ES2015+ Cheatsheet

Table of Contents

A quick overview of new JavaScript features in ES2015, ES2016, ES2017, ES2018 and beyond.

Block scoping

let

function fn() {
    let x = 0
    if (true) {
        let x = 1 // only inside this `if`
    }
}

const

const a = 1

let is the new var. Constants work just like let, but can’t be reassigned. See: let and const

Backtick strings

Interpolation

const message = `Hello ${name}`

Multiline strings

const str = `
hello
world
`

Templates and multiline strings. See: Template strings

Binary and octal literals

let bin = 0b1010010
let oct = 0o755

See: Binary and octal literals

New methods

New string methods

'hello'.repeat(3)
'hello'.includes('ll')
'hello'.startsWith('he')
'hello'.padStart(8) // "   hello"
'hello'.padEnd(8) // "hello   "
'hello'.padEnd(8, '!') // hello!!!
'\u1E9B\u0323'.normalize('NFC')

See: New methods

Classes

class Circle extends Shape {

Constructor

  constructor (radius) {
    this.radius = radius
  }

Methods

  getArea () {
    return Math.PI * 2 * this.radius
  }

Calling superclass methods

  expand (n) {
    return super.expand(n) * Math.PI
  }

Static methods

  static createFromDiameter(diameter) {
    return new Circle(diameter / 2)
  }
}

Syntactic sugar for prototypes. See: Classes

Exponent operator

const byte = 2 ** 8
// Same as: Math.pow(2, 8)

Promises

See: JavaScript Promises

Destructuring

Destructuring assignment

Arrays

const [first, last] = ['Nikola', 'Tesla']

Objects

let { title, author } = {
    title: 'The Silkworm',
    author: 'R. Galbraith'
}

Supports for matching arrays and objects. See: Destructuring

Default values

const scores = [22, 33]
const [math = 50, sci = 50, arts = 50] = scores

// Result:
// math === 22, sci === 33, arts === 50

Default values can be assigned while destructuring arrays or objects.

Function arguments

function greet({ name, greeting }) {
    console.log(`${greeting}, ${name}!`)
}

greet({ name: 'Larry', greeting: 'Ahoy' })

Destructuring of objects and arrays can also be done in function arguments.

Default values

function greet({ name = 'Rauno' } = {}) {
    console.log(`Hi ${name}!`)
}

greet() // Hi Rauno!
greet({ name: 'Larry' }) // Hi Larry!

Reassigning keys

function printCoordinates({ left: x, top: y }) {
    console.log(`x: ${x}, y: ${y}`)
}

printCoordinates({ left: 25, top: 90 })

This example assigns x to the value of the left key.

Loops

for (let {title, artist} of songs) {
  ···
}

The assignment expressions work in loops, too.

Object destructuring

const { id, ...detail } = song

Extract some keys individually and remaining keys in the object using rest (…) operator

Spread

Object spread

with Object spread

const options = {
    ...defaults,
    visible: true
}

without Object spread

const options = Object.assign({}, defaults, { visible: true })

The Object spread operator lets you build new objects from other objects.

See: Object spread

Array spread

with Array spread

const users = [...admins, ...editors, 'rstacruz']

without Array spread

const users = admins.concat(editors).concat(['rstacruz'])

The spread operator lets you build new arrays in the same way.

See: Spread operator

Functions

Function arguments

Default arguments

function greet(name = 'Jerry') {
    return `Hello ${name}`
}

Rest arguments

function fn(x, ...y) {
    // y is an Array
    return x * y.length
}

Spread

fn(...[1, 2, 3])
// same as fn(1, 2, 3)

Default, rest, spread. See: Function arguments

Arrow functions

Arrow functions

setTimeout(() => {
  ···
})

With arguments

readFile('text.txt', (err, data) => {
  ...
})

Implicit return

numbers.map(n => n * 2)
// No curly braces = implicit return
// Same as: numbers.map(function (n) { return n * 2 })
numbers.map(n => ({
    result: n * 2
}))
// Implicitly returning objects requires parentheses around the object

Like functions but with this preserved. See: Arrow functions

Objects

Shorthand syntax

module.exports = { hello, bye }
// Same as: module.exports = { hello: hello, bye: bye }

See: Object literal enhancements

Methods

const App = {
    start() {
        console.log('running')
    }
}
// Same as: App = { start: function () {···} }

See: Object literal enhancements

Getters and setters

const App = {
    get closed() {
        return this.status === 'closed'
    },
    set closed(value) {
        this.status = value ? 'closed' : 'open'
    }
}

See: Object literal enhancements

Computed property names

let event = 'click'
let handlers = {
    [`on${event}`]: true
}
// Same as: handlers = { 'onclick': true }

See: Object literal enhancements

Extract values

const fatherJS = { age: 57, name: 'Brendan Eich' }

Object.values(fatherJS)
// [57, "Brendan Eich"]
Object.entries(fatherJS)
// [["age", 57], ["name", "Brendan Eich"]]

Modules

Imports

import 'helpers'
// aka: require('···')

import Express from 'express'
// aka: const Express = require('···').default || require('···')

import { indent } from 'helpers'
// aka: const indent = require('···').indent

import * as Helpers from 'helpers'
// aka: const Helpers = require('···')

import { indentSpaces as indent } from 'helpers'
// aka: const indent = require('···').indentSpaces

import is the new require(). See: Module imports

Exports

export default function () { ··· }
// aka: module.exports.default = ···

export function mymethod () { ··· }
// aka: module.exports.mymethod = ···

export const pi = 3.14159
// aka: module.exports.pi = ···

export is the new module.exports. See: Module exports

Generators

Generators

function* idMaker() {
    let id = 0
    while (true) {
        yield id++
    }
}

let gen = idMaker()
gen.next().value // → 0
gen.next().value // → 1
gen.next().value // → 2

It’s complicated. See: Generators

For..of iteration

for (let i of iterable) {
  ···
}

For iterating through generators and arrays. See: For..of iteration

Related