ECMAScript 2015

This chapter will introduce some of the new features in ES6.

JavaScript let
JavaScript const
JavaScript Arrow Functions
JavaScript Classes
Default parameter values
Array.find()
Array.findIndex()
Exponentiation (**) (EcmaScript 2016)
Browser Support for ES6 (ECMAScript 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 statement 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
JavaScript const
The const statement 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
Read more about let and const in our JavaScript Let / Const Chapter.

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;
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 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 };
Learn more about Arrow Functions in our JavaScript Arrow Function chapter.

Classes
ES6 introduced classes.

A class is a type of function, but instead of using the keyword function to initiate it, we use the keyword class, and the properties are assigned inside a constructor() method.

Use the keyword class to create a class, and always add a constructor method.

The constructor method is called each time the class object is initialized.

Example
A simple class definition for a class named "Car":

class Car {
  constructor(brand) {
    this.carname = brand;
  }
}
Now you can create objects using the Car class:

Example
Create an object called "mycar" based on the Car class:

class Car {
  constructor(brand) {
    this.carname = brand;
  }
}
mycar = new Car("Ford");

Learn more about classes in our JavaScript Classes chapter.

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

function myFunction(value, index, array) {
  return value > 18;
}
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
var numbers = [4, 9, 16, 25, 29];
var first = numbers.findIndex(myFunction);

function myFunction(value, index, array) {
  return value > 18;
}
Note that the function takes 3 arguments:

The item value
The item index
The array itself
New Number Properties
ES6 added the following properties to the Number object:

EPSILON
MIN_SAFE_INTEGER
MAX_SAFE_INTEGER
Example
var x = Number.EPSILON;
Example
var x = Number.MIN_SAFE_INTEGER;
Example
var x = Number.MAX_SAFE_INTEGER;
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
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
Safe integers are all integers from -(253 - 1) to +(253 - 1).
This is safe: 9007199254740991. This is not safe: 9007199254740992.

New Global Methods
ES6 also 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
The isNaN() Method
The global isNaN() method returns true if the argument is NaN. Otherwise it returns false:

Example
isNaN("Hello");       // returns true
Exponentiation Operator
The exponentiation operator (**) raises the first operand to the power of the second operand.

Example
var x = 5;
var z = x ** 2;          // result is 25
x ** y produces the same result as Math.pow(x,y):

Example
var x = 5;
var z = Math.pow(x,2);   // result is 25