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
|
Desktop |
|
Chrome |
Edge |
Firefox |
Internet Explorer |
Opera |
Safari |
compareExchange |
68
- 68
- 60 — 63
- Chrome disabled
SharedArrayBuffer on January 5, 2018 to help reduce the efficacy of speculative side-channel attacks. This was a temporary removal while mitigations were put in place.
|
79
|
78
- 78
- 57
Disabled
- Support was disabled by default to mitigate speculative execution side-channel attacks (Mozilla Security Blog).
- Disabled 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.
- 55 — 57
- 46 — 55
Disabled
- Disabled 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.
|
No |
No |
10.1 — 11.1 |
|
Mobile |
|
Android webview |
Chrome for Android |
Firefox for Android |
Opera for Android |
Safari on iOS |
Samsung Internet |
compareExchange |
60 — 63
- 60 — 63
- 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.
|
60 — 63
- 60 — 63
- 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.
|
57 Disabled
- 57
Disabled
- Support was disabled by default to mitigate speculative execution side-channel attacks (Mozilla Security Blog).
- Disabled 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.
- 55 — 57
- 46 — 55
Disabled
- Disabled 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.
|
No |
10.3 — 11.3 |
No
- No
- 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.
|
|
Server |
|
Node.js |
compareExchange |
8.10.0 |
See also