Classes & OOP

Class Declaration

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

  speak() {
    console.log(`${this.name} says ${this.sound}!`);
  }
}

const dog = new Animal("Rex", "Woof");
dog.speak(); // "Rex says Woof!"

Getters & Setters

class Temperature {
  #celsius;
  constructor(c) { this.#celsius = c; }

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

  toString() { return `${this.#celsius}°C`; }
}

const temp = new Temperature(100);
console.log(temp.fahrenheit); // 212
temp.fahrenheit = 32;
console.log(temp.toString()); // 0°C

Private Fields (#)

ES2022 private fields (prefix #) cannot be accessed outside the class:

class BankAccount {
  #balance = 0;

  deposit(amount) { this.#balance += amount; }
  withdraw(amount) {
    if (amount > this.#balance) throw new Error("Insufficient funds");
    this.#balance -= amount;
  }
  get balance() { return this.#balance; }
}

const acc = new BankAccount();
acc.deposit(100);
console.log(acc.balance); // 100

Static Methods & Properties

class MathUtils {
  static PI = 3.14159;

  static circleArea(r) {
    return MathUtils.PI * r * r;
  }
}

console.log(MathUtils.PI);            // 3.14159
console.log(MathUtils.circleArea(5)); // 78.53...

Inheritance with extends

class Shape {
  constructor(color) { this.color = color; }
  describe() { return `A ${this.color} shape`; }
}

class Circle extends Shape {
  constructor(color, radius) {
    super(color);
    this.radius = radius;
  }
  area() { return Math.PI * this.radius ** 2; }
  describe() { return `${super.describe()} (circle r=${this.radius})`; }
}

const c = new Circle("red", 5);
console.log(c.describe()); // "A red shape (circle r=5)"