Lesson 2Beginner 2 min read

Variables and Data Types

What are variables?

A variable is a named container for a value. You store something in a variable so you can use it later, change it, or pass it to a function.

JavaScript has two modern ways to declare variables: let and const.

let vs const

const — use this by default. It means the variable can't be reassigned:

JavaScript

const name = "Manish";
const pi = 3.14159;

name = "Someone"; // Error! Can't reassign a const

let — use this when you need to reassign the value:

JavaScript

let score = 0;
score = score + 10; // Works fine
score += 5;         // Shorthand — score is now 15

What about var? You'll see it in older code, but avoid it. var has confusing scoping behavior that let and const fix. Always use const first; switch to let only if you need to reassign.

Data types

JavaScript has several built-in types:

Strings — text wrapped in quotes:

JavaScript

const greeting = "Hello, world!";
const name = 'Manish';
const message = `Welcome, ${name}!`; // Template literal (backticks)

Template literals (backticks) let you embed expressions with `$

` — extremely useful.

Numbers — integers and decimals are the same type:

JavaScript

const age = 25;
const price = 9.99;
const total = price * 3; // 29.97

Booleans — true or false:

JavaScript

const isLoggedIn = true;
const hasPermission = false;

null and undefined:

JavaScript

let user = null;       // Explicitly "no value"
let address;           // undefined — declared but not assigned

Arrays — ordered lists:

JavaScript

const colors = ["red", "green", "blue"];
console.log(colors[0]); // "red"
console.log(colors.length); // 3

Objects — key-value pairs:

JavaScript

const person = {
  name: "Manish",
  age: 25,
  isStudent: true,
};

console.log(person.name); // "Manish"

Naming conventions

Use camelCase for variables: firstName, isLoggedIn, totalPrice.
Make names descriptive: userCount is better than x.
Constants that never change are sometimes written in UPPERSNAKECASE: const MAX_RETRIES = 3.

Common mistakes

Using == instead of ===:

JavaScript

console.log(2 == "2");  // true  (loose equality — converts types!)
console.log(2 === "2"); // false (strict equality — compares type too)

Always use === (strict equality). The loose == operator has surprising type coercion rules that lead to bugs.

Key takeaway

Start with const for everything. Switch to let only when you need to reassign. Learn the basic types — strings, numbers, booleans, arrays, and objects — and you'll have the foundation for everything else in JavaScript.