How to use Array.prototype.filter with async?

Solution 1:

There is no way to use filter with an async function (at least that I know of). The simplest way that you have to use filter with a collection of promises is to use Promise.all and then apply the function to your collection of results. It would look something like this:

const results = await Promise.all(your_promises)
const filtered_results = results.filter(res => //do your filtering here)

Hope it helps.

Solution 2:

Adapted from the article How to use async functions with Array.filter in Javascript by Tamás Sallai, you basically have 2 steps:

1. One that creates the conditions for an object to pass
2. One that receives the objects and returns true or false according to conditions

Here's an example

const arr = [1, 2, 3, 4, 5];

function sleep(ms) {
      return new Promise((resolve) => setTimeout(resolve, ms));
    }

const asyncFilter = async (arr, predicate) => {
    const results = await Promise.all(arr.map(predicate));

    return arr.filter((_v, index) => results[index]);
}

const asyncRes = await asyncFilter(arr, async (i) => {
    await sleep(10);
    return i % 2 === 0;
});

console.log(asyncRes);
// 2,4

Solution 3:

Use Scramjet fromArray/toArray methods...

const result = await scramjet.fromArray(arr)
                             .filter(async (item) => somePromiseReturningMethod(item))
                             .toArray();

as simple as that - here's a ready example to copy/paste:

const scramjet = require('../../');

async function myAsyncFilterFunc(data) {
    return new Promise(res => {
        process.nextTick(res.bind(null, data % 2));
    });
}

async function x() {
    const x = await scramjet.fromArray([1,2,3,4,5])
        .filter(async (item) => myAsyncFilterFunc(item))
        .toArray();
    return x;
}

x().then(
    (out) => console.log(out),
    (err) => (console.error(err), process.exit(3)) // eslint-disable-line
);

Disclamer: I am the author of scramjet. :)

Solution 4:

Build a parallel array to your array which you want to call filter on. Await all of the promises from your filter func, in my eg, isValid. In the callback in filter, use the 2nd arg, index, to index into your parallel array to determine if it should be filtered.

// ===============================================
// common
// ===============================================
const isValid = async (value) => value >= 0.5;
const values = [0.2, 0.3, 0.4, 0.5, 0.6];


// ===============================================
// won't filter anything
// ===============================================
const filtered = values.filter(async v => await isValid(v));
console.log(JSON.stringify(filtered));


// ===============================================
// filters
// ===============================================
(async () => {
  const shouldFilter = await Promise.all(values.map(isValid));
  const filtered2 = values.filter((value, index) => shouldFilter[index]);

  console.log(JSON.stringify(filtered2));
})();

This behavior makes sense since any Promise instance has a truthy value, but it's not intuitive at a glance.