Modern javascript (ES6) introduces a new keyword for the declaration of variables let and const which is an addition to the var keyword in javascript. In this article, i will introduce you to the basic difference between the let, var, and const keywords.

Firstly all three different types of variable declaration differ in the scope of the variable.

1. var - The scope of the var keyword within a function. let's see this in code

     function say(){
        for(var i=0;i<5;i++){
           console.log(i);
       }
       console.log(i);
     }
     say();
   

   In the above code when we call say() function it will print 5. but we have declared var in for loop and we print the value of i outside the scope of i that is for loop. This is how var works.

2. Let - Now let's try the above code with the let keyword

     function say(){
        for(let i=0;i<5;i++){
           console.log(i);
       }
       console.log(i);
     }
     say();
   

   If we run the above code we will get some error (shown below) which means we are using i now out of scope. Hence we can say that when we have to preserve the scope of the variable up to a specific block we can use 'var'.

    console.log(i);
                    ^
   
    ReferenceError: i is not defined
   

3. Const - As the name suggests this keyword is used to declare constant value. The scope of a constant variable is the same as let within the scope of block.

const a=1;    //declaration
// a=3

We can't modify the const variable if we try to run the second line of the above code and modify the value of const we will get an error (shown below).

a=3;
 ^
TypeError: Assignment to constant variable.

We can this is the basic difference between this 3 keywords.