从承诺返回 then() [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();

我的 var 测试总是有一个未定义的值.我认为这是因为承诺还没有解决......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 are not resolved yet..there is a way to return a value from a promise?

推荐答案

当你从 then() 回调中返回一些东西时，这有点神奇.如果您返回一个值，则使用该值调用下一个 then().但是，如果您返回类似 promise 的内容，则下一个 then() 会等待它，并且仅在该 promise 稳定(成功/失败)时才会调用.

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).

来源:https://web.dev/promises/#queuing-asynchronous-actions

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