Atomics.wait()

The Atomics.wait() static method verifies that a shared memory location contains a given value and if so sleeps, awaiting a wake-up notification or a time out. It returns a string which is "not-equal" if the memory location does not match the given value, "ok" if woken by Atomics.notify(), or "timed-out" if the timeout expires.

Atomics.wait() and Atomics.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 an Int32Array or BigInt64Array that views a SharedArrayBuffer. It is blocking and cannot be used in the main thread. For a non-blocking, asynchronous version of this method, see Atomics.waitAsync().

Syntax

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

Parameters

typedArray: An Int32Array or BigInt64Array that views a SharedArrayBuffer.
index: The position in the typedArray to wait on.
value: The expected value to test.
timeout Optional: Time to wait in milliseconds. NaN (and values that get converted to NaN, such as undefined) becomes Infinity. Negative values become 0.

Return value

A string which is either "not-equal", "ok", or "timed-out".

"not-equal" is returned immediately if the initial value does not equal what is stored at index.
"ok" is returned if woken up by a call to Atomics.notify(), regardless of whether the expected value has changed.
"timed-out" is returned if a sleeping wait exceeds the specified timeout without being woken up by Atomics.notify().

Exceptions

TypeError

Thrown in one of the following cases:

If typedArray is not an Int32Array or BigInt64Array that views a SharedArrayBuffer.
If the current thread cannot be blocked (for example, because it's the main thread).

RangeError

Thrown if index is out of bounds in the typedArray.

Examples

Note that these examples cannot be run directly from the console or an arbitrary web page, because SharedArrayBuffer is not defined unless its security requirements are met.

Using Atomics.wait()

Given a shared Int32Array:

// Create a SharedArrayBuffer with a size in bytes
const sab = new SharedArrayBuffer(1024);
const int32 = new Int32Array(sab);

A reading thread is sleeping and waiting on location 0 because the provided value matches what is stored at the provided index. The reading thread will not move on until the writing thread has called Atomics.notify() on position 0 of the provided typedArray. Note that if, after being woken up, the value of location 0 has not been changed by the writing thread, the reading thread will not go back to sleep, but will continue on.

Atomics.wait(int32, 0, 0);
console.log(int32[0]); // 123

A writing thread stores a new value and notifies the waiting thread once it has written:

console.log(int32[0]); // 0;
Atomics.store(int32, 0, 123);
Atomics.notify(int32, 0, 1);

Specifications

ECMAScript® 2027 Language Specification