrayBufferDetachKey]] ».
  2. If maxByteLength is present and maxByteLength is not empty, let allocatingResizableBuffer be true; otherwise let allocatingResizableBuffer be false.
  3. If allocatingResizableBuffer is true, then
    1. If byteLength > maxByteLength, throw a RangeError exception.
    2. Append [[ArrayBufferMaxByteLength]] to slots.
  4. Let obj be ? OrdinaryCreateFromConstructor(constructor, "%ArrayBuffer.prototype%", slots).
  5. Let block be ? CreateByteDataBlock(byteLength).
  6. Set obj.[[ArrayBufferData]] to block.
  7. Set obj.[[ArrayBufferByteLength]] to byteLength.
  8. If allocatingResizableBuffer is true, then
    1. If it is not possible to create a Data Block block consisting of maxByteLength bytes, throw a RangeError exception.
    2. NOTE: Resizable ArrayBuffers are designed to be implementable with in-place growth. Implementations may throw if, for example, virtual memory cannot be reserved up front.
    3. Set obj.[[ArrayBufferMaxByteLength]] to maxByteLength.
  9. Return obj.

25.1.3.2 ArrayBufferByteLength ( arrayBuffer, order )

The abstract operation ArrayBufferByteLength takes arguments arrayBuffer (an ArrayBuffer or SharedArrayBuffer) and order (seq-cst or unordered) and returns a non-negative integer. It performs the following steps when called:

  1. If IsSharedArrayBuffer(arrayBuffer) is true and arrayBuffer has an [[ArrayBufferByteLengthData]] internal slot, then
    1. Let bufferByteLengthBlock be arrayBuffer.[[ArrayBufferByteLengthData]].
    2. Let rawLength be GetRawBytesFromSharedBlock(bufferByteLengthBlock, 0, biguint64, true, order).
    3. Let isLittleEndian be the value of the [[LittleEndian]] field of the surrounding agent's Agent Record.
    4. Return ℝ(RawBytesToNumeric(biguint64, rawLength, isLittleEndian)).
  2. Assert: IsDetachedBuffer(arrayBuffer) is false.
  3. Return arrayBuffer.[[ArrayBufferByteLength]].

25.1.3.3 ArrayBufferCopyAndDetach ( arrayBuffer, newLength, preserveResizability )

The abstract operation ArrayBufferCopyAndDetach takes arguments arrayBuffer (an ECMAScript language value), newLength (an ECMAScript language value), and preserveResizability (preserve-resizability or fixed-length) and returns either a normal completion containing an ArrayBuffer or a throw completion. It performs the following steps when called:

  1. Perform ? RequireInternalSlot(arrayBuffer, [[ArrayBufferData]]).
  2. If IsSharedArrayBuffer(arrayBuffer) is true, throw a TypeError exception.
  3. If newLength is undefined, then
    1. Let newByteLength be arrayBuffer.[[ArrayBufferByteLength]].
  4. Else,
    1. Let newByteLength be ? ToIndex(newLength).
  5. If IsDetachedBuffer(arrayBuffer) is true, throw a TypeError exception.
  6. If preserveResizability is preserve-resizability and IsFixedLengthArrayBuffer(arrayBuffer) is false, then
    1. Let newMaxByteLength be arrayBuffer.[[ArrayBufferMaxByteLength]].
  7. Else,
    1. Let newMaxByteLength be empty.
  8. If arrayBuffer.[[ArrayBufferDetachKey]] is not undefined, throw a TypeError exception.
  9. Let newBuffer be ? AllocateArrayBuffer(%ArrayBuffer%, newByteLength, newMaxByteLength).
  10. Let copyLength be min(newByteLength, arrayBuffer.[[ArrayBufferByteLength]]).
  11. Let fromBlock be arrayBuffer.[[ArrayBufferData]].
  12. Let toBlock be newBuffer.[[ArrayBufferData]].
  13. Perform CopyDataBlockBytes(toBlock, 0, fromBlock, 0, copyLength).
  14. NOTE: Neither creation of the new Data Block nor copying from the old Data Block are observable. Implementations may implement this method as a zero-copy move or a realloc.
  15. Perform ! DetachArrayBuffer(arrayBuffer).
  16. Return newBuffer.

25.1.3.4 IsDetachedBuffer ( arrayBuffer )

The abstract operation IsDetachedBuffer takes argument arrayBuffer (an ArrayBuffer or a SharedArrayBuffer) and returns a Boolean. It performs the following steps when called:

  1. If arrayBuffer.[[ArrayBufferData]] is null, return true.
  2. Return false.

25.1.3.5 DetachArrayBuffer ( arrayBuffer [ , key ] )

The abstract operation DetachArrayBuffer takes argument arrayBuffer (an ArrayBuffer) and optional argument key (anything) and returns either a normal completion containing unused or a throw completion. It performs the following steps when called:

  1. Assert: IsSharedArrayBuffer(arrayBuffer) is false.
  2. If key is not present, set key to undefined.
  3. If arrayBuffer.[[ArrayBufferDetachKey]] is not key, throw a TypeError exception.
  4. Set arrayBuffer.[[ArrayBufferData]] to null.
  5. Set arrayBuffer.[[ArrayBufferByteLength]] to 0.
  6. Return unused.
 Note

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.

25.1.3.6 CloneArrayBuffer ( srcBuffer, srcByteOffset, srcLength )

The abstract operation CloneArrayBuffer takes arguments srcBuffer (an ArrayBuffer or a SharedArrayBuffer), srcByteOffset (a non-negative integer), and srcLength (a non-negative integer) and returns either a normal completion containing an ArrayBuffer or a throw completion. 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:

  1. Assert: IsDetachedBuffer(srcBuffer) is false.
  2. Let targetBuffer be ? AllocateArrayBuffer(%ArrayBuffer%, srcLength).
  3. Let srcBlock be srcBuffer.[[ArrayBufferData]].
  4. Let targetBlock be targetBuffer.[[ArrayBufferData]].
  5. Perform CopyDataBlockBytes(targetBlock, 0, srcBlock, srcByteOffset, srcLength).
  6. Return targetBuffer.

25.1.3.7 GetArrayBufferMaxByteLengthOption ( options )

The abstract operation GetArrayBufferMaxByteLengthOption takes argument options (an ECMAScript language value) and returns either a normal completion containing either a non-negative integer or empty, or a throw completion. It performs the following steps when called:

  1. If options is not an Object, return empty.
  2. Let maxByteLength be ? Get(options, "maxByteLength").
  3. If maxByteLength is undefined, return empty.
  4. Return ? ToIndex(maxByteLength).

25.1.3.8 HostResizeArrayBuffer ( buffer, newByteLength )

The host-defined abstract operation HostResizeArrayBuffer takes arguments buffer (an ArrayBuffer) and newByteLength (a non-negative integer) and returns either a normal completion containing either handled or unhandled, or a throw completion. It gives the host an opportunity to perform implementation-defined resizing of buffer. If the host chooses not to handle resizing of buffer, it may return unhandled for the default behaviour.

The implementation of HostResizeArrayBuffer must conform to the following requirements:

  • The abstract operation does not detach buffer.
  • If the abstract operation completes normally with handled, buffer.[[ArrayBufferByteLength]] is newByteLength.

The default implementation of HostResizeArrayBuffer is to return NormalCompletion(unhandled).

25.1.3.9 IsFixedLengthArrayBuffer ( arrayBuffer )

The abstract operation IsFixedLengthArrayBuffer takes argument arrayBuffer (an ArrayBuffer or a SharedArrayBuffer) and returns a Boolean. It performs the following steps when called:

  1. If arrayBuffer has an [[ArrayBufferMaxByteLength]] internal slot, return false.
  2. Return true.

25.1.3.10 IsUnsignedElementType ( type )

The abstract operation IsUnsignedElementType takes argument type (a TypedArray element type) and returns a Boolean. It verifies if the argument type is an unsigned TypedArray element type. It performs the following steps when called:

  1. If type is one of uint8, uint8clamped, uint16, uint32, or biguint64, return true.
  2. Return false.

25.1.3.11 IsUnclampedIntegerElementType ( type )

The abstract operation IsUnclampedIntegerElementType takes argument type (a TypedArray element type) and returns a Boolean. It verifies if the argument type is an Integer TypedArray element type not including uint8clamped. It performs the following steps when called:

  1. If type is one of int8, uint8, int16, uint16, int32, or uint32, return true.
  2. Return false.

25.1.3.12 IsBigIntElementType ( type )

The abstract operation IsBigIntElementType takes argument type (a TypedArray element type) and returns a Boolean. It verifies if the argument type is a BigInt TypedArray element type. It performs the following steps when called:

  1. If type is either biguint64 or bigint64, return true.
  2. Return false.

25.1.3.13 IsNoTearConfiguration ( type, order )

The abstract operation IsNoTearConfiguration takes arguments type (a TypedArray element type) and order (seq-cst, unordered, or init) and returns a Boolean. It performs the following steps when called:

  1. If IsUnclampedIntegerElementType(type) is true, return true.
  2. If IsBigIntElementType(type) is true and order is neither init nor unordered, return true.
  3. Return false.

25.1.3.14 RawBytesToNumeric ( type, rawBytes, isLittleEndian )

The abstract operation RawBytesToNumeric takes arguments type (a TypedArray element type), rawBytes (a List of byte values), and isLittleEndian (a Boolean) and returns a Number or a BigInt. It performs the following steps when called:

  1. Let elementSize be the Element Size value specified in Table 71 for Element Type type.
  2. If isLittleEndian is false, reverse the order of the elements of rawBytes.
  3. If type is float32, then
    1. Let value be the byte elements of rawBytes concatenated and interpreted as a little-endian bit string encoding of an IEEE 754-2019 binary32 value.
    2. If value is an IEEE 754-2019 binary32 NaN value, return the NaN Number value.
    3. Return the Number value that corresponds to value.
  4. If type is float64, then
    1. Let value be the byte elements of rawBytes concatenated and interpreted as a little-endian bit string encoding of an IEEE 754-2019 binary64 value.
    2. If value is an IEEE 754-2019 binary64 NaN value, return the NaN Number value.
    3. Return the Number value that corresponds to value.
  5. If IsUnsignedElementType(type) is true, then
    1. 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,
    1. Let intValue be the byte elements of rawBytes concatenated and interpreted as a bit string encoding of a binary little-endian two's complement number of bit length elementSize × 8.
  7. If IsBigIntElementType(type) is true, return the BigInt value that corresponds to intValue.
  8. Otherwise, return the Number value that corresponds to intValue.

25.1.3.15 GetRawBytesFromSharedBlock ( block, byteIndex, type, isTypedArray, order )

The abstract operation GetRawBytesFromSharedBlock takes arguments block (a Shared Data Block), byteIndex (a non-negative integer), type (a TypedArray element type), isTypedArray (a Boolean), and order (seq-cst or unordered) and returns a List of byte values. It performs the following steps when called:

  1. Let elementSize be the Element Size value specified in Table 71 for Element Type type.
  2. Let execution be the [[CandidateExecution]] field of the surrounding agent's Agent Record.
  3. Let eventsRecord be the Agent Events Record of execution.[[EventsRecords]] whose [[AgentSignifier]] is AgentSignifier().
  4. If isTypedArray is true and IsNoTearConfiguration(type, order) is true, let noTear be true; otherwise let noTear be false.
  5. Let rawValue be a List of length elementSize whose elements are nondeterministically chosen byte values.
  6. 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.
  7. Let readEvent be ReadSharedMemory { [[Order]]: order, [[NoTear]]: noTear, [[Block]]: block, [[ByteIndex]]: byteIndex, [[ElementSize]]: elementSize }.
  8. Append readEvent to eventsRecord.[[EventList]].
  9. Append Chosen Value Record { [[Event]]: readEvent, [[ChosenValue]]: rawValue } to execution.[[ChosenValues]].
  10. Return rawValue.

25.1.3.16 GetValueFromBuffer ( arrayBuffer, byteIndex, type, isTypedArray, order [ , isLittleEndian ] )

The abstract operation GetValueFromBuffer takes arguments arrayBuffer (an ArrayBuffer or SharedArrayBuffer), byteIndex (a non-negative integer), type (a TypedArray element type), isTypedArray (a Boolean), and order (seq-cst or unordered) and optional argument isLittleEndian (a Boolean) and returns a Number or a BigInt. It performs the following steps when called:

  1. Assert: IsDetachedBuffer(arrayBuffer) is false.
  2. Assert: There are sufficient bytes in arrayBuffer starting at byteIndex to represent a value of type.
  3. Let block be arrayBuffer.[[ArrayBufferData]].
  4. Let elementSize be the Element Size value specified in Table 71 for Element Type type.
  5. If IsSharedArrayBuffer(arrayBuffer) is true, then
    1. Assert: block is a Shared Data Block.
    2. Let rawValue be GetRawBytesFromSharedBlock(block, byteIndex, type, isTypedArray, order).
  6. Else,
    1. Let rawValue be a List whose elements are bytes from block at indices in the interval from byteIndex (inclusive) to byteIndex + elementSize (exclusive).
  7. Assert: The number of elements in rawValue is elementSize.
  8. If isLittleEndian is not present, set isLittleEndian to the value of the [[LittleEndian]] field of the surrounding agent's Agent Record.
  9. Return RawBytesToNumeric(type, rawValue, isLittleEndian).

25.1.3.17 NumericToRawBytes ( type, value, isLittleEndian )

The abstract operation NumericToRawBytes takes arguments type (a TypedArray element type), value (a Number or a BigInt), and isLittleEndian (a Boolean) and returns a List of byte values. It performs the following steps when called:

  1. If type is float32, then
    1. Let rawBytes be a List whose elements are the 4 bytes that are the result of converting value to IEEE 754-2019 binary32 format using roundTiesToEven mode. The bytes are arranged in little endian order. If value is NaN, rawBytes may be set to any implementation chosen IEEE 754-2019 binary32 format Not-a-Number encoding. An implementation must always choose the same encoding for each implementation distinguishable NaN value.
  2. Else if type is float64, then
    1. Let rawBytes be a List whose elements are the 8 bytes that are the IEEE 754-2019 binary64 format encoding of value. The bytes are arranged in little endian order. If value is NaN, rawBytes may be set to any implementation chosen IEEE 754-2019 binary64 format Not-a-Number encoding. An implementation must always choose the same encoding for each implementation distinguishable NaN value.
  3. Else,
    1. Let n be the Element Size value specified in Table 71 for Element Type type.
    2. Let convOp be the abstract operation named in the Conversion Operation column in Table 71 for Element Type type.
    3. Let intValue be ℝ(convOp(value)).
    4. If intValue ≥ 0, then
      1. Let rawBytes be a List whose elements are the n-byte binary encoding of intValue. The bytes are ordered in little endian order.
    5. Else,
      1. Let rawBytes be a List whose elements are the n-byte binary two's complement encoding of intValue. The bytes are ordered in little endian order.
  4. If isLittleEndian is false, reverse the order of the elements of rawBytes.
  5. Return rawBytes.

25.1.3.18 SetValueInBuffer ( arrayBuffer, byteIndex, type, value, isTypedArray, order [ , isLittleEndian ] )

The abstract operation SetValueInBuffer takes arguments arrayBuffer (an ArrayBuffer or SharedArrayBuffer), byteIndex (a non-negative integer), type (a TypedArray element type), value (a Number or a BigInt), isTypedArray (a Boolean), and order (seq-cst, unordered, or init) and optional argument isLittleEndian (a Boolean) and returns unused. It performs the following steps when called:

  1. Assert: IsDetachedBuffer(arrayBuffer) is false.
  2. Assert: There are sufficient bytes in arrayBuffer starting at byteIndex to represent a value of type.
  3. Assert: value is a BigInt if IsBigIntElementType(type) is true; otherwise, value is a Number.
  4. Let block be arrayBuffer.[[ArrayBufferData]].
  5. Let elementSize be the Element Size value specified in Table 71 for Element Type type.
  6. If isLittleEndian is not present, set isLittleEndian to the value of the [[LittleEndian]] field of the surrounding agent's Agent Record.
  7. Let rawBytes be NumericToRawBytes(type, value, isLittleEndian).
  8. If IsSharedArrayBuffer(arrayBuffer) is true, then
    1. Let execution be the [[CandidateExecution]] field of the surrounding agent's Agent Record.
    2. Let eventsRecord be the Agent Events Record of execution.[[EventsRecords]] whose [[AgentSignifier]] is AgentSignifier().
    3. If isTypedArray is true and IsNoTearConfiguration(type, order) is true, let noTear be true; otherwise let noTear be false.
    4. Append WriteSharedMemory { [[Order]]: order, [[NoTear]]: noTear, [[Block]]: block, [[ByteIndex]]: byteIndex, [[ElementSize]]: elementSize, [[Payload]]: rawBytes } to eventsRecord.[[EventList]].
  9. Else,
    1. Store the individual bytes of rawBytes into block, starting at block[byteIndex].
  10. Return unused.

25.1.3.19 GetModifySetValueInBuffer ( arrayBuffer, byteIndex, type, value, op )

The abstract operation GetModifySetValueInBuffer takes arguments arrayBuffer (an ArrayBuffer or a SharedArrayBuffer), byteIndex (a non-negative integer), type (a TypedArray element type), value (a Number or a BigInt), and op (a read-modify-write modification function) and returns a Number or a BigInt. It performs the following steps when called:

  1. Assert: IsDetachedBuffer(arrayBuffer) is false.
  2. Assert: There are sufficient bytes in arrayBuffer starting at byteIndex to represent a value of type.
  3. Assert: value is a BigInt if IsBigIntElementType(type) is true; otherwise, value is a Number.
  4. Let block be arrayBuffer.[[ArrayBufferData]].
  5. Let elementSize be the Element Size value specified in Table 71 for Element Type type.
  6. Let isLittleEndian be the value of the [[LittleEndian]] field of the surrounding agent's Agent Record.
  7. Let rawBytes be NumericToRawBytes(type, value, isLittleEndian).
  8. If IsSharedArrayBuffer(arrayBuffer) is true, then
    1. Let execution be the [[CandidateExecution]] field of the surrounding agent's Agent Record.
    2. Let eventsRecord be the Agent Events Record of execution.[[EventsRecords]] whose [[AgentSignifier]] is AgentSignifier().
    3. Let rawBytesRead be a List of length elementSize whose elements are nondeterministically chosen byte values.
    4. 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.
    5. Let rmwEvent be ReadModifyWriteSharedMemory { [[Order]]: seq-cst, [[NoTear]]: true, [[Block]]: block, [[ByteIndex]]: byteIndex, [[ElementSize]]: elementSize, [[Payload]]: rawBytes, [[ModifyOp]]: op }.
    6. Append rmwEvent to eventsRecord.[[EventList]].
    7. Append Chosen Value Record { [[Event]]: rmwEvent, [[ChosenValue]]: rawBytesRead } to execution.[[ChosenValues]].
  9. Else,
    1. Let rawBytesRead be a List of length elementSize whose elements are the sequence of elementSize bytes starting with block[byteIndex].
    2. Let rawBytesModified be op(rawBytesRead, rawBytes).
    3. Store the individual bytes of rawBytesModified into block, starting at block[byteIndex].
  10. Return RawBytesToNumeric(type, rawBytesRead, isLittleEndian).

25.1.4 The ArrayBuffer Constructor

The ArrayBuffer constructor:

  • is %ArrayBuffer%.
  • is the initial value of the "ArrayBuffer" property of the global object.
  • creates and initializes a new ArrayBuffer when called as a constructor.
  • is not intended to be called as a function and will throw an exception when called in that manner.
  • 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 constructor to create and initialize subclass instances with the internal state necessary to support the ArrayBuffer.prototype built-in methods.

25.1.4.1 ArrayBuffer ( length [ , options ] )

This function performs the following steps when called:

  1. If NewTarget is undefined, throw a TypeError exception.
  2. Let byteLength be ? ToIndex(length).
  3. Let requestedMaxByteLength be ? GetArrayBufferMaxByteLengthOption(options).
  4. Return ? AllocateArrayBuffer(NewTarget, byteLength, requestedMaxByteLength).

25.1.5 Properties of the ArrayBuffer Constructor

The ArrayBuffer constructor:

  • has a [[Prototype]] internal slot whose value is %Function.prototype%.
  • has the following properties:

25.1.5.1 ArrayBuffer.isView ( arg )

This function performs the following steps when called:

  1. If arg is not an Object, return false.
  2. If arg has a [[ViewedArrayBuffer]] internal slot, return true.
  3. Return false.

25.1.5.2 ArrayBuffer.prototype

The initial value of ArrayBuffer.prototype is the ArrayBuffer prototype object.

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

25.1.5.3 get ArrayBuffer [ @@species ]

ArrayBuffer[@@species] is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Return the this value.

The value of the "name" property of this function is "get [Symbol.species]".

Note

ArrayBuffer.prototype.slice ( start, end ) normally uses its this value's constructor to create a derived object. However, a subclass constructor may over-ride that default behaviour for the ArrayBuffer.prototype.slice ( start, end ) method by redefining its @@species property.

25.1.6 Properties of the ArrayBuffer Prototype Object

The ArrayBuffer prototype object:

  • is %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.

25.1.6.1 get ArrayBuffer.prototype.byteLength

ArrayBuffer.prototype.byteLength is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? RequireInternalSlot(O, [[ArrayBufferData]]).
  3. If IsSharedArrayBuffer(O) is true, throw a TypeError exception.
  4. If IsDetachedBuffer(O) is true, return +0𝔽.
  5. Let length be O.[[ArrayBufferByteLength]].
  6. Return 𝔽(length).

25.1.6.2 ArrayBuffer.prototype.constructor

The initial value of ArrayBuffer.prototype.constructor is %ArrayBuffer%.

25.1.6.3 get ArrayBuffer.prototype.detached

ArrayBuffer.prototype.detached is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? RequireInternalSlot(O, [[ArrayBufferData]]).
  3. If IsSharedArrayBuffer(O) is true, throw a TypeError exception.
  4. Return IsDetachedBuffer(O).

25.1.6.4 get ArrayBuffer.prototype.maxByteLength

ArrayBuffer.prototype.maxByteLength is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? RequireInternalSlot(O, [[ArrayBufferData]]).
  3. If IsSharedArrayBuffer(O) is true, throw a TypeError exception.
  4. If IsDetachedBuffer(O) is true, return +0𝔽.
  5. If IsFixedLengthArrayBuffer(O) is true, then
    1. Let length be O.[[ArrayBufferByteLength]].
  6. Else,
    1. Let length be O.[[ArrayBufferMaxByteLength]].
  7. Return 𝔽(length).

25.1.6.5 get ArrayBuffer.prototype.resizable

ArrayBuffer.prototype.resizable is an accessor property whose set accessor function is undefined. Its get accessor function performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? RequireInternalSlot(O, [[ArrayBufferData]]).
  3. If IsSharedArrayBuffer(O) is true, throw a TypeError exception.
  4. If IsFixedLengthArrayBuffer(O) is false, return true; otherwise return false.

25.1.6.6 ArrayBuffer.prototype.resize ( newLength )

This method performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? RequireInternalSlot(O, [[ArrayBufferMaxByteLength]]).
  3. If IsSharedArrayBuffer(O) is true, throw a TypeError exception.
  4. Let newByteLength be ? ToIndex(newLength).
  5. If IsDetachedBuffer(O) is true, throw a TypeError exception.
  6. If newByteLength > O.[[ArrayBufferMaxByteLength]], throw a RangeError exception.
  7. Let hostHandled be ? HostResizeArrayBuffer(O, newByteLength).
  8. If hostHandled is handled, return undefined.
  9. Let oldBlock be O.[[ArrayBufferData]].
  10. Let newBlock be ? CreateByteDataBlock(newByteLength).
  11. Let copyLength be min(newByteLength, O.[[ArrayBufferByteLength]]).
  12. Perform CopyDataBlockBytes(newBlock, 0, oldBlock, 0, copyLength).
  13. NOTE: Neither creation of the new Data Block nor copying from the old Data Block are observable. Implementations may implement this method as in-place growth or shrinkage.
  14. Set O.[[ArrayBufferData]] to newBlock.
  15. Set O.[[ArrayBufferByteLength]] to newByteLength.
  16. Return undefined.

25.1.6.7 ArrayBuffer.prototype.slice ( start, end )

This method performs the following steps when called:

  1. Let O be the this value.
  2. Perform ? RequireInternalSlot(O, [[ArrayBufferData]]).
  3. If IsSharedArrayBuffer(O) is true, throw a TypeError exception.
  4. If IsDetachedBuffer(O) is true, throw a TypeError exception.
  5. Let len be O.[[ArrayBufferByteLength]].
  6. Let relativeStart be ? ToIntegerOrInfinity(start).
  7. If relativeStart = -∞, let first be 0.
  8. Else if relativeStart < 0, let first be max(len + relativeStart, 0).
  9. Else, let first be min(relativeStart, len).
  10. If end is undefined, let relativeEnd be len; else let relativeEnd be ? ToIntegerOrInfinity(end).
  11. If relativeEnd = -∞, let final be 0.
  12. Else if relativeEnd < 0, let final be max(len + relativeEnd, 0).
  13. Else, let final be min(relativeEnd, len).
  14. Let newLen be max(final - first, 0).
  15. Let ctor be ? SpeciesConstructor(O, %ArrayBuffer%).
  16. Let new be ? Construct(ctor, « 𝔽(newLen) »).
  17. Perform ? RequireInternalSlot(new, [[ArrayBufferData]]).
  18. If IsSharedArrayBuffer(new) is true, throw a TypeError exception.
  19. If IsDetachedBuffer(new) is true, throw a TypeError exception.
  20. If SameValue(new, O) 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 or resized O.
  23. If IsDetachedBuffer(O) is true, throw a TypeError exception.
  24. Let fromBuf be O.[[ArrayBufferData]].
  25. Let toBuf be new.[[ArrayBufferData]].
  26. Let currentLen be O.[[ArrayBufferByteLength]].
  27. If first < currentLen, then
    1. Let count be min(newLen, currentLen - first).
    2. Perform CopyDataBlockBytes(toBuf, 0, fromBuf, first, count).
  28. Return new.

25.1.6.8 ArrayBuffer.prototype.transfer ( [ newLength ] )

This method performs the following steps when called:

  1. Let O be the this value.
  2. Return ? ArrayBufferCopyAndDetach(O, newLength, preserve-resizability).

25.1.6.9 ArrayBuffer.prototype.transferToFixedLength ( [ newLength ] )

This method performs the following steps when called:

  1. Let O be the this value.
  2. Return ? ArrayBufferCopyAndDetach(O, newLength, fixed-length).

25.1.6.10 ArrayBuffer.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "ArrayBuffer".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

25.1.7 Properties of ArrayBuffer Instances

ArrayBuffer instances inherit properties from the ArrayBuffer prototype object. ArrayBuffer instances each have an [[ArrayBufferData]] internal slot, an [[ArrayBufferByteLength]] internal slot, and an [[ArrayBufferDetachKey]] internal slot. ArrayBuffer instances which are resizable each have an [[ArrayBufferMaxByteLength]] 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.

25.1.8 Resizable ArrayBuffer Guidelines

Note 1

The following are guidelines for ECMAScript programmers working with resizable ArrayBuffer.

We recommend that programs be tested in their deployment environments where possible. The amount of available physical memory differs greatly between hardware devices. Similarly, virtual memory subsystems also differ greatly between hardware devices as well as operating systems. An application that runs without out-of-memory errors on a 64-bit desktop web browser could run out of memory on a 32-bit mobile web browser.

When choosing a value for the "maxByteLength" option for resizable ArrayBuffer, we recommend that the smallest possible size for the application be chosen. We recommend that "maxByteLength" does not exceed 1,073,741,824 (230 bytes or 1GiB).

Please note that successfully constructing a resizable ArrayBuffer for a particular maximum size does not guarantee that future resizes will succeed.

 Note 2

The following are guidelines for ECMAScript implementers implementing resizable ArrayBuffer.

Resizable ArrayBuffer can be implemented as copying upon resize, as in-place growth via reserving virtual memory up front, or as a combination of both for different values of the constructor's "maxByteLength" option.

If a host is multi-tenanted (i.e. it runs many ECMAScript applications simultaneously), such as a web browser, and its implementations choose to implement in-place growth by reserving virtual memory, we recommend that both 32-bit and 64-bit implementations throw for values of "maxByteLength" ≥ 1GiB to 1.5GiB. This is to reduce the likelihood a single application can exhaust the virtual memory address space and to reduce interoperability risk.

If a host does not have virtual memory, such as those running on embedded devices without an MMU, or if a host only implements resizing by copying, it may accept any Number value for the "maxByteLength" option. However, we recommend a RangeError be thrown if a memory block of the requested size can never be allocated. For example, if the requested size is greater than the maximium amount of usable memory on the device.