ES6
Getting Started
Block-scoped
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 (const) work just like let, but cannot be reassigned.
See: Let and const
Template Strings
Interpolation
const message = `Hello ${name}`
Multi-line string
const str = `
hello
the world
`
Templates and multiline strings. See: template strings
Binary and octal literals
let bin = 0b1010010
let oct = 0o755
See: Binary and Octal Literals
Exponential Operator
const byte = 2 **8
Same as: Math.pow(2, 8)
New library additions
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")
New Number Methods
Number.EPSILON
Number.isInteger(Infinity) // false
Number.isNaN("NaN") // false
New Math methods
Math.acosh(3) // 1.762747174039086
Math.hypot(3, 4) // 5
Math.imul(Math.pow(2, 32) -1, Math.pow(2, 32) -2) // 2
New Array methods
//return a real array
Array.from(document.querySelectorAll("*"))
//similar to new Array(...), but without the special single-argument behavior
Array.of(1, 2, 3)
kind
class Circle extends Shape {
Constructor
constructor (radius) {
this.radius = radius
}
method
getArea () {
return Math.PI *2 *this.radius
}
Call the superclass method
expand(n) {
return super.expand(n) *Math.PI
}
Static methods
static createFromDiameter(diameter) {
return new Circle(diameter /2)
}
Syntactic sugar for prototypes. See: classes
Private class
The javascript default field is public (public), if you need to indicate private, you can use (#)
class Dog {
#name;
constructor(name) {
this.#name = name;
}
printName() {
// Only private fields can be called inside the class
console.log(`Your name is ${this.#name}`)
}
}
const dog = new Dog("putty")
//console.log(this.#name)
//Private identifiers are not allowed outside class bodies.
dog.printName()
Static private class
class ClassWithPrivate {
static #privateStaticField;
static #privateStaticFieldWithInitializer = 42;
static #privateStaticMethod() {
// …
}
}
Promises
make the commitment
new Promise((resolve, reject) => {
if (ok) { resolve(result) }
else { reject(error) }
})
for asynchronous programming. See: Promises
Using Promises
promise
.then((result) => { ··· })
.catch((error) => { ··· })
Using Promises in finally
promise
.then((result) => { ··· })
.catch((error) => { ··· })
.finally(() => {
/*logic independent of success/error */
})
The handler is called when the promise is fulfilled or rejected
Promise function
Promise.all(···)
Promise.race(···)
Promise.reject(···)
Promise.resolve(···)
Async-await
async function run () {
const user = await getUser()
const tweets = await getTweets(user)
return [user, tweets]
}
async functions are another way to use functions.
See: Async Function
Destructuring
Destructuring assignment
Arrays
const [first, last] = ['Nikola', 'Tesla']
Objects
let {title, author} = {
title: 'The Silkworm',
author: 'R. Galbraith'
}
Supports matching arrays and objects. See: Destructuring
Defaults
const scores = [22, 33]
const [math = 50, sci = 50, arts = 50] = scores
//Result:
//math === 22, sci === 33, arts === 50
A default value can be assigned when destructuring an array or object
Function parameters
function greet({ name, greeting }) {
console.log(`${greeting}, ${name}!`)
}
greet({ name: 'Larry', greeting: 'Ahoy' })
Destructuring of objects and arrays can also be done in function parameters
Defaults
function greet({ name = 'Rauno' } = {}) {
console.log(`Hi ${name}!`);
}
greet() // Hi Rauno!
greet({ name: 'Larry' }) // Hi Larry!
Reassign 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
Loop
for (let {title, artist} of songs) {
···
}
Assignment expressions also work in loops
Object Deconstruction
const { id, ...detail } = song;
Use the rest(...) operator to extract some keys individually and the rest of the keys in the object
Spread operator Spread
Object Extensions
with object extensions
const options = {
...defaults,
visible: true
}
No object extension
const options = Object.assign(
{}, defaults,
{ visible: true })
The object spread operator allows you to build new objects from other objects. See: Object Spread
Array Expansion
with array extension
const users = [
...admins,
...editors,
'rstacruz'
]
No array expansion
const users = admins
.concat(editors)
.concat([ 'rstacruz' ])
The spread operator allows you to build new arrays in the same way. See: Spread operator
Functions
Function parameters
Default parameters
function greet (name = 'Jerry') {
return `Hello ${name}`
}
Rest parameters
function fn(x, ...y) {
// y is an array
return x * y.length
}
Extensions
fn(...[1, 2, 3])
//same as fn(1, 2, 3)
Default (default), rest, spread (extension). See: function parameters
Arrow function
Arrow functions
setTimeout(() => {
···
})
with parameters
readFile('text.txt', (err, data) => {
...
})
implicit return
arr.map(n => n*2)
//no curly braces = implicit return
//Same as: arr.map(function (n) { return n*2 })
arr.map(n => ({
result: n*2
}))
//Implicitly returning an object requires parentheses around the object
Like a function, but preserves this.
See: Arrow functions
Parameter setting default value
function log(x, y = 'World') {
console.log(x, y);
}
log('Hello') // Hello World
log('Hello', 'China') // Hello China
log('Hello', '') // Hello
Used in conjunction with destructuring assignment defaults
function foo({x, y = 5} = {}) {
console.log(x, y);
}
foo() // undefined 5
name attribute
function foo() {}
foo.name // "foo"
length property
function foo(a, b){}
foo.length // 2
Objects
Shorthand Syntax
module.exports = { hello, bye }
same below:
module.exports = {
hello: hello, bye: bye
}
method
const App = {
start () {
console.log('running')
}
}
//Same as: App = { start: function () {···} }
Getters and setters
const App = {
get closed () {
return this.status === 'closed'
},
set closed (value) {
this.status = value ? 'closed' : 'open'
}
}
Computed property name
let event = 'click'
let handlers = {
[`on${event}`]: true
}
//Same as: handlers = { 'onclick': true }
Extract value
const fatherJS = { age: 57, name: "Zhang San" }
Object.values(fatherJS)
//[57, "Zhang San"]
Object.entries(fatherJS)
//[["age", 57], ["name", "Zhang San"]]
Modules module
Imports import
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
export default function () { ··· }
//aka: module.exports.default = ···
export function mymethod () { ··· }
//aka: module.exports.mymethod = ···
export const pi = 3.14159
//aka: module.exports.pi = ···
const firstName = 'Michael';
const lastName = 'Jackson';
const year = 1958;
export { firstName, lastName, year };
export *from "lib/math";
export is the new module.exports.
See: Module exports
as keyword renaming
import {
lastName as surname // import rename
} from './profile.js';
function v1() { ... }
function v2() { ... }
export { v1 as default };
//Equivalent to export default v1;
export {
v1 as streamV1, // export rename
v2 as streamV2, // export rename
v2 as streamLatestVersion // export rename
};
Dynamically load modules
button.addEventListener('click', event => {
import('./dialogBox.js')
.then(dialogBox => {
dialogBox. open();
})
.catch(error => {
/*Error handling */
})
});
ES2020 Proposal introduce import() function
import() allows module paths to be dynamically generated
const main = document.querySelector('main')
import(`./modules/${someVariable}.js`)
.then(module => {
module.loadPageInto(main);
})
.catch(err => {
main.textContent = err.message;
});
import.meta
ES2020 Added a meta property import.meta to the import command, which returns the meta information of the current module
new URL('data.txt', import.meta.url)
In the Node.js environment, import.meta.url always returns a local path, that is, a string of the file:URL protocol, such as file:/// home/user/foo.js
Import Assertions
static import
import json from "./package.json" assert {type: "json"}
//Import all objects in the json file
Dynamic Import
const json =
await import("./package.json", { assert: { type: "json" } })
Generators
Generator function
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 + iterator
let fibonacci = {
[Symbol.iterator]() {
let pre = 0, cur = 1;
return {
next() {
[pre, cur] = [cur, pre + cur];
return { done: false, value: cur }
}
}
}
}
for (var n of fibonacci) {
// truncate sequence at 1000
if (n > 1000) break;
console.log(n);
}
For iterating over generators and arrays. See: For..of iteration
Relationship with Iterator interface
var gen = {};
gen[Symbol.iterator] = function*() {
yield 1;
yield 2;
yield 3;
};
[...gen] // => [1, 2, 3]
The Generator function is assigned to the Symbol.iterator property, so that the gen object has the Iterator interface, which can be traversed by the ... operator
Symbol.iterator property
function*gen() { /*some code */}
var g = gen();
g[Symbol.iterator]() === g // true
gen is a Generator function, calling it will generate a traverser object g. Its Symbol.iterator property, which is also an iterator object generation function, returns itself after execution
see also
- Learn ES2015(babeljs.io)
- ECMAScript 6 Features Overview (github.com)