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What is ES6?

ES6 stands for ECMAScript 6. ECMAScript was created to standardize JavaScript, and ES6 is the 6th version of ECMAScript, it was published in 2015, and is also known as ECMAScript 2015.

React ES6 Classes

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.

Example

A simple class constructor:

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

Notice the case of the class name. We have begun the name, “Car”, with an uppercase character. This is a standard naming convention for classes. Now you can create objects using the Car class:

Example

Create an object called “mycar” based on the Car class.

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

const mycar = new Car("Ford");

React ES6 Arrow Functions

Arrow Functions

Arrow functions allow us to write shorter function syntax:

Before:

hello = function() {
  return "Hello World!";
}

With Arrow Function:

hello = () => {
  return "Hello World!";
}

It gets shorter! If the function has only one statement, and the statement returns a value, you can remove the brackets and the return keyword:

Arrow Functions Return Value by Default:

hello = () => "Hello World!";

Note: This works only if the function has only one statement.

If you have parameters, you pass them inside the parentheses:

Arrow Function With Parameters:

hello = (val) => "Hello " + val;

In fact, if you have only one parameter, you can skip the parentheses as well:

Arrow Function Without Parentheses:

hello = val => "Hello " + val;

What About this?

The handling of this is also different in arrow functions compared to regular functions. In short, with arrow functions there is no binding of this. In regular functions the this keyword represented the object that called the function, which could be the window, the document, a button or whatever. With arrow functions, the this keyword always represents the object that defined the arrow function. Let us take a look at two examples to understand the difference. Both examples call a method twice, first when the page loads, and once again when the user clicks a button. The first example uses a regular function, and the second example uses an arrow function. The result shows that the first example returns two different objects (window and button), and the second example returns the Header object twice.

Example

With a regular function, this represents the object that called the function:

class Header {
  constructor() {
    this.color = "Red";
  }

//Regular function:
  changeColor = function() {
    document.getElementById("demo").innerHTML += this;
  }
}

const myheader = new Header();

//The window object calls the function:
window.addEventListener("load", myheader.changeColor);

//A button object calls the function:
document.getElementById("btn").addEventListener("click", myheader.changeColor);

Example

With an arrow function, this represents the Header object no matter who called the function:

class Header {
  constructor() {
    this.color = "Red";
  }

//Arrow function:
  changeColor = () => {
    document.getElementById("demo").innerHTML += this;
  }
}

const myheader = new Header();


//The window object calls the function:
window.addEventListener("load", myheader.changeColor);

//A button object calls the function:
document.getElementById("btn").addEventListener("click", myheader.changeColor);

React ES6 Variables

Variables

Before ES6 there was only one way of defining your variables: with the var keyword. If you did not define them, they would be assigned to the global object. Unless you were in strict mode, then you would get an error if your variables were undefined. Now, with ES6, there are three ways of defining your variables: var, let, and const.

var

var x = 5.6;

If you use var outside of a function, it belongs to the global scope. If you use var inside of a function, it belongs to that function. If you use var inside of a block, i.e. a for loop, the variable is still available outside of that block. var has a function scope, not a block scope.

let

let x = 5.6;

let is the block scoped version of var, and is limited to the block (or expression) where it is defined. If you use let inside of a block, i.e. a for loop, the variable is only available inside of that loop.

let has a block scope.

const

const x = 5.6;

const is a variable that once it has been created, its value can never change. const has a block scope. The keyword const is a bit misleading. It does not define a constant value. It defines a constant reference to a value.

Because of this you can NOT:

• Reassign a constant value
• Reassign a constant array
• Reassign a constant object

• Change the elements of constant array
• Change the properties of constant object