Atomics.waitAsync()

This feature is not Baseline because it does not work in some of the most widely-used browsers.

TheAtomics.waitAsync() static method verifies that a shared memory location contains a given value, immediately returning an object with thevalue property containing the string"not-equal" if the memory location does not match the given value, or"timed-out" if the timeout was set to zero. Otherwise the method returns an object where thevalue property is aPromise that fulfills with either"ok" whenAtomics.notify() is called, or"timed-out" if the timeout expires.

Atomics.waitAsync() andAtomics.notify() are used together to enable thread synchronization based on a value in shared memory. A thread can proceed immediately if the synchronization value has changed, or it can wait for notification from another thread when it reaches the synchronization point.

This method only works with anInt32Array orBigInt64Array that views aSharedArrayBuffer. It is non-blocking and, unlikeAtomics.wait(), can be used on the main thread. Because it does not block the whole thread, you still need to be careful not to access the shared memory before the promise settles.

Syntax

Atomics.waitAsync(typedArray, index, value)Atomics.waitAsync(typedArray, index, value, timeout)

Parameters

typedArray: AnInt32Array orBigInt64Array that views aSharedArrayBuffer.
index: The position in thetypedArray to wait on.
value: The expected value to test.
timeoutOptional: Time to wait in milliseconds.NaN (and values that get converted toNaN, such asundefined) becomesInfinity. Negative values become0.

Return value

AnObject with the following properties:

async: A boolean indicating whether thevalue property is aPromise or not.
value: Ifasync isfalse, it will be a string which is either"not-equal" or"timed-out" (only when thetimeout parameter is0). Ifasync istrue, it will be aPromise which is fulfilled with a string value, either"ok" or"timed-out". The promise is never rejected.

Exceptions

TypeError: Thrown iftypedArray is not anInt32Array orBigInt64Array that views aSharedArrayBuffer.
RangeError: Thrown ifindex is out of bounds in thetypedArray.

Examples

Using waitAsync()

Given a sharedInt32Array.

const sab = new SharedArrayBuffer(1024);const int32 = new Int32Array(sab);

A reading thread is sleeping and waiting on location 0 which is expected to be 0.Theresult.value will be a promise.

const result = Atomics.waitAsync(int32, 0, 0, 1000);// { async: true, value: Promise {<pending>} }

In the reading thread or in another thread, the memory location 0 is called and the promise can be resolved with"ok".

Atomics.notify(int32, 0);// { async: true, value: Promise {<fulfilled>: 'ok'} }

If it isn't resolving to"ok", the value in the shared memory location wasn't the expected (thevalue would be"not-equal" instead of a promise) or the timeout was reached (the promise will resolve to"time-out").