Sallar Kaboli

Promisified Arrays

December 16, 2015

I like Javascript Promises a lot, so I’m trying to fit them everywhere in my apps. If you haven’t heard about Promises, or haven’t used them yet, I suggest you check out this great guide on them.

I also like ES5 array functions and in my opinion they’re a few of the best additions to the language. My favorites are filter and map. The problem with these functions is they are synchronous, and I find myself trying to do something like this:

var result = {
    // Do something Async here.

Which clearly isn’t going to work out since the callback function passed to map needs to have a return value. A possible solution to this is turn each array item into a Promise object and wait for all promises to resolve using Promise.all. Something like:

var arr = [1, 2, 3];
var promisified = {
    return new Promise(function(resolve) {
        resolve(item * 10);

Promise.all(promisified).then(function(result) {
    console.log(result); // [10, 20, 30];

This solution is alright, until you need to do it repeatedly then it will get verbose pretty fast. So I made a small helper library to do it.


This super small library provides async versions of map and filter which return a Promise with the result. Take this example:

var PromiseArrays = require('promise-arrays');
var arr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

PromiseArrays.filter(arr, function(item) {
    return new Promise(function(resolve) {
        window.setTimeout(function() {
            resolve(item > 3 && item < 8);
        }, 100);
}).then(function(result) {
    console.log(result); // [4, 5, 6, 7] after about ~100ms.

It’s possible to return a new Promise or a normal value from the callback.

promise-arrays is available to use with Node.JS, UMD and browser globals. Check it out on Github and please report any issues. All suggestions are also welcome :-)

Sallar Kaboli

Thoughts on Software Engineering, IoT and more. Written by Sallar Kaboli who lives in Helsinki. Follow me on Twitter and GitHub.