如何在.then（）链中访问先前的promise结果？ [英] How do I access previous promise results in a .then() chain?

问题描述

我已将我的代码重组为承诺，并构建了一个很棒的扁平承诺链 ，由多个 .then（）回调组成。最后我想返回一些复合值，并且需要访问多个中间承诺结果。但是，序列中间的分辨率值不在最后一个回调范围内，如何访问它们？

I have restructured my code to promises, and built a wonderful long flat promise chain, consisting of multiple .then() callbacks. In the end I want to return some composite value, and need to access multiple intermediate promise results. However the resolution values from the middle of the sequence are not in scope in the last callback, how do I access them?

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?
    });
}

推荐答案

ECMAScript Harmony

当然，这个问题也得到了语言设计师的认可。他们做了很多工作，异步功能提案终于进入了

您不需要单个然后调用或回调函数，如在异步函数中（在被调用时返回一个promise），你可以简单地等待promises直接解析。它还具有任意控制结构，如条件，循环和try-catch-clause，但为了方便起见，我们在这里不需要它们：

You don't need a single then invocation or callback function any more, as in an asynchronous function (that returns a promise when being called) you can simply wait for promises to resolve directly. It also features arbitrary control structures like conditions, loops and try-catch-clauses, but for the sake of convenience we don't need them here:

async function getExample() {
    var resultA = await promiseA(…);
    // some processing
    var resultB = await promiseB(…);
    // more processing
    return // something using both resultA and resultB
}

ECMAScript 6

当我们等待ES8时，我们已经使用了非常类似的语法。 ES6附带生成器功能，允许在任意放置的 yield 关键字。这些切片可以相互独立地运行，甚至是异步运行 - 这正是我们在运行下一步之前等待承诺解析时所做的事情。

ECMAScript 6

While we were waiting for ES8, we already did use a very similar kind of syntax. ES6 came with generator functions, which allow to break the execution apart in pieces at arbitrarily placed yield keywords. Those slices can be run after each other, independently, even asynchronously - and that's just what we do when we want to wait for a promise resolution before running the next step.

有专门的库（如 co <​​/a>或 task.js ），但许多promise库都有辅助函数（ Q ， Bluebird ，当，...）

There are dedicated libraries (like co or task.js), but also many promise libraries have helper functions (Q, Bluebird, when, …) that do this async step-by-step execution for you when you give them a generator function that yields promises.

var getExample = Promise.coroutine(function* () {
//               ^^^^^^^^^^^^^^^^^ Bluebird syntax
    var resultA = yield promiseA(…);
    // some processing
    var resultB = yield promiseB(…);
    // more processing
    return // something using both resultA and resultB
});

自版本4.0以来，它在Node.js中运行，也有一些浏览器（或它们的开发版本）确实相对较早地支持生成器语法。

This did work in Node.js since version 4.0, also a few browsers (or their dev editions) did support generator syntax relatively early.

但是，如果你想/需要倒退 - 兼容你不能使用没有转换器的那些。当前工具支持生成器函数和异步函数，例如参见Babel上的文档生成器和异步功能。

However, if you want/need to be backwards-compatible you cannot use those without a transpiler. Both generator functions and async functions are supported by the current tooling, see for example the documentation of Babel on generators and async functions.

然后，还有很多其他 compile-to-JS语言
致力于简化异步编程。它们通常使用类似于 await 的语法，（例如 Iced CoffeeScript ），但也有其他一些像Haskell一样的做 -notation（例如 LatteJs ， monadic ， PureScript 或 LispyScript ）。

And then, there are also many other compile-to-JS languages that are dedicated to easing asynchronous programming. They usually use a syntax similar to await, (e.g. Iced CoffeeScript), but there are also others that feature a Haskell-like do-notation (e.g. LatteJs, monadic, PureScript or LispyScript).

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