Understanding Var, Let and Const | ES6 Guide

Before the release of ES6, var was the only option to declare variables in javascript. ES6(ES2015) introduced some new features like let and const for declaring variables in javascript.

Let’s understand the usage of each notation how and where it is used :

1. Var

• Var keyword was previously used for declaring a variable in javascript.
• Variables declared with var can be re-initialized and re-declared too.
• It is not recommended to use var after release of let and const.

    var a = 10;

    for(var i=0;i<5;i++){
    		var a = 20;
    		console.log(a); //Returns 20
    }

    console.log(a); // Returns 20

2. Let

• “let” is used when you have to change the value of the variable later in the code.
• It has block scope.
• It can be re-initialized but not re-declared.

    let a = 10;

    // re-initialization
    a = 30; // Updating a value to 30.

    //re-declartion
    let a = 20; // Throws Error

    // Block 1
    {
    	 let c = 10;
    	 console.log(c); // c=10 ( Block scoped)
    }

    console.log(c); // Throws Error, c not defined.

3. Const

• Const is used to define a constant variable which can’t be changed throughout the code.
• It has block scope.
• You can neither be re-initialized nor re-declared.

    const a = 10;

    // re-initialization
    a = 30; // Throws Error, CONST variable can't be changed

    //re-declartion
    const a = 20; // Throws Error

    // Block 1
    {
    	 const c = 10;
    	 console.log(c); // c=10
    }

    console.log(c); // Throws Error, c not defined.

ES6 has a lot of new features like Template literals, Default Arguments, Arrow Functions, Array and Object Destructuring, Map, Reduce and Filter, Rest and Spread Operator, Object Literals, Classes and a lot more.

If you write code in a javascript and still not familiar with ES6 concepts. You should consider googling these topics.

Here I have written a comprehensive guide to learn ES6, please consider checking out.

dipakkr/ES6-Guide

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