To share memory usingSharedArrayBuffer 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 andstructured cloning is used.

The structured clone algorithm acceptsSharedArrayBuffer objects and typed arrays mapped ontoSharedArrayBuffer objects. In both cases, theSharedArrayBuffer object is transmitted to the receiver resulting in a new, privateSharedArrayBuffer object in the receiving agent (just as forArrayBuffer). However, the shared data block referenced by the twoSharedArrayBuffer objects is the same data block, and a side effect to the block in one agent will eventually become visible in the other agent.

const sab = new SharedArrayBuffer(1024);worker.postMessage(sab);

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.

SharedArrayBuffer objects are used by some web APIs, such as:

• WebGLRenderingContext.bufferData()
• WebGLRenderingContext.bufferSubData()
• WebGL2RenderingContext.getBufferSubData()

Shared memory and high-resolution timers were effectivelydisabled at the start of 2018 in light ofSpectre.In 2020, a new, secure approach has been standardized to re-enable shared memory.

To use shared memory your document must be in asecure context andcross-origin isolated.You can use theWindow.crossOriginIsolated andWorkerGlobalScope.crossOriginIsolated properties to check if the document is cross-origin isolated:

const myWorker = new Worker("worker.js");if (crossOriginIsolated) {  const buffer = new SharedArrayBuffer(16);  myWorker.postMessage(buffer);} else {  const buffer = new ArrayBuffer(16);  myWorker.postMessage(buffer);}

When cross-origin isolated,postMessage() no longer throws forSharedArrayBuffer objects, and shared memory across threads is therefore available.

Depending on whether the above security measures are taken, the various memory-sharing APIs have different availabilities:

• TheAtomics object is always available.
• SharedArrayBuffer objects are in principle always available, but unfortunately the constructor on the global object is hidden, unless the two headers mentioned above are set, for compatibility with web content. There is hope that this restriction can be removed in the future.WebAssembly.Memory can still be used to get an instance.
• Unless the two headers mentioned above are set, the variouspostMessage() APIs will throw forSharedArrayBuffer objects. If they are set,postMessage() onWindow objects and dedicated workers will function and allow for memory sharing.

WebAssembly.Memory objects can be created with theshared constructor flag. When this flag is set totrue, the constructedMemory object can be shared between workers viapostMessage(), just likeSharedArrayBuffer, and the backingbuffer of theMemory object is aSharedArrayBuffer. Therefore, the requirements listed above for sharing aSharedArrayBuffer between workers also apply to sharing aWebAssembly.Memory.

The WebAssembly Threads proposal also defines a new set ofatomic instructions. Just asSharedArrayBuffer and its methods are unconditionally enabled (and only sharing between threads is gated on the new headers), the WebAssembly atomic instructions are also unconditionally allowed.

SharedArrayBuffer objects can be made growable by including themaxByteLength option when calling theSharedArrayBuffer() constructor. You can query whether aSharedArrayBuffer is growable and what its maximum size is by accessing itsgrowable andmaxByteLength properties, respectively. You can assign a new size to a growableSharedArrayBuffer with agrow() call. New bytes are initialized to 0.

These features make growingSharedArrayBuffers more efficient — otherwise, you have to make a copy of the buffer with a new size. It also gives JavaScript parity with WebAssembly in this regard (Wasm linear memory can be resized withWebAssembly.Memory.prototype.grow()).

For security reasons,SharedArrayBuffers cannot be reduced in size, only grown.

SharedArrayBuffer()

Creates a newSharedArrayBuffer object.

SharedArrayBuffer[Symbol.species]

Returns the constructor used to construct return values fromSharedArrayBuffer methods.

These properties are defined onSharedArrayBuffer.prototype and shared by allSharedArrayBuffer instances.

SharedArrayBuffer.prototype.byteLength

The size, in bytes, of the array. This is established when the array is constructed and can only be changed using theSharedArrayBuffer.prototype.grow() method if theSharedArrayBuffer is growable.

SharedArrayBuffer.prototype.constructor

The constructor function that created the instance object. ForSharedArrayBuffer instances, the initial value is theSharedArrayBuffer constructor.

SharedArrayBuffer.prototype.growable

Read-only. Returnstrue if theSharedArrayBuffer can be grown, orfalse if not.

SharedArrayBuffer.prototype.maxByteLength

The read-only maximum length, in bytes, that theSharedArrayBuffer can be grown to. This is established when the array is constructed and cannot be changed.

SharedArrayBuffer.prototype[Symbol.toStringTag]

The initial value of the[Symbol.toStringTag] property is the string"SharedArrayBuffer". This property is used inObject.prototype.toString().

SharedArrayBuffer.prototype.grow()

Grows theSharedArrayBuffer to the specified size, in bytes.

SharedArrayBuffer.prototype.slice()

Returns a newSharedArrayBuffer whose contents are a copy of thisSharedArrayBuffer's bytes frombegin, inclusive, up toend, exclusive. If eitherbegin orend is negative, it refers to an index from the end of the array, as opposed to from the beginning.

• Atomics
• ArrayBuffer
• JavaScript typed arrays guide
• Web Workers
• Shared Memory – a brief tutorial in the TC39 ecmascript-sharedmem proposal
• A Taste of JavaScript's New Parallel Primitives on hacks.mozilla.org (2016)
• COOP and COEP explained by the Chrome team (2020)
• Cross-Origin-Opener-Policy
• Cross-Origin-Embedder-Policy
• Cross-Origin-Resource-Policy
• Window.crossOriginIsolated andWorkerGlobalScope.crossOriginIsolated
• SharedArrayBuffer updates in Android Chrome 88 and Desktop Chrome 92 on developer.chrome.com (2021)