JavaScript Variables

In JavaScript, variables are like containers that store information — such as numbers, text, or any kind of data.

They let your program store, use, and update values dynamically.

Example:

let name = "Ajay";
const age = 25;
var city = "Lucknow";

Types of Variables in JavaScript

Variables are declared using three keywords — var, let, and const.

Each behaves differently in terms of scope, redeclaration, and reassignment.

1. var

• Introduced in early versions of JavaScript.
• Function-scoped or globally scoped.
• Can be redeclared and reassigned.
• Is hoisted, meaning it’s moved to the top of its scope during execution.
• Can cause unexpected bugs due to its loose scoping behavior.

Example:

var x = 10;
var x = 20; // Redeclaration allowed
x = 30; // Reassignment allowed
console.log(x); // 30

Output : 

2. let

• Introduced in ES6 (2015).
• Block-scoped (only accessible within { } where it’s declared).
• Can be reassigned, but not redeclared in the same scope.
• Not hoisted in a usable way (accessing before declaration causes an error).

Example:

let y = 15;
y = 25; // Reassignment allowed
// let y = 35;  Error: Cannot redeclare 'y'
console.log(y); // 25

Output :

3. const

• Also introduced in ES6.
• Block-scoped, just like let.
• Must be initialized immediately.
• Cannot be reassigned or redeclared.
• Note: If the variable holds an object or array, the reference is fixed, but the contents can still change.

Example:

const z = 100;
// z = 200;  Error: Assignment to constant variable

const obj = { a: 1 };
obj.a = 2; // Allowed — property change, not reassignment
console.log(obj.a); // 2

Output :

Comparison of var, let, and const

Feature	var	let	const
Scope	Function or global scope	Block scope	Block scope
Reassignment	Allowed	Allowed	Not allowed
Redeclaration	Allowed in same scope	Not allowed in same scope	Not allowed in same scope
Initialization	Optional (defaults to undefined)	Optional	Required at declaration
Hoisting	Hoisted, initialized as undefined	Hoisted, not initialized (TDZ)	Hoisted, not initialized (TDZ)
Use Case	Legacy code, loose scoping	Reassignable variables, loops	Constants, immutable references