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var vs let vs const

In javascript, you can create/declare variables using keywords var, let, and const.

Let’s see the differences between these keywords to have a better understanding of what to use and where.

Scope

Scope essentially means where these variables are available for use.

There are two types of scopes in JS:

 

  • Function Scope: Visibility is limited to the function.
 function myFn() {

var x = 10;

console.log(x); //prints 10

}

console.log(x); // ReferenceError: x is not defined


 
  • Block Scope: Visibility is limited to the block of code.
     if (true) { 

    let x = 10;

    console.log(x); //prints 10

    }

    console.log(x); // ReferenceError: x is not defined
     
     

Now, that we have idea of scope. We can discuss the scope of var, let and const.

  • var declarations are function scoped.
  • let declarations are block scoped.
  • const declarations are block scoped.
     

Redefining and Redeclaring feature

A variable declared using ‘var’ can be redefined and even redeclared anywhere throughout its scope.

var x = 30;
console.log(x); //prints 30
x = "Hi"; //redefining or re-assigning (works without any error)
console.log(x); //prints "Hi"

var y = 10;
console.log(y); //prints 10
var y = "Hello"; //Redeclaring (works without any error)
console.log(y) //Prints "Hello"

 

A variable declared using ‘let’ can be redefined within its scope but cannot be re-declared within its scope.

let x = 11;
console.log(x); //prints 11
x = "IB"; //works without any error
console.log(x); //prints "IB"

let y = 12;
console.log(y); //prints 12
let y = "Scaler"; // error: Identifier y has already been declared

let z = 13;
if(true){
let z = "Fun"; //works without any error as scope is different.
console.log(z) //prints "Fun"
}
console.log(z) //prints 13
 

 

A variable declared using ‘const’ cannot be redefined or re-declared within its scope.

const x = 10;
console.log(x); //prints 10
x = 11; // error: Assignment to constant variable.

const y;
y = 2; //error

const z = 12;
console.log(z) //prints 12
const z= 13; // error: Identifier 'y' has already been declared


Every const declaration must be initialized at the time of declaration.

 

Hoisting

Hoisting is a mechanism where variables and function declarations are moved to the top of their scope before code execution.

 

console.log(x); // prints undefined
var x = 100;
console.log(x); //prints 100


 

  • Variables declared using var are hoisted to the top of their scope and initialized with a value of undefined(special type).
  • Variables declared using let are hoisted to the top of their scope but are not initialized with any value.
  • Variables declared using const are hoisted to the top of their scope but are not initialized with any value.

 

 

console.log(x); // prints undefined
var x = 100;
console.log(x); //prints 100

console.log(y); //Reference error
let y = 200;
console.log(y); //prints 200

console.log(z); //Reference error
const z = 300;
console.log(z); //prints 300
 

Try the following example in the editor below.

Predict the output of the following code:

var a = 10;
{
    var a = -10;
}
let b = a;
{
    let b = -20;
}

console.log(b)
Hints
  • Hints are not available for this problem

Discussion


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