使用返回承诺的函数过滤数组 [英] Filtering an array with a function that returns a promise

问题描述

给定

let arr = [1,2,3];

function filter(num) {
  return new Promise((res, rej) => {
    setTimeout(() => {
      if( num === 3 ) {
        res(num);
      } else {
        rej();
      }
    }, 1);
  });
 }

 function filterNums() {
   return Promise.all(arr.filter(filter));
 }

 filterNums().then(results => {
   let l = results.length;
   // length should be 1, but is 3
 });

长度为3，因为返回的是Promises，而不是值.有没有办法用返回 Promise 的函数过滤数组?

The length is 3 because Promises are returned, not values. Is there a way to filter the array with a function that returns a Promise?

注意:对于这个例子，fs.stat 已经被 setTimeout 替换，参见 https://github.com/silenceisgolden/learn-esnext/blob/array-filter-async-function/tutorials/array-filter-with-async-function.js具体代码.

Note: For this example, fs.stat has been replaced with setTimeout, see https://github.com/silenceisgolden/learn-esnext/blob/array-filter-async-function/tutorials/array-filter-with-async-function.js for the specific code.

推荐答案

正如评论中提到的，Array.prototype.filter 是同步的，因此不支持 Promises.

As mentioned in the comments, Array.prototype.filter is synchronous and therefore does not support Promises.

既然你现在(理论上)可以用 ES6 子类化内置类型，你应该能够添加你自己的异步方法来包装现有的过滤器函数:

Since you can now (theoretically) subclass built-in types with ES6, you should be able to add your own asynchronous method which wraps the existing filter function:

注意:我已经注释掉了子类化，因为 Babel 目前还不支持数组

class AsyncArray /*extends Array*/ {
  constructor(arr) {
    this.data = arr; // In place of Array subclassing
  }

  filterAsync(predicate) {
     // Take a copy of the array, it might mutate by the time we've finished
    const data = Array.from(this.data);
    // Transform all the elements into an array of promises using the predicate
    // as the promise
    return Promise.all(data.map((element, index) => predicate(element, index, data)))
    // Use the result of the promises to call the underlying sync filter function
      .then(result => {
        return data.filter((element, index) => {
          return result[index];
        });
      });
  }
}
// Create an instance of your subclass instead
let arr = new AsyncArray([1,2,3,4,5]);
// Pass in your own predicate
arr.filterAsync(async (element) => {
  return new Promise(res => {
    setTimeout(() => {
      res(element > 3);
    }, 1);
  });
}).then(result => {
  console.log(result)
});

Babel REPL 演示

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