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)

See: New library additions

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
}

See: Object Literals Enhanced

method

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

See: Object Literals Enhanced

Getters and setters

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

See: Object Literals Enhanced

Computed property name

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

See: Object Literals Enhanced

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

config  format