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SharedArrayBuffer

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Syntax
1. Parameters
2. Return value
Description
Properties
SharedArrayBuffer prototype object
1. Properties
2. Methods
Specifications
Browser compatibility
See also

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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

ArrayBuffers vs. SharedArrayBuffers

JavaScript offers ArrayBuffer and SharedArrayBuffer objects. They are constructed like this:

var ab = new ArrayBuffer(1024);
var sab = new SharedArrayBuffer(1024);

Web content uses Web Workers to run scripts in background threads. Data gets sent to and from the worker by using the postMessage() method and certain types are so-called transferable objects, that are transferred from one context to another with a zero-copy operation, resulting in high performance.

When transferring an ArrayBuffer from your main app to a worker script, the original ArrayBuffer is cleared and no longer usable. Its content is (quite literally) transferred to the worker context.

var ab = new ArrayBuffer(1024);
var uInt8Array = new Uint8Array(ab);
for (var i = 0; i < uInt8Array.length; ++i) {
  uInt8Array[i] = i;
}

var worker = new Worker("worker.js");

// before transferring
console.log(uInt8Array.byteLength); // 1024

worker.postMessage(uInt8Array.buffer, [uInt8Array.buffer]);

// after transferring
console.log(uInt8Array.byteLength); // 0

Now with a SharedArrayBuffer, you can share this memory with the worker by transferring it using the same postMessage() call.

var sab = new SharedArrayBuffer(1024);

// before transferring
console.log(sab.byteLength); // 1024

worker.postMessage(sab, [sab]);

// after transferring
console.log(sab.byteLength); // 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);

Properties

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