Day 2

Variables — let, const, var

10 min

JavaScript 100 Days

Variable ek container hai jisme data store karte hain. JavaScript mein teen tarike se variables declare hote hain: var, let, aur const.

let — changeable value

let se declare ki gayi value baad mein change ho sakti hai. Yeh modern JavaScript ka preferred way hai.

variables.js

javascript

let name = "Zohaib";
console.log(name); // Zohaib

name = "Ali"; // Value change kar sakte hain
console.log(name); // Ali

let age = 25;
let isStudent = true;

const — fixed value

const se declare ki gayi value change nahi ho sakti. Isko use karo jab value fix ho.

variables.js

javascript

const PI = 3.14159;
const siteName = "CodingSkillUp";

// Yeh error dega:
// PI = 3; // TypeError: Assignment to constant variable

var — purana tarika (avoid karo)

var globally scoped hota hai jis se unexpected bugs aate hain. Modern code mein let/const use karo.

variables.js

javascript

// var ka problem — function scope vs block scope
function example() {
  var x = 10;
  if (true) {
    var x = 20; // Same variable!
    console.log(x); // 20
  }
  console.log(x); // 20 (surprise!)
}

// let ke saath sahi behavior:
function example2() {
  let y = 10;
  if (true) {
    let y = 20; // Different variable
    console.log(y); // 20
  }
  console.log(y); // 10 (correct!)
}

Naming Rules

Variables ke naam meaningful hone chahiye. JavaScript case-sensitive hai.

naming.js

javascript

// ✅ Good names
let firstName = "Ahmed";
let totalPrice = 500;
let isLoggedIn = false;

// ❌ Bad names
let x = "Ahmed";   // kya hai yeh?
let tp = 500;      // unclear
let a1b2 = false;  // unreadable

// camelCase use karo: firstName, lastName, totalPrice

🎯 Practice Challenge

Apne baare mein variables banao: name, age, city, isStudent. Console mein print karo.

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