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.


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


A shared integer typed array. One of Int8Array, Uint8Array, Int16Array, Uint16Array, Int32Array, or Uint32Array.
The position in the typedArray to exchange a value.
The value to check for equality.
The number to exchange.

Return value

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


  • Throws a TypeError, if typedArray is not one of the allowed integer types.
  • Throws a TypeError, if typedArray is not a shared typed array type.
  • Throws a RangeError, if index is out of bounds in the typedArray.


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

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


Specification Status Comment
ECMAScript Latest Draft (ECMA-262)
The definition of 'Atomics.compareExchange' in that specification.
Living Standard Initial definition in ES2017.

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support6016


46 — 541

No No10.1
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidIE mobileOpera AndroidiOS Safari
Basic support6060 ?


46 — 541

No No No

1. From version 46 until version 54 (exclusive): this feature is behind the javascript.options.shared_memory preference (needs to be set to true). To change preferences in Firefox, visit about:config.

See also

Document Tags and Contributors

 Contributors to this page: fscholz, lth
 Last updated by: fscholz,