JavaScript Series — Phase 1 day 3 — JavaScript Fundamentals

📅 Day 3 — Variables in JavaScript

1. 📦 What is a Variable?

A variable is a named container used to store data.

let name = "Tuhin";
let age = 20;

console.log(name);
console.log(age);

You can think of it as:

name → "Tuhin"
age  → 20

2. 🔑 var, let, and const

JavaScript provides three keywords for declaring variables.

var city = "Kolkata";
let age = 20;
const country = "India";

var

• Older way of declaring variables

• Function-scoped

• Can be redeclared and reassigned

let

• Modern way to declare a variable

• Block-scoped

• Can be reassigned

• Cannot be redeclared in the same scope

const

• Block-scoped

• Cannot be reassigned

• Cannot be redeclared

• Used when the variable reference should remain constant

3. ✏️ Variable Naming Rules

JavaScript variable names have some important rules:

let firstName = "Tuhin";
let user_age = 20;
let $price = 500;
let _value = 100;

Rules:

• Can contain letters, numbers, _, and $

• Cannot start with a number

• Cannot contain spaces

• Cannot use reserved JavaScript keywords

• JavaScript is case-sensitive

❌ Invalid:

let 1name = "Tuhin";
let user name = "Tuhin";

✅ Valid:

let name1 = "Tuhin";
let userName = "Tuhin";

A common convention is camelCase:

let firstName = "Tuhin";
let totalAmount = 5000;

4. 🔄 Reassigning Variables

Reassignment means giving an existing variable a new value.

With let:

let age = 20;

age = 21;

console.log(age);

Output:

21

With const:

const country = "India";

country = "Japan"; // Error

So:

let   → can be reassigned
const → cannot be reassigned

5. ⚖️ let vs const vs var

Feature	var	let	const
Reassign	✅	✅	❌
Redeclare	✅	❌	❌
Block scoped	❌	✅	✅
Modern usage	⚠️ Avoid generally	✅	✅
Common use	Legacy code	Changing values	Fixed references

⭐ Best Practice

For modern JavaScript:

const name = "Tuhin";
let age = 20;

Use const by default, and use let when you know the value needs to change.

Generally, avoid var in new code unless you specifically need its older scoping behavior.