🚀 OOPs in JavaScript

A beginner-friendly and interview-ready repository to understand Object-Oriented Programming (OOP) concepts in JavaScript with clear definitions, practical examples, and real-world use cases.

---

🎯 What You'll Learn

• Core OOP principles in JavaScript
• ES6 Classes & Objects
• Encapsulation, Abstraction, Inheritance, Polymorphism
• Static methods & this keyword
• Real-world OOP-based examples
• Clean folder structure for easy learning

---

📚 Ideal For

• JavaScript beginners
• MERN / Full Stack developers
• Interview preparation
• Students & freshers
• Developers revising OOP concepts

---

---

---

🔥 OOPs in JavaScript – Complete Notes/pdf with Practical Examples

📌 What is OOP?

Object-Oriented Programming (OOP) is a programming paradigm based on objects, which contain:

• Properties (data)
• Methods (functions)

Why OOP?

OOP helps in:

• Code reusability
• Better structure
• Easy maintenance
• Real-world modeling

---

1️⃣ Object

📖 Definition

An object is a collection of related data and functions stored as key–value pairs.

✅ Example

const user = {
  name: "Kunal",
  role: "Full Stack Developer",
  greet() {
    console.log(`Hello, I am ${this.name}`);
  }
};

user.greet();

---

2️⃣ Class

📖 Definition A class is a blueprint for creating objects.

✅ Example

class User {
  constructor(name, role) {
    this.name = name;
    this.role = role;
  }

  greet() {
    console.log(`Hi, I am ${this.name}`);
  }
}

const user1 = new User("Kunal", "Developer");
user1.greet();

---

3️⃣ Constructor

📖 Definition A constructor is a special method that runs automatically when an object is created.

✅ Example

class Car {
  constructor(brand, speed) {
    this.brand = brand;
    this.speed = speed;
  }
}

const car1 = new Car("BMW", 220);

---

4️⃣ Encapsulation

📖 Definition Encapsulation means binding data and methods together and restricting direct access to data.

✅ Example

class BankAccount {
  #balance = 0; // private field

  deposit(amount) {
    this.#balance += amount;
  }

  getBalance() {
    return this.#balance;
  }
}

const acc = new BankAccount();
acc.deposit(5000);
console.log(acc.getBalance());

✔ Data hiding

✔ Access via methods only

---

5️⃣ Abstraction

📖 Definition Abstraction means hiding internal implementation details and exposing only necessary functionality.

✅ Example

class Payment {
  pay() {
    console.log("Processing payment...");
  }
}

class UPI extends Payment {
  pay() {
    console.log("Paid using UPI");
  }
}

const payment = new UPI();
payment.pay();

✔ User does not care how it works

✔ User only cares what it does

---

6️⃣ Inheritance

📖 Definition Inheritance allows one class to reuse properties and methods of another class.

✅ Example

class Person {
  constructor(name) {
    this.name = name;
  }

  walk() {
    console.log("Walking...");
  }
}

class Developer extends Person {
  code() {
    console.log("Coding...");
  }
}

const dev = new Developer("Kunal");
dev.walk();
dev.code();

---

7️⃣ Polymorphism

📖 Definition Polymorphism means same method name but different behavior.

✅ Example

class Shape {
  area() {
    return 0;
  }
}

class Square extends Shape {
  area() {
    return 5 * 5;
  }
}

class Circle extends Shape {
  area() {
    return 3.14 * 5 * 5;
  }
}

const shapes = [new Square(), new Circle()];
shapes.forEach(shape => console.log(shape.area()));

---

8️⃣ this Keyword

📖 Definition this refers to the current object.

✅ Example

class User {
  constructor(name) {
    this.name = name;
  }

  show() {
    console.log(this.name);
  }
}

---

9️⃣ Static Methods

📖 Definition Static methods belong to the class, not to objects created from the class.

✅ Example

class MathUtils {
  static add(a, b) {
    return a + b;
  }
}
console.log(MathUtils.add(5, 3));

🔁 Real-World Example (Mini Project) User Management System

class User {
  constructor(username) {
    this.username = username;
  }

  login() {
    console.log(`${this.username} logged in`);
  }
}

class Admin extends User {
  deleteUser(user) {
    console.log(`${user.username} deleted`);
  }
}

const admin = new Admin("Admin1");
const user = new User("Kunal");

admin.login();
admin.deleteUser(user);

---

📁 Recommended Folder Structure

oops-js/
│
├── 01_object/
│   └── object.js
│
├── 02_class/
│   └── class.js
│
├── 03_constructor/
│   └── constructor.js
│
├── 04_encapsulation/
│   └── encapsulation.js
│
├── 05_abstraction/
│   └── abstraction.js
│
├── 06_inheritance/
│   └── inheritance.js
│
├── 07_polymorphism/
│   └── polymorphism.js
│
├── 08_static/
│   └── static.js
│
├── 09_real_world_examples/
│   └── user-management.js
│
├── NOTES.md
└── README.md

📌 Notes for Interview Preparation

• OOP improves scalability and maintainability

• JavaScript uses prototype-based inheritance

• class is syntactic sugar over prototypes

• Private fields use #

• Polymorphism is achieved using method overriding

---

🚀 Author

Kunal Sahu Full Stack Developer | MERN | JavaScript

---

🙌 Connect with Me

• 🔗 LinkedIn
• 💼 GitHub
• 🌍 Portfolio
• 📧 ksahu0103@gmail.com

---