What are the different ways to define a variable in JavaScript? And when to pick which one to use?

There are three possible ways of defining a variable in JavaScript(ES6/2015):

1. Var – The var keyword is used to declare a variable and, optionally, initialize the value of that variable globally.
   Example:
   

       var userName = "rajat";
   
       // username can be used here
   
       function myFunction() {
           // userName can also be used here
       }
   

2. Let – Introduced in ES2015. The let keyword is used to define a variable that will be used only in the block it is defined in. Usually provides a hint that the variable may have been reassigned, i.e. a counter in a loop, or a value swap in an algo.
   The let provide Block{} Scope variables. Hence, it can't be accessed outside the scope. Example:
   

       {
           let a = 2;
       }
       // a can NOT be used here
       
   

3. Const – Introduced in ES2015. Declaring a variable with const is similar to that of let when it comes to Block Scope, i.e not accessible outside the block variables. The const variable must be assigned a value when they are declared. For example:
   Incorrect Way
   

       const PI;
       PI = 3.14159265359;
   

   
   Correct Way
   

       const PI = 3.14159265359;
   

   
   The const variables cannot be reassigned
   

       const PI = 3.141592653589793;
       PI = 3.14;  // This will give an error
       PI = PI + 10;  // This will also give an error