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
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")
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
Promises
Making promises
new Promise((resolve, reject) => {
if (ok) { resolve(result) }
else { reject(error) }
})
For asynchronous programming. See: Promises
Using promises
promise
.then((result) => { ··· })
.catch((error) => { ··· })
Using promises with finally
promise
.then((result) => { ··· })
.catch((error) => { ··· })
.finally(() => { // logic independent of success/error })
The handler is called when the promise is fulfilled or rejected.
Promise functions
Promise.all(···)
Promise.race(···)
Promise.reject(···)
Promise.resolve(···)
Async-await
async function run () {
const user = await getUser()
const tweets = await getTweets(user)
return [user, tweets]
}
async functions are another way of using functions.
See: async function
Destructuring
De-structuring 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 be 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 leftkey.
Loops
for (let {title, artist} of songs) {
···
}
The assignment expressions work in loops, too.
Object de-structuring
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 ${name}`
}
Rest arguments
function fn(x, ...y) {
// y is an Array
return x * y.length
}
Spread
fn(...[1, 2, 3])
// same as fn(1, 2, 3)
Default, rest, spread. See: Function arguments
Fat arrows
setTimeout(() => {
···
})
With arguments
readFile('text.txt', (err, data) => {
...
})
Implicit return
numbers.map(n => n * 2)
// No curly braces = implicit return
// Same as: numbers.map(function (n) { return n * 2 })
numbers.map(n => ({
result: n * 2
})
// Implicitly returning objects requires parentheses around the object
Like functions but with this preserved. See: Fat arrows
Objects
Shorthand syntax
module.exports = { hello, bye }
// Same as: module.exports = { hello: hello, bye: bye }
See: Object literal enhancements
Methods
const App = {
start () {
console.log('running')
}
}
// Same as: App = { start: function () {···} }
See: Object literal enhancements
Getters and setters
const App = {
get closed () {
return this.status === 'closed'
},
set closed (value) {
this.status = value ? 'closed' : 'open'
}
}
See: Object literal enhancements
Computed property names
let event = 'click'
let handlers = {
[`on${event}`]: true
}
// Same as: handlers = { 'onclick': true }
See: Object literal enhancements
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