从承诺返回然后（） [英] Return from a promise then()

问题描述

我有一个这样的javascript代码：

I have got a javascript code like this:

function justTesting() {
  promise.then(function(output) {
    return output + 1;
  });
}

var test = justTesting();

var测试总是有一个未定义的值。我认为这是因为承诺没有得到解决。有没有办法从承诺中返回一个值？

I have got always an undefined value for the var test. I think that it is because the promises is not resolved jet..there is a way to return a value from a promises?

推荐答案

当你从then（）回调中返回一些东西时，它有点神奇。如果返回一个值，则使用该值调用next then（）。但是，如果你返回类似promise的东西，那么next（）会等待它，并且只有在该promise承诺结束时才会被调用（成功/失败）。

When you return something from a then() callback, it's a bit magic. If you return a value, the next then() is called with that value. However, if you return something promise-like, the next then() waits on it, and is only called when that promise settles (succeeds/fails).

via < a href =https://developers.google.com/web/fundamentals/getting-started/primers/promises#queuing_asynchronous_actions\"rel =noreferrer> https://developers.google.com/web/fundamentals/getting-开始/ primer / promises＃queuing_asynchronous_actions

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