queueMicrotask(callback)

• callback <Function> 要排队的函数。

queueMicrotask() 方法将微任务排队以调用 callback。 如果 callback 抛出异常，则将触发 process 对象的 'uncaughtException' 事件。

微任务队列由 V8 管理，并且可以以类似于 process.nextTick() 队列的方式使用，后者由 Node.js 管理。 在 Node.js 事件循环的每次轮询中，process.nextTick() 队列总是在微任务队列之前处理。

// 在这里，`queueMicrotask()` 用于确保 'load' 事件总是异步地触发，
// 并且因此始终如一。
// 在这里使用 `process.nextTick()` 会导致 'load' 事件
// 总是在任何其他 promise 作业之前触发。

DataHandler.prototype.load = async function load(key) {
  const hit = this._cache.get(key);
  if (hit !== undefined) {
    queueMicrotask(() => {
      this.emit('load', hit);
    });
    return;
  }

  const data = await fetchData(key);
  this._cache.set(key, data);
  this.emit('load', data);
};

• callback <Function> Function to be queued.

The queueMicrotask() method queues a microtask to invoke callback. If callback throws an exception, the process object 'uncaughtException' event will be emitted.

The microtask queue is managed by V8 and may be used in a similar manner to the process.nextTick() queue, which is managed by Node.js. The process.nextTick() queue is always processed before the microtask queue within each turn of the Node.js event loop.

// Here, `queueMicrotask()` is used to ensure the 'load' event is always
// emitted asynchronously, and therefore consistently. Using
// `process.nextTick()` here would result in the 'load' event always emitting
// before any other promise jobs.

DataHandler.prototype.load = async function load(key) {
  const hit = this._cache.get(key);
  if (hit !== undefined) {
    queueMicrotask(() => {
      this.emit('load', hit);
    });
    return;
  }

  const data = await fetchData(key);
  this._cache.set(key, data);
  this.emit('load', data);
};