Javascript 2015 (ES6)

ECMAScript 2015

The second major revision to JavaScript.

ECMAScript 2015 is also known as ES6.

New Features in JavaScript 2015 (ES6)

Feature	Description
The let keyword	Declares a variable with block scope
The const keyword	Declare a contant immutable variable
Arrow Functions	Allows short syntax for writing function expressions
{a,b} = Operator	Assigns object properties to variables (object destructuring)
[a,b] = Operator	Assigns array values to variables (array destructuring)
... Operator	Spreads an array or iterable into individual elements
For/of	Loops through the values of iterable objects
Map Objects	Object with key-value pairs, similar but different from objects
Set Objects	Array that stores unique values
Classes	Templates for JavaScript Objects
Promises	Object representing the completion of an asynchronous operation
Symbol	A unique "hidden" identifier that no code can access
Default Parameters	Allows default values for function parameters
Rest Parameters	Allows functions to treat an indefinite number of arguments
String.includes()	Returns true if a string contains a specified value
String.startsWith()	Returns true if a string begins with a specified value
String.endsWith()	Returns true if a string ends with a specified value
Array entries()	Returns an iterator key/value pairs from an array
Array.from()	Creates an array from a string
Array keys()	Returns an iterator with the keys of an array
Array find()	Returns the value of the first element that passes a test
Array findIndex()	Returns the index of the first element that passes a test
Object.assign()	Copies properties from a source object to a target object
RegExp /u	Enables full Unicode support in a regular expression
RegExp /y	Performs a "sticky" search from the lastIndex property
isFinite()	returns true if the argument is not Infinity or NaN
IsNaN()	Returns true if the argument is NaN
Modules	Allows for breaking up your code into separate files
Reflect	Object for low-lewel operation on objects
Proxy	Object that wraps other objects for control operations

Math Features

Feature	Description
Math.trunc(x)	Returns the integer part of x
Math.sign(x)	Returns -1, 0 or 1 (x is negative, null or positive)
Math.cbrt(x)	Returns the cube root of x
Math.log2(x)	Returns the base 2 logarithm of x
Math.log10(x)	Returns the base 10 logarithm of x

Number Features

Feature	Description
Number.EPSILON	The difference between 1 and the smallest number greater than 1
Number.MIN_SAFE_INTEGER	Minimum value that can be precisely represented
Number.MAX_SAFE_INTEGER	Maximum value that can be precisely represented
Number.isInteger()	Returns true if the argument is an integer
Number.isSafeInteger()	Returns true if the argument is a safe integer

Browser Support

JavaScript 2015 is supported in all modern browsers sinceJune 2017:


Chrome 51	Edge 15	Firefox 54	Safari 10	Opera 38
May 2016	Apr 2017	Jun 2017	Sep 2016	Jun 2016

ES6 is not supported in Internet Explorer.

JavaScript let

Thelet 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 aboutlet in the chapter:JavaScript Let.

JavaScript const

Theconst 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 aboutconst in the chapter:JavaScript Const.

Arrow Functions

Arrow functions is a short syntax for writingfunction expressions.

You don't need thefunction keyword, thereturn keyword, or thecurly brackets.

Before Arrow:

Function to compute the product of a and b

let myFunction = function(a, b) {return a * b}

Try it Yourself »

With Arrow

let myFunction = (a, b) => a * b;

Try it Yourself »

Note

Arrow functions do not have their ownthis.They are not well suited for definingobject methods.

Arrow functions are not hoisted. They must be definedbefore they are used.

You can only omit thereturn keyword and thecurly brackets if the function is a singlestatement. Because of this, it might be a good habit to always keep them:

Example

// This will not work
let myFunction = (x, y) => { x * y } ;

// This will not work
let myFunction = (x, y) => return x * y ;

// Only this will work
let myFunction = (x, y) => { return x * y };

Try it Yourself »

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

Object Destructuring

Destructuring assignment makes it easy to assign array values and object properties to variables.

Example

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

// Destructuring Assignment
let { firstName, age } = person;

Try it Yourself »

Note:

When destructuring an object, you must use the same name for the variablesas the corresponding object keys (names).

The order of the keys (names) does not matter.

Array Destructuring

Destructuring assignment makes it easy to assign array values and object properties to variables.

Example

// Create an Array
const fruits = ["Banana", "Orange", "Apple", "Mango"];

// Destructuring Assignment
let [fruit1, fruit2] = fruits;

Try it Yourself »

The Spread (...) Operator

The... operator spreads an array or iterable into individual elements.

Example

The... operator can pass arguments to a function:

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

Try it Yourself »

Example

The... operator can be used to join arrays:

const arr1 = [1, 2, 3];
const arr2 = [4, 5, 6];
const arr3 = [...arr1, ...arr2];

Try it Yourself »

Example

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

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

Try it Yourself »

The For/Of Loop

The JavaScriptfor/of statement loops through the values of iterable objects.

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

Thefor/of loop has the following syntax:

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

variable - For every iteration the value of the next property is assigned to the variable.Variable can be declared withconst,let, orvar.

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

A Map is an object that stores key-value pairs, similar to objects, but with differences:

Keys can be of any data type (objects, functions, primitive values),unlike plain objects where keys are strings.
Maintains the original insertion order of keys.

Note

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

A Set is an object that stores unique values of any type (primitive values, functions, objects).

A Set can only contain unique values. An attempt to add a duplicate value will be ignored.

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 keywordclass to create a class.

Always add a method namedconstructor():

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 isnot an object.

It is atemplate 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 JavaScript Promise is an object representing the completion or failure of an asynchronousoperation and its values.

It is a placeholder for a value that may not yet be available,providing a structured way to handle asynchronous code.

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 data type 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()

Theincludes() method returnstrue if a string contains a specified value,otherwisefalse:

Example

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

Try it Yourself »

String.startsWith()

ThestartsWith() method returnstrueif a string begins with a specified value, otherwisefalse:

Example

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

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

Try it Yourself »

String.endsWith()

TheendsWith() method returnstrueif a string ends with a specified value, otherwisefalse:

Example

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

Try it Yourself »

Array 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 »

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

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

Theentries() method does not change the original array.

Array.from()

TheArray.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()

Thekeys() 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()

Thefind() 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()

ThefindIndex() 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

Modules

Modules are imported in two different 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.

JavaScript Reflect

Before Reflect,object operations were scattered:

Using the keywordin
Using the keyworddelete
Using methods likeObject.defineProperty
Using language mechanisms like[[Get]] and[[Set]]

Reflect brings all these into cleanReflect methods.

Reflect methods unifies everything
Reflect methods has predictable behavior
Reflect methods works perfectly with Proxy

Reflect.has()

Example

// Create an Object
const person = {name: "John", lastname: "Doe"};

let answer = Reflect.has(person, "name");

Try it Yourself »

Same as using thein operator:

let answer = "name" in person;

Try it Yourself »

Learn More...

JavaScript Reflect Tutorial

JavaScript Proxy

Example

// Create an Object
const user = { name: "Jan", age: 40 };

//Create a Proxy
const proxy = new Proxy(user, {
  get(target, property) {
    log("Getting: " + property);
    return target[property];
  },
  set(target, property, value) {
    log("Setting: " + property);
    return target[property];
  }
});

proxy.name = "John";
let text = proxy.name;

Try it Yourself »

Learn More...

JavaScript Proxy Tutorial

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 -1, 0, or 1 (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

The difference between 1 and the smallest floating-point number greater than 1:

let x = Number.EPSILON;

Try it Yourself »

MIN_SAFE_INTEGER Example

Minimum integer value that can be precisely represented:

let x = Number.MIN_SAFE_INTEGER;

Try it Yourself »

MAX_SAFE_INTEGER Example

Maximum integer value that can be precisely represented:

let x = Number.MAX_SAFE_INTEGER;

Try it Yourself »

The Number.isInteger() Method

TheNumber.isInteger() method returnstrue 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.

TheNumber.isSafeInteger() method returnstrue 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 globalisFinite() method returnsfalse if the argument isInfinity orNaN.

Otherwise it returnstrue:

Example

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

Try it Yourself »

The isNaN() Method

The globalisNaN() method returnstrue if the argument isNaN. Otherwise it returnsfalse:

Example

isNaN("Hello"); // returns true

Try it Yourself »

JavaScript Object.assign()

TheObject.assign() method copies properties fromone or more source objects to a target object.

Example

// Create Target Object
const person1 = {
  firstName: "John",
  lastName: "Doe",
  age: 50,
  eyeColor: "blue"
};

// Create Source Object
const person2 = {firstName: "Anne",lastName: "Smith"};

// Assign Source to Target
Object.assign(person1, person2);

Try it Yourself »

RegExp u Modifier

Examples

let text = "Hello 😄";

let pattern = /\p{Emoji}/u;
let result = pattern.test(text);

Try it Yourself »

let text = "Hello 😄";

let pattern = /\p{Emoji}/;
let result = pattern.test(text);

Try it Yourself »

Description

Theu (unicode) flag enables full Unicode support in the regular expression.

By default, JavaScript and regex treats 4-byte Unicode characters(like emojis or less common symbols) as two separate 2-byte "surrogate" code units.

Theu flag treats the pattern as a sequence of Unicode code points, which is important for correctlyhandling characters outside the Basic Multilingual Plane (BMP).

RegExp y Modifier

Examples

let text = "abc def ghi";
let pattern = /\w+/y;

// Start match from position 4
pattern.lastIndex = 4;
let result = text.match(pattern);

Try it Yourself »

let text = "abc def ghi";
let pattern = /\w+/;

// Start match from position 4
pattern.lastIndex = 4;
let result = text.match(pattern);

Try it Yourself »

Description

They (Sticky) flag performs a "sticky" search that matches only from the lastIndexproperty of the RegExp object.

They flag ensures that subsequent matches start immediatelyafter the previous one, without skipping characters.

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Basic JavaScript

JS Advanced

Javascript 2015 (ES6)

ECMAScript 2015

New Features in JavaScript 2015 (ES6)

Math Features

Number Features

Browser Support

JavaScript let

Example

JavaScript const

Example

Arrow Functions

Before Arrow:

With Arrow

Note

Example

Object Destructuring

Example

Note:

Array Destructuring

Example

The Spread (...) Operator

Example

Example

Example

The For/Of Loop

Looping over an Array

Example

Looping over a String

Example

JavaScript Maps

Note

Example

JavaScript Sets

Example

JavaScript Classes

Syntax

Example

Using a Class

Example

JavaScript Promises

Promise Syntax

Example Using a Promise

The Symbol Type

Example

Note

Default Parameter Values

Example

Function Rest Parameter

Example

String.includes()

Example

String.startsWith()

Example

String.endsWith()

Example

Array entries()

Example

Array.from()

Example

Array keys()

Example

Array find()

Example

Array findIndex()

Example

Modules

Import from named exports

Import from default exports

JavaScript Reflect

Reflect.has()

Example

Learn More...

JavaScript Proxy

Example

Learn More...

New Math Methods

The Math.trunc() Method