数组的承诺与结果分开 [英] Array of promise with results separately

问题描述

基本上，我正在寻找类似 Promise.all（）的东西，但是在然后（）每个结果，一旦结果准备就绪。使用这样的标准回调（文件阅读示例）编写它是微不足道的：

Basically, I'm looking for something like Promise.all(), but one that calls the function in then() for each result, as soon as that result is ready. It's trivial to write using standard callbacks like this (file reading example):

function foo(files, bar) {
  for (let file of files) {
    const fr = new FileReader();
    fr.onload = e => bar(e.target.result);
    fr.readAsDataURL(file);
  }
}

我确定有一个图书馆我想要的，但我想在纯JS中这样做。

I'm sure there's a library out there that does what I want, but I'd like to do that in pure JS.

为了澄清，我想要这样：

To clarify, I would like to have something like this:

function foo(files) {
  // do something here;
}

foo(['1','2','3']).then(/* do something here with separate results */);`

推荐答案

只需使用 map ：

let read = file => return new Promise(resolve => {
  const fr = new FileReader();
  fr.onload = e => resolve(e.target.result);
  fr.readAsDataURL(file);
});

Promise.all(files.map(file => read(file).then(bar)))
  .then(allResultsFromBar => { /* use results array */ });

请注意， .then 促进从它承诺执行的功能（即它隐式调用 Promise.resolve ），所以 Promise.all 是无论什么 bar 返回，保证收到一系列承诺。

Note that .then promotes anything returned from a function it executes to a promise (i.e. it calls Promise.resolve on it implicitly), so Promise.all is guaranteed to receive an array of promises no matter what bar returns.

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