New Upcoming JavaScript features in 2020

New Upcoming JavaScript features in 2020

You should know if you use JavaScript everyday. That is "New Upcoming JavaScript features" in this post

Upcoming new JavaScript features You should know if you use JavaScript everyday

Since ECMAScript2015 (also called ES6) was released, JavaScript has changed and improved widely. This is excellent news for all of the JavaScript developers. Furthermore, a new ECMAScript version has released every year. You likely didn’t notice what features were added in the latest ECMAScript, which was released in June 2019. I will briefly show you the new features added in the latest version and talk about the new features for the future version.

The features that I will show you are NOT yet decided to be in the next version. All of what I will talk about in this post are currently in stage 3. Check out this repo if you want to get more details.

Features in ECMAScript2019 (ES10)

1. Array.prototype.flat

A method that creates a new array with all sub-array elements concatenated into it recursively up to the specified depth.

const array = [1, 2, [3, 4]];
array.flat(); // [1, 2, 3, 4];

This is very useful, especially when you want to flatten your nested array. If the depth of your array is deeper than one depth, calling flat once can’t entirely flatten your array. flat takes a parameter for depth, which refers to how many depths you want it to go into to flatten the array.

// Crazy example
const crazyArray = [1, 2, [3, 4], [[5], [6, [7,8]]]];
crazyArray.flat(Infinity); // [1, 2, 3, 4, 5, 6, 7, 8];
// The parameter must be the number type

The deeper you want to search the array, the more computing time will be required to flatten it. Note that IEs and Edge do not support this feature.

2. Array.prototype.flatMap

A method first maps each element using a mapping function, then flattens the result into a new array.

const arr = ["it's Sunny in", "", "California"];
arr.flatMap(x => x.split(" "));
// ["it's","Sunny","in", "", "California"]

The difference between flat and flatMap is that you can put a custom function in flatMap to manipulate each value. Additionally, unlike flat, flatMap flattens one depth array, only. The return value should be an array type. This would be very useful when you should do something before flattening the array.

There were more features added to ES10. if you want to know more about them.

New Features In Stage 3

In stage 3, there are a few interesting features suggested. I will introduce some of them to you briefly.

Numeric Separators

When you assigned a big number to a variable, weren’t you confused on how big that number is or if you wrote it right? This proposal allows you to put an underscore between numbers so you can count it easier.

1_000_000_000           // Ah, so a billion
101_475_938.38          // And this is hundreds of millions

let fee = 123_00;       // $123 (12300 cents, apparently)
let fee = 12_300;       // $12,300 (woah, that fee!)
let amount = 12345_00;  // 12,345 (1234500 cents, apparently)
let amount = 123_4500;  // 123.45 (4-fixed financial)
let amount = 1_234_500; // 1,234,500

let budget = 1_000_000_000_000;
// What is the value of `budget`? It's 1 trillion!
// Let's confirm:
console.log(budget === 10 ** 12); // true

It will be up to each developer whether to use this feature once it’s released, but one thing’s for sure, this feature would reduce your headaches for counting how big a number is!

Top-level await

Top-level _await_ enables modules to act as big async functions: With top-level _await_, ECMAScript Modules (ESM) can _await_ resources, causing other modules who _import_ them to wait before they start evaluating their body.

The motivation of this feature was that when you import a module which has async function, the output of the async function is undefined.

// awaiting.mjs
import { process } from "./some-module.mjs";
const dynamic = import(computedModuleSpecifier);
const data = fetch(url);
export const output = process((await dynamic).default, await data);

There are two files. output could be undefined if it’s called before the Promises tasks are done.

// usage.mjs
import { output } from "./awaiting.mjs";
export function outputPlusValue(value) { return output + value }
setTimeout(() => console.log(outputPlusValue(100), 1000);

usage.mjs will not execute any of the statements in it until the awaits in awaiting.mjs have had their Promises resolved.

Nullish Coalescing for JavaScript

This would be one of the most useful features amongst proposals in stage 3. We often wrote this kind of code.

const obj = { 
  name: 'James'

const name = || 'Jane'; // James

If is falsy, then return ‘Jane’, so undefined won’t be returned. But the problem is, an empty string(‘’) is also considered falsy in this case. Then we should rewrite it again like this below.

const name = ( && !== '') || 'Jane';

It is a pain in the neck to write the code like that every time. This proposal allows you to check null and undefined only.

const response = {
  settings: {
    nullValue: null,
    height: 400,
    animationDuration: 0,
    headerText: '',
    showSplashScreen: false

const undefinedValue = response.settings.undefinedValue ?? 'some other default'; // result: 'some other default'
const nullValue = response.settings.nullValue ?? 'some other default'; // result: 'some other default'
const headerText = response.settings.headerText ?? 'Hello, world!'; // result: ''
const animationDuration = response.settings.animationDuration ?? 300; // result: 0
const showSplashScreen = response.settings.showSplashScreen ?? true; // result: false

Optional Chaining

This proposal goes with Nullish Coalescing for JavaScript, especially in TypeScript. TypeScript has announced that they will include Nullish Coalescing for JavaScript and this proposal in their next released version, 3.7.0.

const city = country &&; 
// undefined if city doesn't exist

Look at the example code. To get city, which is in country object, we should check if country exists and if city exists in country.

With Optional Chaining, this code can be refactored like this.

const city = country?.city; // undefined if city doesn't exist

This feature seems very handy and useful for this situation.

import { fetch } from '../yourFetch.js';
(async () => {
  const res = await fetch();
  // res && && || undefined
  const cities = res?.data?.cities;


_Promise.any_ accepts an iterable of promises and returns a promise that is fulfilled by the first given promise to be fulfilled, or rejected with an array of rejection reasons if all of the given promises are rejected.

With async-await,

try {
  const first = await Promise.any(promises);
  // Any of the promises was fulfilled.
} catch (error) {
  // All of the promises were rejected.

With Promise pattern,

  (first) => {
    // Any of the promises was fulfilled.
  (error) => {
    // All of the promises were rejected.

Since there were Promise all, allSettled, and race, there wasn’t any. So this feature is simple but powerful for a needed situation.

However, this proposal wasn’t tested yet, so this proposal might have a longer time to be accepted in a future version of ECMAScript.


There are so many interesting proposals in stage 3. I can’t wait to meet them in ES11 or ES12. Of course, I won’t need all of them, but some of them would definitely make my codes more elegant.

Thank you ! If you liked this post, share it with all of your programming buddies!

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JavaScript developers should you be using Web Workers?

JavaScript developers should you be using Web Workers?

Do you think JavaScript developers should be making more use of Web Workers to shift execution off of the main thread?

Originally published by David Gilbertson at

So, Web Workers. Those wonderful little critters that allow us to execute JavaScript off the main thread.

Also known as “no, you’re thinking of Service Workers”.

Photo by Caleb Jones on Unsplash

Before I get into the meat of the article, please sit for a lesson in how computers work:

Understood? Good.

For the red/green colourblind, let me explain. While a CPU is doing one thing, it can’t be doing another thing, which means you can’t sort a big array while a user scrolls the screen.

This is bad, if you have a big array and users with fingers.

Enter, Web Workers. These split open the atomic concept of a ‘CPU’ and allow us to think in terms of threads. We can use one thread to handle user-facing work like touch events and rendering the UI, and different threads to carry out all other work.

Check that out, the main thread is green the whole way through, ready to receive and respond to the gentle caress of a user.

You’re excited (I can tell), if we only have UI code on the main thread and all other code can go in a worker, things are going to be amazing (said the way Oprah would say it).

But cool your jets for just a moment, because websites are mostly about the UI — it’s why we have screens. And a lot of a user’s interactions with your site will be tapping on the screen, waiting for a response, reading, tapping, looking, reading, and so on.

So we can’t just say “here’s some JS that takes 20ms to run, chuck it on a thread”, we must think about where that execution time exists in the user’s world of tap, read, look, read, tap…

I like to boil this down to one specific question:

Is the user waiting anyway?

Imagine we have created some sort of git-repository-hosting website that shows all sorts of things about a repository. We have a cool feature called ‘issues’. A user can even click an ‘issues’ tab in our website to see a list of all issues relating to the repository. Groundbreaking!

When our users click this issues tab, the site is going to fetch the issue data, process it in some way — perhaps sort, or format dates, or work out which icon to show — then render the UI.

Inside the user’s computer, that’ll look exactly like this.

Look at that processing stage, locking up the main thread even though it has nothing to do with the UI! That’s terrible, in theory.

But think about what the human is actually doing at this point. They’re waiting for the common trio of network/process/render; just sittin’ around with less to do than the Bolivian Navy.

Because we care about our users, we show a loading indicator to let them know we’ve received their request and are working on it — putting the human in a ‘waiting’ state. Let’s add that to the diagram.

Now that we have a human in the picture, we can mix in a Web Worker and think about the impact it will have on their life:


First thing to note is that we’re not doing anything in parallel. We need the data from the network before we process it, and we need to process the data before we can render the UI. The elapsed time doesn’t change.

(BTW, the time involved in moving data to a Web Worker and back is negligible: 1ms per 100 KB is a decent rule of thumb.)

So we can move work off the main thread and have a page that is responsive during that time, but to what end? If our user is sitting there looking at a spinner for 600ms, have we enriched their experience by having a responsive screen for the middle third?


I’ve fudged these diagrams a little bit to make them the gorgeous specimens of graphic design that they are, but they’re not really to scale.

When responding to a user request, you’ll find that the network and DOM-manipulating part of any given task take much, much longer than the pure-JS data processing part.

I saw an article recently making the case that updating a Redux store was a good candidate for Web Workers because it’s not UI work (and non-UI work doesn’t belong on the main thread).

Chucking the data processing over to a worker thread sounds sensible, but the idea struck me as a little, umm, academic.

First, let’s split instances of ‘updating a store’ into two categories:

  1. Updating a store in response to a user interaction, then updating the UI in response to the data change
  2. Not that first one

If the first scenario, a user taps a button on the screen — perhaps to change the sort order of a list. The store updates, and this results in a re-rendering of the DOM (since that’s the point of a store).

Let me just delete one thing from the previous diagram:

In my experience, it is rare that the store-updating step goes beyond a few dozen milliseconds, and is generally followed by ten times that in DOM updating, layout, and paint. If I’ve got a site that’s taking longer than this, I’d be asking questions about why I have so much data in the browser and so much DOM, rather than on which thread I should do my processing.

So the question we’re faced with is the same one from above: the user tapped something on the screen, we’re going to work on that request for hopefully less than a second, why would we want to make the screen responsive during that time?

OK what about the second scenario, where a store update isn’t in response to a user interaction? Performing an auto-save, for example — there’s nothing more annoying than an app becoming unresponsive doing something you didn’t ask it to do.

Actually there’s heaps of things more annoying than that. Teens, for example.

Anyhoo, if you’re doing an auto-save and taking 100ms to process data client-side before sending it off to a server, then you should absolutely use a Web Worker.

In fact, any ‘background’ task that the user hasn’t asked for, or isn’t waiting for, is a good candidate for moving to a Web Worker.

The matter of value

Complexity is expensive, and implementing Web Workers ain’t cheap.

If you’re using a bundler — and you are — you’ll have a lot of reading to do, and probably npm packages to install. If you’ve got a create-react-app app, prepare to eject (and put aside two days twice a year to update 30 different packages when the next version of Babel/Redux/React/ESLint comes out).

Also, if you want to share anything fancier than plain data between a worker and the main thread you’ve got some more reading to do (comlink is your friend).

What I’m getting at is this: if the benefit is real, but minimal, then you’ve gotta ask if there’s something else you could spend a day or two on with a greater benefit to your users.

This thinking is true of everything, of course, but I’ve found that Web Workers have a particularly poor benefit-to-effort ratio.

Hey David, why you hate Web Workers so bad?

Good question.

This is a doweling jig:

I own a doweling jig. I love my doweling jig. If I need to drill a hole into the end of a piece of wood and ensure that it’s perfectly perpendicular to the surface, I use my doweling jig.

But I don’t use it to eat breakfast. For that I use a spoon.

Four years ago I was working on some fancy animations. They looked slick on a fast device, but janky on a slow one. So I wrote fireball-js, which executes a rudimentary performance benchmark on the user’s device and returns a score, allowing me to run my animations only on devices that would render them smoothly.

Where’s the best spot to run some CPU intensive code that the user didn’t request? On a different thread, of course. A Web Worker was the correct tool for the job.

Fast forward to 2019 and you’ll find me writing a routing algorithm for a mapping application. This requires parsing a big fat GeoJSON map into a collection of nodes and edges, to be used when a user asks for directions. The processing isn’t in response to a user request and the user isn’t waiting on it. And so, a Web Worker is the correct tool for the job.

It was only when doing this that it dawned on me: in the intervening quartet of years, I have seen exactly zero other instances where Web Workers would have improved the user experience.

Contrast this with a recent resurgence in Web Worker wonderment, and combine that contrast with the fact that I couldn’t think of anything else to write about, then concatenate that combined contrast with my contrarian character and you’ve got yourself a blog post telling you that maybe Web Workers are a teeny-tiny bit overhyped.

Thanks for reading

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Further reading

An Introduction to Web Workers

JavaScript Web Workers: A Beginner’s Guide

Using Web Workers to Real-time Processing

How to use Web Workers in Angular app

Using Web Workers with Angular CLI

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