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

There is a one liner to to do that.

const filterPromise = (values, fn) => 
    Promise.all(values.map(fn)).then(booleans => values.filter((_, i) => booleans[i]));

Pass the array into values and the function into fn.

More description on how this one liner works is available here.