Mastering JavaScript Functional Programming - Second Edition

Filtering with async calls

Filtering with an async function is a tad more complicated. We will have to use mapAsync() to produce an array of true/false values, and then use the standard filter() method to pick values out of the original array depending on what the async filtering function returned. Let's try out a simple example, calling the API and accepting only even results by means of a fakeFilter() function, which, for our example, accepts even numbers and rejects odd ones:

const filterAsync = (arr, fn) =>  mapAsync(arr, fn).then(arr2 => arr.filter((v, i) => Boolean(arr2[i])));const fakeFilter = (value) =>  new Promise(resolve =>    setTimeout(() => resolve(value % 2 === 0), 1000)  );(async () => {  console.log("START FILTER"); const filtered ...