Atomics.compareExchange()

The static Atomics.compareExchange() 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.

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 value.
expectedValue
The value to check for equality.
replacementValue
The number to exchange.

Return value

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

Exceptions

  • Throws a TypeError, if typedArray is not one of the allowed integer types.
  • Throws a RangeError, 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 (ECMA-262)
The definition of 'Atomics.compareExchange' in that specification.

Browser compatibility

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See also