Classes & Objects¶

JavaScript supports object-oriented programming (OOP) using classes (ES6+) and constructor functions. Classes help organize related data and behavior into reusable blueprints.

1. Creating a Class¶

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

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

2. Creating Objects (Instances)¶

const user1 = new Person("Alice", 25);
user1.greet(); // Hi, I'm Alice

3. Constructor Method¶

constructor() is called automatically when using new
Used to initialize object properties

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

4. Adding Methods¶

Define methods directly inside the class:

class Circle {
  constructor(radius) {
    this.radius = radius;
  }

  area() {
    return Math.PI * this.radius ** 2;
  }
}

5. Class Expression (Anonymous or Named)¶

const Animal = class {
  constructor(type) {
    this.type = type;
  }
};

6. Inheritance (`extends`)¶

One class can inherit from another.

class Animal {
  speak() {
    console.log("Generic sound");
  }
}

class Dog extends Animal {
  speak() {
    console.log("Bark");
  }
}

const d = new Dog();
d.speak(); // Bark

7. `super()` Keyword¶

Calls the constructor or method from the parent class.

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

class Cat extends Animal {
  constructor(name, color) {
    super(name);
    this.color = color;
  }
}

8. Static Methods¶

Static methods belong to the class, not instances.

class MathUtil {
  static square(x) {
    return x * x;
  }
}

MathUtil.square(4); // 16

9. Object vs Class¶

Object literal: used for single instances
Class: used for creating multiple objects with shared structure

// Object literal
const user = {
  name: "Bob",
  greet() {
    console.log("Hi");
  }
};

Use object literals for simple structures, classes for reusable templates.