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Atomics

The Atomics object provides atomic operations as static methods. They are used with SharedArrayBuffer objects.

The Atomic operations are installed on an Atomics module. Unlike the other global objects, Atomics is not a constructor. You cannot use it with a new operator or invoke the Atomics object as a function. All properties and methods of Atomics are static (as is the case with the Math object, for example).

Properties

Atomics[Symbol.toStringTag]: The value of this property is "Atomics".

Methods

Atomic operations

When memory is shared, multiple threads can read and write the same data in memory. Atomic operations make sure that predictable values are written and read, that operations are finished before the next operation starts and that operations are not interrupted.

Atomics.add(): Adds a given value at a given position in the array. Returns the old value at that position.
Atomics.and(): Computes a bitwise AND at a given position in the array. Returns the old value at that position.
Atomics.compareExchange(): Stores a given value at a given position in the array, if it equals a given value. Returns the old value.
Atomics.exchange(): Stores a given value at a given position in the array. Returns the old value.

Atomics.load(): Returns the value at the given position in the array.
Atomics.or(): Computes a bitwise OR at a given position in the array. Returns the old value at that position.
Atomics.store(): Stores a given value at the given position in the array. Returns the value.
Atomics.sub(): Subtracts a given value at a given position in the array. Returns the old value at that position.
Atomics.xor(): Computes a bitwise XOR at a given position in the array. Returns the old value at that position.

Wait and wake

The wait() and wake() methods are modeled on Linux futexes ("fast user-space mutex") and provide ways for waiting until a certain condition becomes true and are typically used as blocking constructs.

Atomics.wait(): Verifies that a given position in the array still contains a given value and sleeps awaiting or times out. Returns either "ok", "not-equal", or "timed-out". If waiting is not allowed in the calling agent then it throws an Error exception (most browsers will not allow wait() on the browser's main thread).
Atomics.wake(): Wakes up some agents that are sleeping in the wait queue on the given array position. Returns the number of agents that were woken up.
Atomics.isLockFree(size): An optimization primitive that can be used to determine whether to use locks or atomic operations. Returns true, if an atomic operation on arrays of the given element size will be implemented using a hardware atomic operation (as opposed to a lock). Experts only.

Specifications

Specification	Status	Comment
ECMAScript Latest Draft (ECMA-262) The definition of 'Atomics' in that specification.	Draft	Initial definition in ES2017.
ECMAScript 2017 (ECMA-262) The definition of 'Atomics' in that specification.	Standard

Browser compatibility

Feature	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari
Basic support	60¹	No²	57³ ⁴ 55 — 57 46 — 55⁵	No	No	10.1 —?
`add`	60¹	No²	57³ ⁴ 55 — 57 46 — 55⁵	No	No	10.1 —?
`and`	60¹	No²	57³ ⁴ 55 — 57 46 — 55⁵	No	No	10.1 —?
`compareExchange`	60¹	No²	57³ ⁴ 55 — 57 46 — 55⁵	No	No	10.1 —?
`exchange`	60¹	No²	57³ ⁴ 55 — 57 46 — 55⁵	No	No	10.1 —?
`isLockFree`	60¹	No²	57³ ⁴ 55 — 57 46 — 55⁵	No	No	10.1 —?
`load`	60¹	No²	57³ ⁴ 55 — 57 46 — 55⁵	No	No	10.1 —?
`or`	60¹	No²	57³ ⁴ 55 — 57 46 — 55⁵	No	No	10.1 —?
`store`	60¹	No²	57³ ⁴ 55 — 57 46 — 55⁵	No	No	10.1 —?
`sub`	60¹	No²	57³ ⁴ 55 — 57 46 — 55⁵	No	No	10.1 —?
`wait`	60¹	No²	57³ ⁴ 55 — 57 46 — 55⁵	No	No	10.1 —?
`wake`	60¹	No²	57³ ⁴ 55 — 57 46 — 55⁵	No	No	10.1 —?
`xor`	60¹	No²	57³ ⁴ 55 — 57 46 — 55⁵	No	No	10.1 —?

Feature	Android webview	Chrome for Android	Edge mobile	Firefox for Android	Opera Android	iOS Safari	Samsung Internet
Basic support	60¹	60¹	?	57³ ⁴ 55 — 57 46 — 55⁵	No	No	?
`add`	60¹	60¹	?	57³ ⁴ 55 — 57 46 — 55⁵	No	No	?
`and`	60¹	60¹	?	57³ ⁴ 55 — 57 46 — 55⁵	No	No	?
`compareExchange`	60¹	60¹	?	57³ ⁴ 55 — 57 46 — 55⁵	No	No	?
`exchange`	60¹	60¹	?	57³ ⁴ 55 — 57 46 — 55⁵	No	No	?
`isLockFree`	60¹	60¹	?	57³ ⁴ 55 — 57 46 — 55⁵	No	No	?
`load`	60¹	60¹	?	57³ ⁴ 55 — 57 46 — 55⁵	No	No	?
`or`	60¹	60¹	?	57³ ⁴ 55 — 57 46 — 55⁵	No	No	?
`store`	60¹	60¹	?	57³ ⁴ 55 — 57 46 — 55⁵	No	No	?
`sub`	60¹	60¹	?	57³ ⁴ 55 — 57 46 — 55⁵	No	No	?
`wait`	60¹	60¹	?	57³ ⁴ 55 — 57 46 — 55⁵	No	No	?
`wake`	60¹	60¹	?	57³ ⁴ 55 — 57 46 — 55⁵	No	No	?
`xor`	60¹	60¹	?	57³ ⁴ 55 — 57 46 — 55⁵	No	No	?

1. Chrome disabled SharedArrayBuffer on January 5, 2018 to help reduce the efficacy of speculative side-channel attacks. This is intended as a temporary measure until other mitigations are in place.

2. Support was removed to mitigate speculative execution side-channel attacks (Windows blog).

3. Support was disabled by default to mitigate speculative execution side-channel attacks (Mozilla Security Blog).

4. From version 57: this feature is behind the javascript.options.shared_memory preference (needs to be set to true). To change preferences in Firefox, visit about:config.

5. From version 46 until version 55 (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.

Compatibility notes

Prior to Firefox 48, the latest API names and semantics weren't implemented yet. The changes between Firefox version 46 and version 48 are:

The methods Atomics.futexWakeOrRequeue() and Atomics.fence() are now removed entirely (bug 1259544 and bug 1225028).
The methods Atomics.wait() and Atomics.wake() were named Atomics.futexWait() and Atomics.futexWake() (bug 1260910). Note: The old names have been removed in version 49 and later (bug 1262062).
The properties Atomics.OK, Atomics.TIMEDOUT, Atomics.NOTEQUAL have been removed. The Atomics.wait() method now returns the strings "ok", "timed-out" and "not-equal" (bug 1260835).
The count parameter of the Atomics.wake() method has been changed: it now defaults to +Infinity, not 0 (bug 1253350).