SharedArrayBuffer

This is an experimental technology
Because this technology's specification has not stabilized, check the compatibility table for usage in various browsers. Also note that the syntax and behavior of an experimental technology is subject to change in future versions of browsers as the specification changes.

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.

Syntax

new SharedArrayBuffer(length)

Parameters

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.

Description

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);
worker.postMessage(sab);

In earlier versions of the specification, SharedArrayBuffers needed to be explicitly transferred during structured cloning. However, a SharedArrayBuffer is not a Transferable object in the sense of HTML. Thus if a SharedArrayBuffer object is present in the transfer list, then postMessage will throw a DataCloneError:

var sab = new SharedArrayBuffer(1024);
worker.postMessage(sab, [sab]); 
// TypeError: SharedArrayBuffer must not be in the transfer list

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);

Properties

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

SharedArrayBuffer prototype object

All SharedArrayBuffer instances inherit from SharedArrayBuffer.prototype.

Properties

SharedArrayBuffer.prototype.constructor
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.

Methods

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.

Specifications

Specification Status Comment
ECMAScript Shared Memory and Atomics
The definition of 'SharedArrayBuffer' in that specification.
Draft Initial definition.
ECMAScript 2017 Draft (ECMA-262) Draft  

Browser compatibility

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari
Basic support No support [2] No support 46 (46) [1] No support No support No support
slice() No support No support 52 (52) [1] No support No support No support
SAB in DataView No support No support 53 (53) [1] No support No support No support
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support No support No support 46.0 (46) [1] No support No support No support
slice() No support No support 52.0 (52) [1] No support No support No support
SAB in DataView No support No support 53.0 (53) [1] No support No support No support

[1] 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

Document Tags and Contributors

 Contributors to this page: fscholz, keffidesign, TheJaredWilcurt, marco-c, lth
 Last updated by: fscholz,