ES 2015

Also known as ECMAScript 2015 or ES6

Modules / import

Provides an alternative to CommonJS
Instead of

var React = require('react')  

you can have

import * as React from 'react'  

var, let and const

var declaration are function scoped when declared inside a function and globally scoped when declared outside
var variables are hoisted to the top of its scope.
var variables are initialized with a value of undefined.
var variables can be used before they have been declared.
var variables can be re-declared within the scope.

let declarations are block scoped.
let variables are hoisted to the top of its scope.
let variables are not initialised (and are only initialised when they are assigned a value).
let variables cannot be used before they have been declared.
let variables cannot be re-declared within its scope.

const declarations are block scoped .
const variables are hoisted to the top of its scope.
const variables cannot be used before they have been declared.
const variables have the same value throughout the scope.
const variables cannot be re-declared or updated within its scope.

Classes

inheritance
static methods
private methods
get and set

Collections

Arrow Functions

Instead of:

const myFunction = function() { 
} 

you can have

const myFunction = () => { 
} 

For single statement arrow functions there is an implicit return

const myFunction = () => ( { value : 'text' } )  

Not to be used inside classes to declare constructors or methods.
For single statement functions you can condense to one line
Instead of

const myFunction = function (param1) { 
   dosomething() 
} 
const myFunction = function (param1, param2) { 
   dosomething() 
} 

you can have

const myFunction = (param1) => dosomething() 
const myFunction = (param1, param2) => dosomething() 

or

const myFunction = param1 => dosomething() 

This last notation can only be used when you have one parameter.

implicit return

this keyword

when used inside an object method
(declared using without arrow functions)
this refers to the object

When used in combination with arrow functions 'this' will be undefined
For this reason do not use arrow functions inside objects

Promises

They provide one way to deal with asynchronous code

Object Destructuring

Object Spread Operator

Array Destructuring

Array Spread Operator

This operator is three dots.

const a = [1,2,3] 
const b = [...a, 4,5,6] 

Array Rest Elements

This is an improved way to handle function parameters.
The rest parameter represents an indefinite number of arguments as an array.

Template Literals

The syntax uses backticks instead of single or double quotes.

const mystring = `text` 

Strings Multi-Line

Instead of having to use the '\' character at the end

const string = "first part\ 
              second part" 

you can now use a back tick character
This is a feature that accompanies template literals

const string = `first part 
                  second part` 

Template Tags

In styled components, template tags are used to define CSS strings

const Button = styled.button 
   font-size : 1.5em 
   color : white 

Interpolation

Arrays

Array.find -
Array.findIndex -

Function Parameter Default Values

Numbers

Number.MIN_SAFE_INTEGER
Number.MAX_SAFE_INTEGER
Number.EPSILON
Number.IsInteger -
Number.IsSafeInteger -

New Global Methods

isFinite()
isNaN()