What's new in JavaScript ES6?

13 February, 2018

Recently I had to go through my company’s code to figure out why certain browsers (Cough Internet Explorer Cough) weren’t working - here’s a kind of post-mortem:

We used webpack and babel to build our sites - but normally we only specify node and react as targets. The thing is, ES6 never got implemented on Internet Explorer 11, so I had to dig through and figure out what isn’t supported.

Here’s a partial list of all the awesome things you can’t do for your users (without a polyfill) on IE11:

  • Arrow functions

Just your average function, except they share the same ‘this’ as the code surrounding it

  • Classes

Classes in ES6 are basically just functions that allow you to use classic inheritance, instance methods, and constructors. They can also have instances.

  • Object Literals

Basically just let you have adhoc functions in your objects, and perform ‘super’ calls on the object’s parents. Brings more of an Object Oriented approach to Javascript.

  • Template Strings

Just lets you construct strings in a syntactically nice way. Things like multilining, inserting variables via interpolation, etc.

  • Destructuring

Lets you pull in a variable name out of an object, for direct use (doesn’t remove the variable from the object, just gives you access to its name directly) i.e

let x = { this: 123, that:345}
const { this } = x
// 123
  • Default Parameters

Lets you optionally pass/skip arguments. i.e:

function blah(x, y=5)
{ return x \* y }
console.log(blah(5)) //25
  • Rest Parameters

Allows you to pass in infinite arguments to a function via an array i.e

function sum(...args){
  return args.reduce((previous, current) => {
      return previous + current
  • Spread Syntax

allows an array, object, or string to be deconstructed out or expanded into places where zero or more are expected. Use case: spread a smaller object into a bigger one, without nesting the object: i.e.

let myObject = {a: b, c: d}
let Item = {
  x: y,
  ...myObject }
  • Let + Const

Let is the new var, except its scope is limited to its block Const is a variable you can only define once

  let y = 5
y = 5 //syntax error, y is undefined
  • Map object

Lets you store key:value pairs

Maps can store anything against anything, while object keys have to be strings

Map has a size property

Maps are iterable Objects

Maps have a prototype, so technically you could collide with those defaultkeys

Maps may perform better than objects when frequently adding or removing key/value pairs

  • Object.assign()

Usage: Object.assign({}, object)

Method to copy values from a source object to a target object Returns the target object

  • Promises

a promise is a representation of a value that may be available in the future - particularly useful for asynchronous programming

  • Set object

Lets you store a list of unique values of any type

const set2 = new Set([1,2,3,])
//trade-off is that ensuring uniqueness costs you speed

It also doesn’t allow index-based access, as you would have in an Array

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