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

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

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

Comparison of var, let, and const