Differences between declaring variables using var, let and const

 Before the ES6 version of javascript, only the keyword var was used to declare variables.

With the ES6 Version, keywords let and const were introduced to declare variables.

keyword	const	let	var
global scope	no	no	yes
function scope	yes	yes	yes
block scope	yes	yes	no
can be reassigned	no	yes	yes

Let’s understand the differences with examples:

Let’s understand the differences with examples:

var variable1 = 23;

let variable2 = 89;

function catchValues(){
  console.log(variable1);
  console.log(variable2);

// Both the variables can be accessed anywhere since they are declared in the global scope
}

window.variable1; // Returns the value 23

window.variable2; // Returns undefined    

The variables declared with the let keyword in the global scope behave just like the variable declared with the var keyword in the global scope.

Variables declared in the global scope with var and let keywords can be accessed from anywhere in the code.

But, there is one difference !

Variables that are declared with the var keyword in the global scope are added to the window/global object. Therefore, they can be accessed using window.variableName.

Whereas, the variables declared with the let keyword are not added to the global object, therefore, trying to access such variables using window.variableName results in an error.

var vs let in functional scope

function varVsLetFunction(){
  let awesomeCar1 = "Audi";
  var awesomeCar2 = "Mercedes";
}

console.log(awesomeCar1); // Throws an error
console.log(awesomeCar2); // Throws an error

Variables declared in a functional/local scope using var and let keywords behave exactly the same, meaning , they cannot be accessed from outside of the scope.

{
  var variable3 = [1, 2, 3, 4];
}

console.log(variable3); // Outputs [1,2,3,4]

{
  let variable4 = [6, 55, -1, 2];
}

console.log(variable4); // Throws error

for(let i = 0; i < 2; i++){
  //Do something
}

console.log(i); // Throws error

for(var j = 0; j < 2; i++){
  // Do something
}

console.log(j) // Outputs 2

In javascript, a block means the code written inside the curly braces {} .

Variables declared with var keyword do not have block scope. It means a variable declared in block scope {} with the var keyword is the same as declaring the variable in the global scope.

Variables declared with let keyword inside the block scope cannot be accessed from outside of the block.

Const keyword

Variables with the const keyword behave exactly like a variable declared with the let keyword with only one difference, any variable declared with the const keyword cannot be reassigned.

Example:

const x = {name:"Vijay"};

x = {address: "India"}; // Throws an error

x.name = "Mukesh"; // No error is thrown

const y = 23;

y = 44; // Throws an error

In the code above, although we can change the value of a property inside the variable declared with const keyword, we cannot completely reassign the variable itself.