📦 Lesson 18: Variables and Data Types

A variable in JavaScript is like a labeled box. You can store something inside it — a name, a number, a state — and pull it out whenever you need it.

🔑 Declaring Variables

There are three main ways to declare variables in JavaScript:

let name = "Joe";
const age = 25;
var isStudent = true;

• let: Used for variables that can change later. This is the most common choice today.
• const: Use this when you don’t plan to reassign the variable. It’s a constant.
• var: The old-school way. It still works but has weird scoping behavior. Avoid it unless you know what you’re doing.

🎨 Data Types in JavaScript

JavaScript supports several basic data types. Here’s a breakdown with examples:

1. String

Text values surrounded by quotes.

let city = "Lagos";

2. Number

Just numbers — integers or decimals.

let score = 98;
let price = 14.99;

3. Boolean

Either true or false. Used for logic, conditionals, etc.

let isLoggedIn = true;
let hasPaid = false;

4. Null

A deliberate “nothing.” You assign it when you want to empty a variable.

let user = null;

5. Undefined

A variable that exists but hasn’t been given a value yet.

let notSet;
console.log(notSet); // undefined

6. Object

A collection of key-value pairs. Think of it like a dictionary or real-world object.

let person = {
  name: "Joe",
  age: 25,
  isStudent: true
};

7. Array

A list of items stored in a single variable. Each item has an index (starting from 0).

let fruits = ["apple", "banana", "orange"];

📌 Tip: Type Checking

You can check the type of a variable using the typeof operator:

typeof "hello"     // "string"
typeof 10          // "number"
typeof false       // "boolean"
typeof undefined   // "undefined"
typeof null        // "object" (weird quirk)
typeof {}          // "object"
typeof []          // "object"

🚀 What’s Next?

Now that you understand how data is stored and the types of data JavaScript uses, we’ll move on to Conditionals and loops — the building blocks of interaction.