Filtering an array with a function that returns a promise

Question

Given

let arr = [1,2,3];

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


        

Answer 1

A valid way to do this (but it seems too messy):

let arr = [1,2,3];

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

async function check(num) {
  try {
    await filter(num);
    return true;
  } catch(err) {
    return false;
  }
}

(async function() {
  for( let num of arr ) {
    let res = await check(num);
    if(!res) {
      let index = arr.indexOf(num);
      arr.splice(index, 1);
    }
  }
})();

Again, seems way too messy.

Answer 2

For production purposes you probably want to use a lib like lodasync:

import { filterAsync } from 'lodasync'

const result = await filterAsync(async(element) => {
  await doSomething()
  return element > 3
}, array)

Under the hood, it maps your array by invoking the callback on each element and filters the array using the result. But you should not reinvent the wheel.

Answer 3

Here's a way:

var wait = ms => new Promise(resolve => setTimeout(resolve, ms));
var filter = num => wait(1).then(() => num == 3);

var filterAsync = (array, filter) =>
  Promise.all(array.map(entry => filter(entry)))
  .then(bits => array.filter(entry => bits.shift()));

filterAsync([1,2,3], filter)
.then(results => console.log(results.length))
.catch(e => console.error(e));

The filterAsync function takes an array and a function that must either return true or false or return a promise that resolves to true or false, what you asked for (almost, I didn't overload promise rejection because I think that's a bad idea). Let me know if you have any questions about it.

var wait = ms => new Promise(resolve => setTimeout(resolve, ms));
var filter = num => wait(1).then(() => num == 3);

var filterAsync = (array, filter) =>
  Promise.all(array.map(entry => filter(entry)))
  .then(bits => array.filter(entry => bits.shift()));

filterAsync([1,2,3], filter)
.then(results => console.log(results.length))
.catch(e => console.error(e));

var console = { log: msg => div.innerHTML += msg + "<br>",
                error: e => console.log(e +", "+ (e.lineNumber-25)) };

<div id="div"></div>

Answer 4

Promise Reducer to the rescue!

[1, 2, 3, 4].reduce((op, n) => {
    return op.then(filteredNs => {
        return new Promise(resolve => {
            setTimeout(() => {
                if (n >= 3) {
                    console.log("Keeping", n);
                    resolve(filteredNs.concat(n))
                } else {
                    console.log("Dropping", n);
                    resolve(filteredNs);
                }
            }, 1000);
        });
    });
}, Promise.resolve([]))
.then(filteredNs => console.log(filteredNs));

Reducers are awesome. "Reduce my problem to my goal" seems to be a pretty good strategy for anything more complex than what the simple tools will solve for you, i.e. filtering an array of things that aren't all available immediately.

Answer 5

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

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:

Note: I've commented out the subclassing, because it's not supported by Babel just yet for 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 Demo

Answer 6

Here's a shorter version of @pie6k's Typescript version:

async function filter<T>(arr: T[], callback: (val: T) => Promise<Boolean>) {
  const fail = Symbol()
  const result = (await Promise.all(arr.map(async item => (await callback(item)) ? item : fail))).filter(i => i !== fail)
  return result as T[] // the "fail" entries are all filtered out so this is OK
}