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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

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()

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

Specification
ECMAScript Language Specification # sec-atomics.compareexchange

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