Javascript ES6

ECMAScript 2015 was the second major revision to JavaScript.

ECMAScript 2015 is also known as ES6 and ECMAScript 6.

This chapter describes the most important features of ES6.

New Features in ES6

The let keyword
The const keyword
Arrow Functions
The ... Operator
For/of
Map Objects
Set Objects
Classes
Promises
Symbol
Default Parameters
Function Rest Parameter
String.includes()
String.startsWith()
String.endsWith()
Array.from()
Array keys()
Array find()
Array findIndex()
New Math Methods
New Number Properties
New Number Methods
New Global Methods
Object entries
JavaScript Modules

Browser Support for ES6 (2015)

Safari 10 and Edge 14 were the first browsers to fully support ES6:


Chrome 58	Edge 14	Firefox 54	Safari 10	Opera 55
Jan 2017	Aug 2016	Mar 2017	Jul 2016	Aug 2018

JavaScript let

The let keyword allows you to declare a variable with block scope.

Example

var x = 10;
// Here x is 10
{
let x = 2;
// Here x is 2
}
// Here x is 10

Try it Yourself »

Read more about let in the chapter: JavaScript Let.

JavaScript const

The const keyword allows you to declare a constant (a JavaScript variable with a constant value).

Constants are similar to let variables, except that the value cannot be changed.

Example

var x = 10;
// Here x is 10
{
const x = 2;
// Here x is 2
}
// Here x is 10

Try it Yourself »

Read more about const in the chapter: JavaScript Const.

Arrow Functions

Arrow functions allows a short syntax for writing function expressions.

You don't need the function keyword, the return keyword, and the curly brackets.

Example

// ES5
var x = function(x, y) {
return x * y;
}

// ES6
const x = (x, y) => x * y;

Try it Yourself »

Arrow functions do not have their own this. They are not well suited for defining object methods.

Arrow functions are not hoisted. They must be defined before they are used.

Using const is safer than using var, because a function expression is always a constant value.

You can only omit the return keyword and the curly brackets if the function is a single statement. Because of this, it might be a good habit to always keep them:

Example

const x = (x, y) => { return x * y };

Try it Yourself »

Learn more about Arrow Functions in the chapter: JavaScript Arrow Function.

The Spread (...) Operator

The ... operator expands an iterable (like an array) into more elements:

Example

const q1 = ["Jan", "Feb", "Mar"];
const q2 = ["Apr", "May", "Jun"];
const q3 = ["Jul", "Aug", "Sep"];
const q4 = ["Oct", "Nov", "May"];

const year = [...q1, ...q2, ...q3, ...q4];

Try it Yourself »

The ... operator can be used to expand an iterable into more arguments for function calls:

Example

const numbers = [23,55,21,87,56];
let maxValue = Math.max(...numbers);

Try it Yourself »

The For/Of Loop

The JavaScript for/of statement loops through the values of an iterable objects.

for/of lets you loop over data structures that are iterable such as Arrays, Strings, Maps, NodeLists, and more.

The for/of loop has the following syntax:

for ( variable of iterable) {
// code block to be executed
}

variable - For every iteration the value of the next property is assigned to the variable. Variable can be declared with const, let, or var.

iterable - An object that has iterable properties.

Looping over an Array

Example

const cars = ["BMW", "Volvo", "Mini"];
let text = "";

for (let x of cars) {
text += x + " ";
}

Try it Yourself »

Looping over a String

Example

let language = "JavaScript";
let text = "";

for (let x of language) {
text += x + " ";
}

Try it Yourself »

Learn more in the chapter: JavaScript Loop For/In/Of.

JavaScript Maps

Being able to use an Object as a key is an important Map feature.

Example

const fruits = new Map([
["apples", 500],
["bananas", 300],
["oranges", 200]
]);

Try it Yourself »

Learn more about Map objects, and the difference between a Map and an Array, in the the chapter: JavaScript Maps.

JavaScript Sets

Example

// Create a Set
const letters = new Set();

// Add some values to the Set
letters.add("a");
letters.add("b");
letters.add("c");

Try it Yourself »

Learn more about Set objects in the the chapter: JavaScript Sets.

JavaScript Classes

JavaScript Classes are templates for JavaScript Objects.

Use the keyword class to create a class.

Always add a method named constructor():

Syntax

class ClassName {
constructor() { ... }
}

Example

class Car {
constructor(name, year) {
this.name = name;
this.year = year;
}
}

The example above creates a class named "Car".

The class has two initial properties: "name" and "year".

A JavaScript class is not an object.

It is a template for JavaScript objects.

Using a Class

When you have a class, you can use the class to create objects:

Example

const myCar1 = new Car("Ford", 2014);
const myCar2 = new Car("Audi", 2019);

Try it Yourself »

Learn more about classes in the the chapter: JavaScript Classes.

JavaScript Promises

A Promise is a JavaScript object that links "Producing Code" and "Consuming Code".

"Producing Code" can take some time and "Consuming Code" must wait for the result.

Promise Syntax

Example Using a Promise

Try it Yourself »

Learn more about Promises in the the chapter: JavaScript Promises.

The Symbol Type

A JavaScript Symbol is a primitive datatype just like Number, String, or Boolean.

It represents a unique "hidden" identifier that no other code can accidentally access.

For instance, if different coders want to add a person.id property to a person object belonging to a third-party code, they could mix each others values.

Using Symbol() to create a unique identifiers, solves this problem:

Example

const person = {
firstName: "John",
lastName: "Doe",
age: 50,
eyeColor: "blue"
};

let id = Symbol('id');
person[id] = 140353;
// Now person[id] = 140353
// but person.id is still undefined

Try it Yourself »

Note

Symbols are always unique.

If you create two symbols with the same description they will have different values:

Symbol("id") == Symbol("id"); // false

Default Parameter Values

ES6 allows function parameters to have default values.

Example

function myFunction(x, y = 10) {
// y is 10 if not passed or undefined
return x + y;
}
myFunction(5); // will return 15

Try it Yourself »

Function Rest Parameter

The rest parameter (...) allows a function to treat an indefinite number of arguments as an array:

Example

function sum(...args) {
let sum = 0;
for (let arg of args) sum += arg;
return sum;
}

let x = sum(4, 9, 16, 25, 29, 100, 66, 77);

Try it Yourself »

String.includes()

The includes() method returns true if a string contains a specified value, otherwise false:

Example

let text = "Hello world, welcome to the universe.";
text.includes("world") // Returns true

Try it Yourself »

String.startsWith()

The startsWith() method returns true if a string begins with a specified value, otherwise false:

Example

let text = "Hello world, welcome to the universe.";

text.startsWith("Hello") // Returns true

Try it Yourself »

String.endsWith()

The endsWith() method returns true if a string ends with a specified value, otherwise false:

Example

var text = "John Doe";
text.endsWith("Doe") // Returns true

Try it Yourself »

Array.from()

The Array.from() method returns an Array object from any object with a length property or any iterable object.

Example

Create an Array from a String:

Array.from("ABCDEFG") // Returns [A,B,C,D,E,F,G]

Try it Yourself »

Array keys()

The keys() method returns an Array Iterator object with the keys of an array.

Example

Create an Array Iterator object, containing the keys of the array:

const fruits = ["Banana", "Orange", "Apple", "Mango"];
const keys = fruits.keys();

let text = "";
for (let x of keys) {
text += x + "<br>";
}

Try it Yourself »

Array find()

The find() method returns the value of the first array element that passes a test function.

This example finds (returns the value of ) the first element that is larger than 18:

Example

const numbers = [4, 9, 16, 25, 29];
let first = numbers.find(myFunction);

function myFunction(value, index, array) {
return value > 18;
}

Try it Yourself »

Note that the function takes 3 arguments:

The item value
The item index
The array itself

Array findIndex()

The findIndex() method returns the index of the first array element that passes a test function.

This example finds the index of the first element that is larger than 18:

Example

const numbers = [4, 9, 16, 25, 29];
let first = numbers.findIndex(myFunction);

function myFunction(value, index, array) {
return value > 18;
}

Try it Yourself »

Note that the function takes 3 arguments:

The item value
The item index
The array itself

New Math Methods

ES6 added the following methods to the Math object:

Math.trunc()
Math.sign()
Math.cbrt()
Math.log2()
Math.log10()

The Math.trunc() Method

Math.trunc(x) returns the integer part of x:

Example

Math.trunc(4.9);    // returns 4
Math.trunc(4.7);    // returns 4
Math.trunc(4.4);    // returns 4
Math.trunc(4.2);    // returns 4
Math.trunc(-4.2);    // returns -4

Try it Yourself »

The Math.sign() Method

Math.sign(x) returns if x is negative, null or positive:

Example

Math.sign(-4);    // returns -1
Math.sign(0);    // returns 0
Math.sign(4);    // returns 1

Try it Yourself »

The Math.cbrt() Method

Math.cbrt(x) returns the cube root of x:

Example

Math.cbrt(8);    // returns 2
Math.cbrt(64);    // returns 4
Math.cbrt(125);    // returns 5

Try it Yourself »

The Math.log2() Method

Math.log2(x) returns the base 2 logarithm of x:

Example

Math.log2(2); // returns 1

Try it Yourself »

The Math.log10() Method

Math.log10(x) returns the base 10 logarithm of x:

Example

Math.log10(10); // returns 1

Try it Yourself »

New Number Properties

ES6 added the following properties to the Number object:

EPSILON
MIN_SAFE_INTEGER
MAX_SAFE_INTEGER

EPSILON Example

let x = Number.EPSILON;

Try it Yourself »

MIN_SAFE_INTEGER Example

let x = Number.MIN_SAFE_INTEGER;

Try it Yourself »

MAX_SAFE_INTEGER Example

let x = Number.MAX_SAFE_INTEGER;

Try it Yourself »

New Number Methods

ES6 added 2 new methods to the Number object:

Number.isInteger()
Number.isSafeInteger()

The Number.isInteger() Method

The Number.isInteger() method returns true if the argument is an integer.

Example

Number.isInteger(10); // returns true
Number.isInteger(10.5); // returns false

Try it Yourself »

The Number.isSafeInteger() Method

A safe integer is an integer that can be exactly represented as a double precision number.

The Number.isSafeInteger() method returns true if the argument is a safe integer.

Example

Number.isSafeInteger(10); // returns true
Number.isSafeInteger(12345678901234567890); // returns false

Try it Yourself »

Safe integers are all integers from -(2⁵³ - 1) to +(2⁵³ - 1).
This is safe: 9007199254740991. This is not safe: 9007199254740992.

New Global Methods

ES6 added 2 new global number methods:

isFinite()
isNaN()

The isFinite() Method

The global isFinite() method returns false if the argument is Infinity or NaN.

Otherwise it returns true:

Example

isFinite(10/0); // returns false
isFinite(10/1); // returns true

Try it Yourself »

The isNaN() Method

The global isNaN() method returns true if the argument is NaN. Otherwise it returns false:

Example

isNaN("Hello"); // returns true

Try it Yourself »

Object entries()

Example

Create an Array Iterator, and then iterate over the key/value pairs:

const fruits = ["Banana", "Orange", "Apple", "Mango"];
const f = fruits.entries();

for (let x of f) {
document.getElementById("demo").innerHTML += x;
}

Try it Yourself »

The entries() method returns an Array Iterator object with key/value pairs:

[0, "Banana"]
[1, "Orange"]
[2, "Apple"]
[3, "Mango"]

The entries() method does not change the original array.

Modules

Modules are imported in two differen ways:

Import from named exports

Import named exports from the file person.js:

import { name, age } from "./person.js";

Try it Yourself »

Import from default exports

Import a default export from the file message.js:

import message from "./message.js";

Try it Yourself »

Learn more about Modules in: JavaScript Modules.

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