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);
      } 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
 });

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?

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.

Answer 1

Here is a 2017 elegant solution using async/await :

Very straightforward usage:

const results = await filter(myArray, async num => {
  await doAsyncStuff()
  return num > 2
})

The helper function (copy this into your web page):

async function filter(arr, callback) {
  const fail = Symbol()
  return (await Promise.all(arr.map(async item => (await callback(item)) ? item : fail))).filter(i=>i!==fail)
}

Demo:

// Async IIFE
(async function() {
  const myArray = [1, 2, 3, 4, 5]

  // This is exactly what you'd expect to write 
  const results = await filter(myArray, async num => {
    await doAsyncStuff()
    return num > 2
  })

  console.log(results)
})()


// Arbitrary asynchronous function
function doAsyncStuff() {
  return Promise.resolve()
}


// The helper function
async function filter(arr, callback) {
  const fail = Symbol()
  return (await Promise.all(arr.map(async item => (await callback(item)) ? item : fail))).filter(i=>i!==fail)
}

I'll even throw in a CodePen.