Atomics.compareExchange()

¥Syntax

Atomics.compareExchange(typedArray, index, expectedValue, replacementValue)

¥Parameters

typedArray

整数类型数组。Int8Array、Uint8Array、Int16Array、Uint16Array、Int32Array、Uint32Array、BigInt64Array 或 BigUint64Array 之一。

index

把 typedArray 的位置换成 replacementValue。

expectedValue

用于检查相等性的值。

replacementValue

要交换的号码。

¥Return value

给定位置 (typedArray[index]) 的旧值。如果返回值等于 expectedValue，则交换成功；否则，交换失败。

¥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

如果 typedArray 不是允许的整数类型之一，则抛出该错误。

RangeError

如果 index 在 typedArray 中出界，则抛出该球。

¥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

比较和交换 保证新值是根据最新信息计算的；如果在此期间另一个线程已更新该值，则写入将失败。因此，你应该检查 compareExchange() 的返回值以检查它是否失败，并在必要时重试。

¥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.

以下是原子加法器（与 Atomics.add() 功能相同）的一个示例，改编自链接的 Wikipedia 文章：

¥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;
}

它首先读取给定索引处的值，然后尝试使用新值更新它。它会不断重试，直到成功更新值。

¥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.

¥See also

• Atomics
• Atomics.exchange()