javascript variable declaration: var vs let vs const

We have 3 different ways to declare variables in java script

1. var - Variables declared with var keyword are function scoped. What do I mean by that ? let's take an example -

   
   function sayHello(){
     for(var i=0;i<4;i++){
       console.log(i);
     }
     // As var is function scoped, i is available outside for loop.
     console.log(i); // This prints 4.
   }
   sayHello();
   
   

   Here, i is declared with var keyword in a for loop ( block ) and it is incremented util i < 4. When i becomes 4, the for loop ends and the last console.log(i) is executed. Output

   
   0
   1
   2
   3
   4
   
   

   Because of the var keyword, i is function scoped hence i is available outside the for loop as well and last console.log() prints 4.
2. let - This is part of ES6 and addresses the problem related to var keyword. Variables declared with let keyword are block scoped. Let's consider the previous code with let

   
   function sayHello(){
     for(let i=0;i<4;i++){
       console.log(i);
     }
     // let is block scoped hence i is only visible inside for block.
     console.log(i); // This line will throw an error.
   }
   sayHello();
   
   

   Because i is declared with let, i is visible inside the for loop only and the last console.log() will throw an error ReferenceError: i is not defined Output

   
   0
   1
   2
   3
   Uncaught ReferenceError: i is not defined
       at sayHello (:6:15)
       at :8:1
   
   

3. const - const keyword is used to define constants in java scripts and are block scoped. e.g.

   
   function sayHello(){
       const i = 1;
       console.log(i);// This prints 1
       i = 2;  // i is declared as constant hence this line will throw an error.
   }
   sayHello();
   
   

   if we try to reassign i with a different value within the same block then java script engine will throw an error TypeError: Assignment to constant variable Output

   
   1
   Uncaught TypeError: Assignment to constant variable.
       at sayHello (:4:4)
       at :6:1
   
   

Conclusion: With var keyword we may get unexpected output, so if the value of a variable is not expected to change then const keyword should be preferred else use let keyword to define variables.

Cheers!