1️⃣ JavaScript OOP (Overview)

OOP is a way of writing code using objects and classes to organize and reuse logic.

Instead of writing everything in functions, we create blueprints (classes) and objects (instances).

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2️⃣ Classes

A class is a blueprint to create objects.

Example:

class Person {
  greet() {
    console.log("Hello!");
  }
}const p1 = new Person();
p1.greet(); // Hello!

👉 Person is a class
👉 p1 is an object (instance)

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3️⃣ Constructor

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

Used to initialize values.

Example:

class Person {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }  show() {
    console.log(this.name, this.age);
  }
}const p1 = new Person("Aditya", 25);
p1.show(); // Aditya 25

👉 this refers to the current object

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4️⃣ Inheritance

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

Example:

class Animal {
  speak() {
    console.log("Animal makes sound");
  }
}class Dog extends Animal {
  bark() {
    console.log("Dog barks");
  }
}const d = new Dog();
d.speak(); // inherited
d.bark();  // own method

👉 extends is used for inheritance

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5️⃣ Encapsulation

Encapsulation means hiding data and controlling access.

In JavaScript, we use private fields (#).

Example:

class BankAccount {
  #balance = 0; // private  deposit(amount) {
    this.#balance += amount;
  }  getBalance() {
    return this.#balance;
  }
}const acc = new BankAccount();
acc.deposit(1000);console.log(acc.getBalance()); // 1000
// console.log(acc.#balance); ❌ Error (private)

👉 Data is protected from direct access

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6️⃣ Polymorphism

Polymorphism means same method, different behavior.

Example:

class Animal {
  speak() {
    console.log("Animal sound");
  }
}class Dog extends Animal {
  speak() {
    console.log("Dog barks");
  }
}class Cat extends Animal {
  speak() {
    console.log("Cat meows");
  }
}const animals = [new Dog(), new Cat()];animals.forEach(a => a.speak());

👉 Output:

Dog barks
Cat meows

👉 Same method speak() behaves differently

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🔥 Quick Summary

Concept	Meaning
Class	Blueprint for objects
Constructor	Initialize values
Inheritance	Reuse code from parent class
Encapsulation	Hide and protect data
Polymorphism	Same method, different behavior