Inheritance and encapsulation

Inheritance: extends and super

A class can inherit from another to reuse and specialize its behavior, using extends:

class Animal {
  constructor(name) {
    this.name = name;
  }
  describe() {
    return `${this.name} makes a sound`;
  }
}

class Dog extends Animal {
  describe() {
    return `${this.name} barks`;
  }
}

If the subclass has its own constructor, it must call super(...) to initialize the inherited part:

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

super.method() also lets you call a method of the parent class.

Encapsulation: private fields

By default, all properties are accessible from outside. For data that should not be touched directly, use private fields with #:

class BankAccount {
  #balance = 0;
  deposit(amount) {
    this.#balance += amount;
  }
  get balance() {        // getter: read like a property
    return this.#balance;
  }
}
const c = new BankAccount();
c.deposit(100);
console.log(c.balance); // 100  (c.#balance would throw an error)

Getters and setters

They let you run logic when reading (get) or assigning (set) a property, while keeping a simple syntax:

class Temperature {
  #celsius = 0;
  get fahrenheit() {
    return this.#celsius * 9 / 5 + 32;
  }
  set fahrenheit(f) {
    this.#celsius = (f - 32) * 5 / 9;
  }
}

Examples

Inheritance with super and a getter

class Vehicle {
  constructor(brand) {
    this.brand = brand;
  }
  describe() {
    return "Vehicle " + this.brand;
  }
}
class Car extends Vehicle {
  constructor(brand, doors) {
    super(brand);
    this.doors = doors;
  }
  describe() {
    return super.describe() + " with " + this.doors + " doors";
  }
}
console.log(new Car("Seat", 5).describe());