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Atomics.compareExchange()

The Atomics.compareExchange() static method exchanges a given replacement value at a given position in the array, if a given expected value equals the old value. It returns the old value at that position whether it was equal to the expected value or not. This atomic operation guarantees that no other write happens until the modified value is written back.

Try it

Syntax

js
Atomics.compareExchange(typedArray, index, expectedValue, replacementValue)

Parameters

typedArray

An integer typed array. One of Int8Array, Uint8Array, Int16Array, Uint16Array, Int32Array, Uint32Array, BigInt64Array, or BigUint64Array.

index

The position in the typedArray to exchange a replacementValue.

expectedValue

The value to check for equality.

replacementValue

The number to exchange.

Return value

The old value at the given position (typedArray[index]).

Exceptions

TypeError

Thrown if typedArray is not one of the allowed integer types.

RangeError

Thrown if index is out of bounds in the typedArray.

Examples

Using compareExchange()

js
const sab = new SharedArrayBuffer(1024);
const ta = new Uint8Array(sab);
ta[0] = 7;

Atomics.compareExchange(ta, 0, 7, 12); // returns 7, the old value
Atomics.load(ta, 0); // 12

Specifications

Browser compatibility

Desktop Mobile Server
Chrome Edge Firefox Opera Safari Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet WebView Android Deno Node.js
compareExchange 68 79 78 55 15.2 89 79 63 15.2 15.0 89 1.0 8.10.0

See also

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Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Atomics/compareExchange