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ECMAScript 6 - ECMAScript 2015


What is ECMAScript 6?

ECMAScript 6 is also known as ES6 and ECMAScript 2015

Some people like to call it JavaScript 6.

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

  • JavaScript let
  • JavaScript const
  • JavaScript default parameter values
  • Array.find()
  • Array.findIndex()

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
Try it Yourself »

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
Try it Yourself »

Read more about let and const in JS Let / Const.


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 »


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;
}
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

var numbers = [4, 9, 16, 25, 29];
var 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 Number Properties

ES6 added the following properties to the Number object:

  • EPSILON
  • MIN_SAFE_INTEGER
  • MAX_SAFE_INTEGER

Example

var x = Number.EPSILON;
Try it Yourself »

Example

var x = Number.MIN_SAFE_INTEGER;
Try it Yourself »

Example

var x = Number.MAX_SAFE_INTEGER;
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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 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
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 »

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 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 »