如果在此之前完成承诺，会发生什么？

问题描述：

假设我有一个承诺，就像这样：

Let's say I have a Promise like this:

var promise = new Promise(function(resolve, reject) {
    // Do some async thing
});
promise.then(function(response) {
    // Then do some other stuff
});

如果 async Promise 在我打电话给 .then（）之前完成？通常情况下，我只会在 Promise 函数中执行长时间运行的任务，但如果它很快完成一次会怎样？

What happens if the async Promise completes before I call .then()? Normally, I'd only have long running tasks in the Promise function, but what if it completes really quickly one time?

答

正如预期的那样：如果在promise已经解决后调用，则会立即调用回调函数。

As expected: then callback will get called immediately in this case if then was called after promise has already resolved.

这很容易测试：

var promise = new Promise(function(resolve, reject) {
    resolve(123);
});

setTimeout(function() {
  promise.then(function(response) {
      alert(response);
  });
}, 1000)

如果在此之前完成承诺，会发生什么？

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