JavaScript ES6 : Difference between var, let and const

   In this tutorial I am going to write about variable declaration in JavaScript.So lets start with some basic difference between var, let and const.

var 

• var is scoped to the nearest function block.
• You can declare the same variable multiple times using var.
• You can use variable before even initialize. 
• Global variables defined with var will be added as properties on the global window object.

    //Example

    function test(){

        for( var i = 0; i < 5; i++ ) {

            //i is visible here
        }

         //i is visible here
     }
     //i is not visible here


   // You can declare the same variable multiple times using var.

    'use strict';
    var a;
    var a;

   //You can use variable before even initialize.

   function fun () {
   typeof a;
   var a = 'big';
   }
   fun();

   /*Global variables defined with var will be added as properties
   on the global window object*/
   

   var  a = 'go';  // globally scoped
   console.log(window.a); // 'go'

let

• let is scoped to the nearest enclosing block (both are global if outside any block), which can be smaller than a function block.
• let says variable will be used only in the block it’s defined in.
• The use case for `let` tends to be for loops or mathematical algorithms.
• Use let where you  need to reassign a variable like in loops.
• Assuming strict mode,you can't declare the same variable multiple times using let.
• Always initialize your identifiers before you try to use them.
• Global variables defined with let will not be added as properties on the global window object

    for( let i = 0; i < 5; i++ ) {
        //i is only visible in here
        //and there is a separate i variable for each iteration of the loop
    }

    //i is not visible here

   // You can't declare the same variable multiple times using let.
   'use strict';
   let a;
   let a; // SyntaxError: Identifier 'b' has already been declared

   //always initialize your identifiers before you try to use them
   function fun () {
   typeof a;
   let a = 'big';
   }  
   fun(); // Uncaught ReferenceError: bar is not defined

   /*Global variables defined with let will not be added as properties
    on the global window object*/
   let a = 'go';  // globally scoped
   console.log(window.a); // undefined

const

• Constants are block-scoped
• The value of a constant cannot change through re-assignment, and it can't be redeclared.
• const says the identifier won’t be reassigned.
• Naming convention for const variable  is to use all-uppercase letters.

   //Example
   // define MY_NUM as a constant and give it the value 17
   const MY_NUM = 17;

   // this will throw an error
   MY_NUM = 20;

   // will print 17
   console.log('my number is: ' + MY_NUM);

   // trying to redeclare a constant throws an error
   const MY_NUM = 20;

   // the name MY_NUM is reserved for constant above, so this will also fail
   var MY_NUM = 20;

   // this throws an error also
   let MY_NUM = 20;

I feel you found this article useful and now you got a clear picture about where and how to  use var, let and const more effectively.

Happy Learning :-)