javascript - 如何访问.then（）链中的先前的诺言结果？(How do I access previous promise results in a .then() chain?)

Question

I have restructured my code to promises , and built a wonderful long flat promise chain , consisting of multiple .then() callbacks.

(我已经将我的代码重组为Promise ，并构建了一个精彩的，长而平坦的Promise链 ，其中包括多个.then()回调。)

In the end I want to return some composite value, and need to access multiple intermediate promise results .

(最后，我想返回一些复合值，并且需要访问多个中间promise结果 。)

However the resolution values from the middle of the sequence are not in scope in the last callback, how do I access them?

(但是，序列中间的分辨率值不在上次回调的范围内，如何访问它们？)

function getExample() {
    return promiseA(…).then(function(resultA) {
        // Some processing
        return promiseB(…);
    }).then(function(resultB) {
        // More processing
        return // How do I gain access to resultA here?
    });
}

  ask by Bergi translate from so

Answer 1

Break the chain(断链)

When you need to access the intermediate values in your chain, you should split your chain apart in those single pieces that you need.

(当需要访问链中的中间值时，应将链分成所需的单个部分。)

Instead of attaching one callback and somehow trying to use its parameter multiple times, attach multiple callbacks to the same promise - wherever you need the result value.

(不必附加一个回调并以某种方式尝试多次使用其参数，而是将多个回调附加到同一promise-无论何时需要结果值。)

Don't forget, a promise just represents (proxies) a future value !

(不要忘记， 承诺只是代表（代理）未来的价值 ！)

Next to deriving one promise from the other in a linear chain, use the promise combinators that are given to you by your library to build the result value.

(除了从线性链中的另一个承诺中推导一个承诺之外，还使用库提供给您的承诺组合器来构建结果值。)

This will result in a very straightforward control flow, clear composition of functionalities and therefore easy modularisation.

(这将导致非常简单的控制流程，清晰的功能组成，因此易于模块化。)

function getExample() {
    var a = promiseA(…);
    var b = a.then(function(resultA) {
        // some processing
        return promiseB(…);
    });
    return Promise.all([a, b]).then(function([resultA, resultB]) {
        // more processing
        return // something using both resultA and resultB
    });
}

Instead of the parameter destructuring in the callback after Promise.all that only became available with ES6, in ES5 the then call would be replaced by a nifty helper method that was provided by many promise libraries ( Q , Bluebird , when , …): .spread(function(resultA, resultB) { … .

(与其在Promise.all之后的回调中进行参数解构（仅在ES6中可用），在ES5中， then调用将由许多promise库（ Q ， Bluebird ， when ，…）提供的漂亮的助手方法代替： .spread(function(resultA, resultB) { … 。)

Bluebird also features a dedicated join function to replace that Promise.all + spread combination with a simpler (and more efficient) construct:

(蓝鸟还具有专用的join功能，可以用更简单（更高效）的结构替换Promise.all + spread组合：)

…
return Promise.join(a, b, function(resultA, resultB) { … });