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

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support601 No2

573 4

55 — 57

46 — 555

No No10.1 —?
add601 No2

573 4

55 — 57

46 — 555

No No10.1 —?
and601 No2

573 4

55 — 57

46 — 555

No No10.1 —?
compareExchange601 No2

573 4

55 — 57

46 — 555

No No10.1 —?
exchange601 No2

573 4

55 — 57

46 — 555

No No10.1 —?
isLockFree601 No2

573 4

55 — 57

46 — 555

No No10.1 —?
load601 No2

573 4

55 — 57

46 — 555

No No10.1 —?
or601 No2

573 4

55 — 57

46 — 555

No No10.1 —?
store601 No2

573 4

55 — 57

46 — 555

No No10.1 —?
sub601 No2

573 4

55 — 57

46 — 555

No No10.1 —?
wait601 No2

573 4

55 — 57

46 — 555

No No10.1 —?
wake601 No2

573 4

55 — 57

46 — 555

No No10.1 —?
xor601 No2

573 4

55 — 57

46 — 555

No No10.1 —?
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidOpera AndroidiOS SafariSamsung Internet
Basic support601601 ?

573 4

55 — 57

46 — 555

No No ?
add601601 ?

573 4

55 — 57

46 — 555

No No ?
and601601 ?

573 4

55 — 57

46 — 555

No No ?
compareExchange601601 ?

573 4

55 — 57

46 — 555

No No ?
exchange601601 ?

573 4

55 — 57

46 — 555

No No ?
isLockFree601601 ?

573 4

55 — 57

46 — 555

No No ?
load601601 ?

573 4

55 — 57

46 — 555

No No ?
or601601 ?

573 4

55 — 57

46 — 555

No No ?
store601601 ?

573 4

55 — 57

46 — 555

No No ?
sub601601 ?

573 4

55 — 57

46 — 555

No No ?
wait601601 ?

573 4

55 — 57

46 — 555

No No ?
wake601601 ?

573 4

55 — 57

46 — 555

No No ?
xor601601 ?

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:

See also

Document Tags and Contributors

 Last updated by: fscholz,