The SharedArrayBuffer object is used to represent a generic, fixed-length raw binary data buffer, similar to the ArrayBuffer object, but in a way that they can be used to create views on shared memory. Unlike an ArrayBuffer, a SharedArrayBuffer cannot become detached.


new SharedArrayBuffer(length)


The size, in bytes, of the array buffer to create.

Return value

A new SharedArrayBuffer object of the specified size. Its contents are initialized to 0.


Allocating and sharing memory

To share memory using SharedArrayBuffer objects from one agent in the cluster to another (an agent is either the web page’s main program or one of its web workers), postMessage and structured cloning is used.

The structured clone algorithm accepts SharedArrayBuffers and TypedArrays mapped onto SharedArrayBuffers. In both cases, the SharedArrayBuffer object is transmitted to the receiver resulting in a new, private SharedArrayBuffer object in the receiving agent (just as for ArrayBuffer). However, the shared data block referenced by the two SharedArrayBuffer objects is the same data block, and a side effect to the block in one agent will eventually become visible in the other agent.

var sab = new SharedArrayBuffer(1024);

Updating and synchronizing shared memory with Atomic operations

Shared memory can be created and updated simultaneously in workers or the main thread. Depending on the system (the CPU, the OS, the Browser) it can take a while until the change is propagated to all contexts. To synchronize, atomic operations are needed.

APIs accepting SharedArrayBuffer objects

Constructing is required with new operator

SharedArrayBuffer constructors require to be constructed with a new operator. Calling a SharedArrayBuffer constructor as a function without new, will throw a TypeError.

var sab = SharedArrayBuffer(1024);
// TypeError: calling a builtin SharedArrayBuffer constructor 
// without new is forbidden
var sab = new SharedArrayBuffer(1024);


The SharedArrayBuffer constructor's length property whose value is 1.
Allows the addition of properties to all SharedArrayBuffer objects.

SharedArrayBuffer prototype object

All SharedArrayBuffer instances inherit from SharedArrayBuffer.prototype.


Specifies the function that creates an object's prototype. The initial value is the standard built-in SharedArrayBuffer constructor.
SharedArrayBuffer.prototype.byteLength Read only
The size, in bytes, of the array. This is established when the array is constructed and cannot be changed. Read only.


SharedArrayBuffer.prototype.slice(begin, end)
Returns a new SharedArrayBuffer whose contents are a copy of this SharedArrayBuffer's bytes from begin, inclusive, up to end, exclusive. If either begin or end is negative, it refers to an index from the end of the array, as opposed to from the beginning.


Specification Status Comment
ECMAScript Latest Draft (ECMA-262)
The definition of 'SharedArrayBuffer' in that specification.
Draft Initial definition in ES2017.

Browser compatibility

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari
Basic support No support [2] No support 55 (55) [1] No support No support 10.1
slice() No support No support 52 (52) [1] No support No support 10.1
SAB in DataView No support No support 53 (53) [1] No support No support 10.1
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support No support No support 55.0 (55) [1] No support No support 10.1
slice() No support No support 52.0 (52) [1] No support No support 10.1
SAB in DataView No support No support 53.0 (53) [1] No support No support 10.1

[1] Enabled by default in Firefox 55. In Firefox version 46 until version 54, this feature is disabled by a preference setting (in about:config, set javascript.options.shared_memory to true).

[2] The implementation is under development and needs these runtime flags: --js-flags=--harmony-sharedarraybuffer --enable-blink-feature=SharedArrayBuffer

See also

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