What is Object-Oriented Programming (OOP)?

OOP is a programming paradigm that organizes code around objects instead of functions. Most modern languages — Java, Python, C#, JavaScript — support OOP. Understanding OOP is essential for writing clean, reusable code.

What is OOP?

OOP structures programs as collections of objects. Each object contains data (properties) and behavior (methods). Think of a car: it has properties like color and speed, and methods like accelerate and brake.

The Four Pillars of OOP

1. Encapsulation

Bundling data and methods together, hiding internal details:

class BankAccount {
  #balance = 0; // Private property

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

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

2. Abstraction

Hiding complex implementation, showing only what's needed:

// User doesn't need to know how email is sent
function sendEmail(to, subject, body) {
  // Complex SMTP logic hidden here
  console.log(`Email sent to ${to}`);
}

3. Inheritance

Creating new classes from existing ones:

class Animal {
  speak() { return "Some sound"; }
}

class Dog extends Animal {
  speak() { return "Woof!"; }
}

class Cat extends Animal {
  speak() { return "Meow!"; }
}

4. Polymorphism

Same method name, different behavior:

const animals = [new Dog(), new Cat()];
animals.forEach(animal => console.log(animal.speak()));
// "Woof!"
// "Meow!"

Classes in JavaScript

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

  greet() {
    return `Hi, I'm ${this.name}`;
  }
}

const john = new Person("John", 25);
console.log(john.greet()); // "Hi, I'm John"

OOP vs Functional Programming

Feature	OOP	Functional
Structure	Objects and classes	Functions
Data	Inside objects	Passed between functions
Best for	Complex systems	Data transformations