Variables

Variables are used to store data that can be referenced and manipulated later.

1. Declaration Keywords

a) var

• Function-scoped
• Hoisted (can be used before declaration, but unsafe)
• Avoid using in modern code

var x = 10;

b) let

• Block-scoped
• Can be reassigned
• Preferred for values that change

let count = 5;
count = 6;

c) const

• Block-scoped
• Cannot be reassigned
• Must be initialized when declared
• Use for constants or fixed references

const PI = 3.14;
// PI = 3.1415 ❌ Error

2. Naming Rules

• Case-sensitive
• Must start with letter, _, or $
• Can’t start with a number
• Use camelCase for naming (userName, totalPrice)

let userName = "Emma";
const MAX_SCORE = 100;

3. Hoisting Behavior

• var is hoisted (declared at top of scope), but value is undefined
• let and const are hoisted but not initialized (Temporal Dead Zone)

console.log(x); // undefined
var x = 10;

console.log(y); // ReferenceError
let y = 20;

4. Reassignment

Keyword	Reassignable	Scope
var	✅	Function
let	✅	Block
const	❌	Block

5. Constants with Objects and Arrays

const prevents reassignment of the variable, not mutation of its contents.

const user = { name: "Rahul" };
user.name = "Dev"; // ✅ Allowed

const arr = [1, 2, 3];
arr.push(4); // ✅ Allowed