Use of Var, Let and Const in Javascript

I have seen lot of people often making mistake in differentiating between "Var", "Let" and "Const"

1. Var (function scope)

It's a keyword used to declare variables that have a scope of Function block. In case we define it globally, it will have a Global Scope

Let's see how "var" works:

 function demo(){  
   var i = 10;  
   console.log("The value of i is:"+i);  
 }  
 demo()  
 console.log("Value of i after function call:"+i);  

In the above example, we are have defined a "var" within the function "demo()". So it means that it will be accessed only within this function. That's why the last line will give you an error as the scope of the variable is within the function.
Var (Global scope)
If we define the var at the top, it would have the global scope which means that it can be accessed in any function.

 var i = 10;  
 function demo(){  
   console.log("The value of i is:"+i);  
   i++;  
 }  
 demo()  
 console.log("Value of i after function call:"+i);  

In the above example we will be able to get the value of i = 11 as the i is have a global scope.
2. let 
Let has a scope within the {} brackets only. Let's take the following examples:
1st Example where we have same variable names defined with "var" and "let"

 var i = 10;  
 function demo(){  
   console.log("The value of i is:"+i);  
   i++;  
 }  
 demo()  
 console.log("Value of i after function call:"+i);  
 function anotherDemo(){  
   for(let i = 1; i <= 4; i++){  
     console.log(i);  
   }  
   console.log(i);  
 }  
 anotherDemo();  

In the above example, there will be no error, but the the value of i being printed in the function anotherDemo() and after loop will be of the "var" i being defined globally.
Let's take another example:

 function anotherDemo(){  
   for(let x = 1; x <= 4; x++){  
     console.log(x);  
   }  
   console.log(x);  
 }  
 anotherDemo();  

In the above example, the value of "x" being printed after the loop will be an error as the scope of the "let" was only inside the for loop.
3. Const
Constant is used to define values that are going to remain same or we can say they are immutable. For e.g. const pie = 3.14
I hope this clearly differentiates between these three keywords and we make more judicial use of these in Javascript.
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