Sandro Turriate

Coder, cook, explorer

Modern Javascript: Everything you missed over the last 10 years (ECMAScript 2020)

JavaScript has come a long way since I knew it as the “D” in DHTML. For anyone like me, who’s been reluctant to use the latest syntax that could require polyfills or a transpiler, I’ve written this cheatsheet to get you caught up on all the goodness that’s widely supported in modern browsers.

I’ve made this page concise, with runnable examples and links to further documentation. If you have any questions or spot any errata, please contact me.

Array functions

Check out all these new built-in array functions! No more need for underscore or lodash!

  • Array.every()
  • Array.filter()
  • Array.find()
  • Array.findIndex()
  • Array.forEach()
  • Array.from()
  • Array.includes()
  • Array.isArray()
  • Array.lastIndexOf()
  • Array.reduce()
  • Array.reduceRight()
  • Array.some()

Array docs


These new keywords declare variables in block scope (as opposed to global or function scope). Using const implies that the value will not change as the reference is immutable. Use let if the value will change.

Let documentation

Nullish coalescing ?? and Optional chaining ?. operators

The ?? operator checks if the value is null or undefined. No more need to use the !! check.

The ?. operator checks if the value is truthy before calling the next property or function. Extremely useful when dealing with optional props.

Optional chaining documentation


The async/await keywords are here to save you from callback hell. Use await to make an asynchronous call resemble a synchronous call, i.e. running await fetchUserName() will not proceed to the next line until fetchUserName() is complete. Note, in order to use await, you have to be executing a function declared as async, i.e.
async function fn(){ await fetchUserName() }.

Async/Await docs.

Arrow functions ()=>{}

These are functions that are bound to the current context. There are three main forms you’ll see in the wild:
single argument, single line, multi-line.

The single argument form does not require parenthesis, and the single line form does not require a return statement; the return is implicit.

const fn = a => a*2
Single argument. Single line.

The multi-line form requires a return statement if the function intends to returns something. Multiple arguments require parenthesis.

const fn = (a,b) => {
  return a*b
Multiple arguments, multiple lines.

Arrow function docs


Used for looping over an iterator. Similar to except you don’t have to check for hasOwnProperty. You cannot use this looping syntax on an Object directly because the Object doesn’t have an iterator. Instead use Object.entries({}) to retrieve an iterable.

for...of docs

for await...of

Asynchronous iteration was introduced In 2018. Much like Promise.all, it can be used to synchronize many asynchronous tasks. The example below shows 3 tasks happening asynchronously. The loop processes one result at a time, in order; in this case, the quickest tasks to complete are only evident at the end of the iteration.

for await...of docs


In 2015, ES6 brought classes to Javascript 🎉. Javascript classes are similar to the classes you know and love from other languages. Inheritance, class methods, getters and setters, properties, etc.

Class documentation


Get and set are functions that are called like properties, i.e. person.age = 16; person.age > 18. These are very convenient when you need a dynamic or computed property. And they can be used with both classes and regular objects.

get/set documentation

Classes with getters and setters

Objects with getters and setters

function default parameters

Yay! You can now specify default parameters in your function definition. Works as you would expect.

Default parameter docs

function named parameters

With a bit of object destructuing magic, functions can now have named parameters.

Named parameter docs

function rest ... parameter

The rest parameter allows a function to accept an arbitrary number of arguments as an array. It’s recommended to use this over arguments.

Rest parameter docs

Object.assign and spread operator

Object.assign(target, source) merges two or more objects into one. It modifies the target object in-place, so if you’d prefer a new object be created, pass an empty object literal as the first argument.

Alternatively, you can use the spread operator ... to merge multiple objects together: {...obj1, ...obj2}, though bear in mind, spread will not call setters on the object, so to be the most portable, consider Object.assign. The spread operator can also be used on arrays as shown in the last code sample.

Spread syntax docs


Destructuring allows you to extract values from objects and arrays through patterns. It is a complex topic with many applications…far too many for me to enumerate, but I've shown some of the most common uses I can think of.

Destructuring docs and MDN docs

Shorthand functions aka Methods

Functions declared on objects can use a new shorthand style that omits the function keyword.

The two functions (shorthand(), long()) are equivalent in the sample below.

Method guide


I’ve mostly skipped over promises because async/await is preferred, but sometimes you need to synchronize multiple asynchronous calls, and Promise.all is the easiest way to do it.

Promise.all documentation

Template literals

Also known as template strings, this new syntax provides easy string interpolation and multi-line strings.

Template literal docs


A Proxy allows you to intercept get/set calls on another object. This could be useful for watching a property for changes, then updating the DOM, or making innovative APIs like the www proxy below.
www proxy program

Proxy docs

Module import/export

Modules allow you to namespace your code and break down functionality into smaller files. In the example below, we have a module named greet.js that gets included in index.html. Note, module loading is always deferred, so it won’t block the HTML from rendering. There are many ways to import/export functionality from js files, read more in the export docs.

Import docs

function greet(msg) {
  console.log("greet:", msg)
export default greet
A file named "greet.js" in the "/js" directory.
<script type="module">
  import greet from "/js/greet.js"

Read More

Okay, so I didn't cover everything that’s changed over the last decade, just the items I find most useful. Check out these other topics.



Apr 15, 2021