This is an experimental technology
Because this technology's specification has not stabilized, check the compatibility table for usage in various browsers. Also note that the syntax and behavior of an experimental technology is subject to change in future versions of browsers as the specification changes.

The static method exchanges a given value at a given position in the array and returns the old value at that position. This atomic operation guarantees that no other write happens until the modified value is written back.

Syntax, index, value)


A shared integer typed array. One of Int8Array, Uint8Array, Int16Array, Uint16Array, Int32Array, or Uint32Array.
The position in the typedArray to exchange a value.
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);, 0, 12); // returns 0, the old value
Atomics.load(ta, 0); // 12


Specification Status Comment
ECMAScript Shared Memory and Atomics
The definition of '' in that specification.
Draft Initial definition.

Browser compatibility

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari
Basic support No support [2] No support 46 (46) [1] No support No support No support
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support No support No support 46.0 (46) [1] No support No support No support

[1] This feature is disabled by a preference setting. In about:config, set javascript.options.shared_memory to true

[2] The implementation is under development and needs these runtime flags: --js-flags=--harmony-sharedarraybuffer --enable-blink-feature=SharedArrayBuffer

See also

Document Tags and Contributors

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