ES6 Syntax and Feature Overview

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Note: A commonly accepted practice is to use const except in cases of loops and reassignment. However, in this resource I'll be using let in place of var for all ES6 examples.

• Variable: x
• Object: obj
• Array: arr
• Function: func
• Parameter, method: a, b, c
• String: str

Table of contents

• Variable declaration
• Constant declaration
• Arrow function syntax
• Template literals
• Implicit returns
• Key/property shorthand
• Method definition shorthand
• Destructuring (object matching)
• Array iteration (looping)
• Default parameters
• Spread syntax
• Classes/constructor functions
• Inheritance
• Modules - export/import
• Promises/callbacks

Variables and constant feature comparison

Understanding Variables, Scope, and Hoisting in JavaScript 

Keyword	Scope	Hoisting	Can Be Reassigned	Can Be Redeclared
var	Function scope	Yes	Yes	Yes
let	Block scope	No	Yes	No
const	Block scope	No	No	No

Variable declaration

ES6 introduced the let keyword, which allows for block-scoped variables which cannot be hoisted or redeclared.

MDN Reference: let

Constant declaration

ES6 introduced the const keyword, which cannot be redeclared or reassigned, but is not immutable.

MDN Reference: const

Arrow functions

The arrow function expression syntax is a shorter way of creating a function expression. Arrow functions do not have their own this, do not have prototypes, cannot be used for constructors, and should not be used as object methods.

# ES5

function func(a, b, c) {} // function declaration
var func = function(a, b, c) {} // function expression

#ES6

let func = a => {} // parentheses optional with one parameter
let func = (a, b, c) => {} // parentheses required with multiple parameters

　　MDN Reference: Arrow functions

Template literals（模板文字）

Concatenation/string interpolation

Expressions can be embedded in template literal strings.

# ES5

var str = 'Release date: ' + date

# ES6

let str = `Release Date: ${date}`

　　MDN Reference: Expression interpolation

Multi-line strings

Using template literal syntax, a JavaScript string can span multiple lines without the need for concatenation.

#ES5

var str = 'This text ' + 'is on ' + 'multiple lines'

# ES6

let str = `This text
            is on
            multiple lines`

　　

Note: Whitespace is preserved in multi-line template literals. See Removing leading whitespace in ES6 template strings.

• MDN Reference: Multi-line strings

Implicit returns

The return keyword is implied and can be omitted if using arrow functions without a block body.

# ES5

function func(a, b, c) {
  return a + b + c
}

# ES6

let func = (a, b, c) => a + b + c // curly brackets must be omitted

　MDN Reference: Function body

Key/property shorthand

ES6 introduces a shorter notation for assigning properties to variables of the same name.

# ES5

var obj = {
  a: a,
  b: b,
}

# ES6

let obj = {
  a,
  b,
}

　　MDN Reference: Property definitions

Method definition shorthand

The function keyword can be omitted when assigning methods on an object.

# ES5

var obj = {
  a: function(c, d) {},
  b: function(e, f) {},
}

# ES6

let obj = {
  a(c, d) {},
  b(e, f) {},
}

obj.a() // call method a

　　MDN Reference: Method definitions

Destructuring (object matching)

Use curly brackets to assign properties of an object to their own variable.

var obj = { a: 1, b: 2, c: 3 }

# ES5

var a = obj.a
var b = obj.b
var c = obj.c

# ES6

let { a, b, c } = obj

　　MDN Reference: Object initializer

Array iteration (looping)

A more concise syntax has been introduced for iteration through arrays and other iterable objects.

var arr = ['a', 'b', 'c']

# ES5

for (var i = 0; i < arr.length; i++) {
  console.log(arr[i])
}

# ES6

for (let i of arr) {
  console.log(i)
}

　　MDN Reference: for...of

Default parameters

Functions can be initialized with default parameters, which will be used only if an argument is not invoked through the function.

# ES5

var func = function(a, b) {
  b = b === undefined ? 2 : b
  return a + b
}

# ES6

let func = (a, b = 2) => {
  return a + b
}

func(10) // returns 12
func(10, 5) // returns 15

　　MDN Reference: Default paramters

Spread syntax

Spread syntax can be used to expand an array.

# ES6

let arr1 = [1, 2, 3]
let arr2 = ['a', 'b', 'c']
let arr3 = [...arr1, ...arr2]

console.log(arr3) // [1, 2, 3, "a", "b", "c"]

Spread syntax can be used for function arguments.

# ES6

let arr1 = [1, 2, 3]
let func = (a, b, c) => a + b + c

console.log(func(...arr1)) // 6

　　MDN Reference: Spread syntax

Classes/constructor functions

ES6 introducess the class syntax on top of the prototype-based constructor function.

# ES5

function Func(a, b) {
  this.a = a
  this.b = b
}

Func.prototype.getSum = function() {
  return this.a + this.b
}

var x = new Func(3, 4)

# ES6

class Func {
  constructor(a, b) {
    this.a = a
    this.b = b
  }

  getSum() {
    return this.a + this.b
  }
}

let x = new Func(3, 4)

x.getSum() // returns 7

　　MDN Reference: Classes

Inheritance

The extends keyword creates a subclass.

# ES5

function Inheritance(a, b, c) {
  Func.call(this, a, b)

  this.c = c
}

Inheritance.prototype = Object.create(Func.prototype)
Inheritance.prototype.getProduct = function() {
  return this.a * this.b * this.c
}

var y = new Inheritance(3, 4, 5)

# ES6

class Inheritance extends Func {
  constructor(a, b, c) {
    super(a, b)

    this.c = c
  }

  getProduct() {
    return this.a * this.b * this.c
  }
}

let y = new Inheritance(3, 4, 5)

y.getProduct() // 60

　　MDN Reference: Subclassing with extends

Modules - export/import

Modules can be created to export and import code between files.

# index.html

<script src="export.js"></script>
<script type="module" src="import.js"></script>

# export.js

let func = a => a + a
let obj = {}
let x = 0

export { func, obj, x }

# import.js

import { func, obj, x } from './export.js'

console.log(func(3), obj, x)

　　

• MDN Reference: export
• MDN Reference: import

Promises/Callbacks

Promises represent the completion of an asynchronous function. They can be used as an alternative to chaining functions.

# ES5 callback

function doSecond() {
  console.log('Do second.')
}

function doFirst(callback) {
  setTimeout(function() {
    console.log('Do first.')

    callback()
  }, 500)
}

doFirst(doSecond)

# ES6 Promise

let doSecond = () => {
  console.log('Do second.')
}

let doFirst = new Promise((resolve, reject) => {
  setTimeout(() => {
    console.log('Do first.')

    resolve()
  }, 500)
})

doFirst.then(doSecond)

　　

An example below using XMLHttpRequest, for demonstrative purposes only (Fetch API would be the proper modern API to use).

# ES5 callback

function makeRequest(method, url, callback) {
  var request = new XMLHttpRequest()

  request.open(method, url)
  request.onload = function() {
    callback(null, request.response)
  }
  request.onerror = function() {
    callback(request.response)
  }
  request.send()
}

makeRequest('GET', 'https://url.json', function(err, data) {
  if (err) {
    throw new Error(err)
  } else {
    console.log(data)
  }
})

# ES6 Promise

function makeRequest(method, url) {
  return new Promise((resolve, reject) => {
    let request = new XMLHttpRequest()

    request.open(method, url)
    request.onload = resolve
    request.onerror = reject
    request.send()
  })
}

makeRequest('GET', 'https://url.json')
  .then(event => {
    console.log(event.target.response)
  })
  .catch(err => {
    throw new Error(err)
  })

　　

• MDN Reference

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