使用返回promise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，因为返回了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：对于此示例，fs.stat已替换为setTimeout，请参阅 https://github.com/silenceisgolden/learn-esnext/blob/array-filter-async-function/tutorials /array-filter-with-async-function.js 了解特定代码。

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

如上所述注释， 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.

因为你现在可以（理论上） ）使用ES6的子类内置类型，您应该能够添加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：我评论过out the subclassing，因为Babel还不支持Arrays

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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