Variables, Data Types & Operators

"If you've ever wondered how apps store your name, your score in a game, or your age — the answer is variables."

Part 1: Variables

Think of a variable as a box where you can store information. You label the box so you know what's inside — and whenever you need that information, you just use the label. In JavaScript, we create variables using three keywords: var, let, and const.

1. `let` — The most common one

let is used when the value can change later. The = sign means 'store this value'.

javascript

let name = "Vivek";
let age = 22;

console.log(name); // Output: Vivek
console.log(age);  // Output: 22

Since we used let, we can change the value later:

javascript

let city = "Chennai";
city = "Bengaluru"; // ✅ Allowed with let
console.log(city); // Output: Bengaluru

2. `const` — When the value won't change

const is short for constant. If you try to change it, JavaScript will throw an error. Use it for things that never change, like a date of birth.

javascript

const pi = 3.14159;
const country = "India";

console.log(pi);
console.log(country);

const birthYear = 2002;
birthYear = 2003; // ❌ Error! Assignment to constant variable.

3. `var` — The old way

var is the old way used before 2015. Today, we prefer let and const because they're safer.

javascript

var score = 100;
console.log(score); // Output: 100

Naming Rules

The golden rule: make your names meaningful. age is better than x!

javascript

// ✅ Good names
let myName      // camelCase — recommended
let userAge     // starts with a letter
let _score      // underscore is fine
let $price      // dollar sign is fine

// ❌ Bad names
// let 1name    // can't start with a number
// let my-name  // hyphens not allowed
// let let      // can't use reserved keywords

Part 2: Data Types

Every value in JavaScript has a type. Let's look at the most important ones.

String (Text)

Wrapped in single quotes, double quotes, or backticks.

javascript

let firstName = "Vivek";
let greeting = 'Hello, World!';
let template = `My name is ${firstName}`;

Number

Both whole numbers and decimals.

javascript

let age = 22;
let price = 99.99;
let temperature = -5;

Boolean

True or false. Like a light switch.

javascript

let isLoggedIn = true;
let hasSubscription = false;

Undefined & Null

Undefined: box exists but is empty. Null: intentionally empty.

javascript

let score; // undefined
let selectedUser = null;

Checking Types with `typeof`

javascript

console.log(typeof "Vivek");    // string
console.log(typeof 22);         // number
console.log(typeof true);       // boolean
console.log(typeof undefined);  // undefined
console.log(typeof null);       // object <- famous JS quirk!

Part 3: Operators

Operators perform actions on values.

1. Arithmetic & Assignment

javascript

let a = 10;
let b = 3;

console.log(a + b);  // 13
console.log(a % b);  // 1 (Remainder/Modulus)

// Assignment
let score = 10;
score += 5; // Same as score = score + 5 (15)

2. Comparison

Always use === (strict equality). == ignores the type and can cause bugs!

javascript

console.log(10 > 5);    // true
console.log(10 === 10); // true (strict equal)

// == vs ===
console.log(5 == "5");  // true  ⚠️ Loose
console.log(5 === "5"); // false ✅ Strict

3. Logical (AND, OR, NOT)

javascript

let isLoggedIn = true;
let hasSubscription = false;

console.log(isLoggedIn && hasSubscription); // false (AND)
console.log(isLoggedIn || hasSubscription); // true (OR)
console.log(!isLoggedIn); // false (NOT)

4. Increment/Decrement & String Concatenation

javascript

let count = 0;
count++; // count is now 1
count--; // count is back to 0

// String Concatenation
let fName = "Vivek", lName = "Kumar";
let full = `${fName} ${lName}`; // Better with backticks!

Variables, Data Types & Operators