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

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.