Atomics.compareExchange()
Baseline Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since December 2021.
The Atomics.compareExchange()
static 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.
Try it
Syntax
Atomics.compareExchange(typedArray, index, expectedValue, replacementValue)
Parameters
typedArray
-
An integer typed array. One of
Int8Array
,Uint8Array
,Int16Array
,Uint16Array
,Int32Array
,Uint32Array
,BigInt64Array
, orBigUint64Array
. index
-
The position in the
typedArray
to exchange areplacementValue
. expectedValue
-
The value to check for equality.
replacementValue
-
The number to exchange.
Return value
The old value at the given position (typedArray[index]
). If the return value is equal to expectedValue
, the exchange was successful; otherwise, the exchange failed.
Exceptions
TypeError
-
Thrown if
typedArray
is not one of the allowed integer types. RangeError
-
Thrown if
index
is out of bounds in thetypedArray
.
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
Checking the return value
Compare-and-swap guarantees that the new value is calculated based on up-to-date information; if the value had been updated by another thread in the meantime, the write would fail. Therefore, you should check the return value of compareExchange()
to check if it has failed, and retry if necessary.
Here is one example of an atomic adder (same functionality as Atomics.add()
), adapted from the linked Wikipedia article:
function add(mem, index, value) {
let done = false;
while (!done) {
const value = Atomics.load(mem, index);
done = Atomics.compareExchange(p, value, value + a) === value;
}
return value + a;
}
It first reads the value at the given index, then tries to update it with the new value. It keeps retrying until it successfully updates the value.
Specifications
Specification |
---|
ECMAScript Language Specification # sec-atomics.compareexchange |
Browser compatibility
BCD tables only load in the browser