JavaScript Basics – Variables and Data Types (JS-01)

Track: JavaScript Basics
Lesson: JS-01 / JS-10

🎯 Lesson Goal

By the end of this lesson, you will understand how to store data in JavaScript using variables and identify different data types used in real-world programming.

📌 Prerequisites

Basic computer knowledge
No prior programming experience required

📘 What is JavaScript?

JavaScript is a programming language used to make web pages interactive.
It allows us to:

Respond to user actions
Change content dynamically
Build web and server-side applications

📘 What are Variables?

A variable is a container used to store data in memory.

Think of it as a labeled box where you keep information for later use.

Example:

let name = "Kavita";
let age = 25;

Here:

name stores text
age stores a number

🧩 Declaring Variables in JavaScript

JavaScript provides three keywords to declare variables:

1️⃣ `var` (old way – avoid)

var city = "Delhi";

Problems with var:

Function-scoped
Can cause bugs
Not recommended in modern JS

2️⃣ `let` (recommended)

let course = "JavaScript";
course = "MERN Stack";

Block-scoped
Value can be changed

3️⃣ `const` (constant value)

const institute = "Programming Empire";

Block-scoped
Value cannot be changed

📘 JavaScript Data Types

Data types specify what kind of data a variable can store.

🔢 1. Number

Stores numeric values (integers and decimals).

let marks = 85;
let price = 199.99;

🔤 2. String

Stores text data (written inside quotes).

let studentName = "Aman";
let city = 'Delhi';

✔️ 3. Boolean

Stores true or false.

let isLoggedIn = true;
let hasPaid = false;

❓ 4. Undefined

A variable declared but not assigned a value.

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

🚫 5. Null

Represents no value intentionally.

let response = null;

📦 6. Object

Stores multiple values in a single variable.

let student = {
  name: "Riya",
  age: 22,
  course: "BCA"
};

📋 7. Array

Stores multiple values in an ordered list.

let subjects = ["HTML", "CSS", "JavaScript"];

🔍 Using `typeof` Operator

To check the data type of a variable:

typeof 10;          // number
typeof "Hello";     // string
typeof true;        // boolean
typeof {};          // object

⚠️ Common Mistakes

Using var instead of let or const
Forgetting quotes for strings
Trying to change a const value
Confusing null with undefined

🧠 Practice Tasks

Declare a variable for your name using let
Declare a constant for your institute name
Create an array of 5 programming languages
Create an object storing student details
Use typeof to check each variable

📝 Summary

Variables store data in memory
let and const are preferred over var
JavaScript supports multiple data types
Objects and arrays store complex data
typeof helps identify data types

➡️ Next Lesson

JavaScript Operators and Expressions (JS-02)

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