JavaScript Classes

A class is a blueprint to create objects. It is the modern, cleaner way to write what constructor functions do.

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

  greet() {
    return "Hi, I am " + this.name;
  }
}

const p = new Person("Robin", 25);
console.log(p.greet());
console.log(p.age);

• constructor() runs once when you use new.
• Methods are written without the function keyword and go on the prototype automatically.

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Getters and setters

class Circle {
  constructor(radius) {
    this.radius = radius;
  }
  get area() {
    return Math.PI * this.radius ** 2;
  }
}

const c = new Circle(2);
console.log(c.area.toFixed(2));

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Static methods

class MathHelper {
  static double(n) {
    return n * 2;
  }
}

console.log(MathHelper.double(5));

A static method belongs to the class itself, so you call it on the class, not on an object.

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Inheritance with extends

class Animal {
  constructor(name) {
    this.name = name;
  }
  speak() {
    return this.name + " makes a sound";
  }
}

class Dog extends Animal {
  speak() {
    return this.name + " barks";
  }
}

const d = new Dog("Tommy");
console.log(d.speak());
console.log(d instanceof Animal);

Use super to call the parent. In a child constructor you must call super() first.

class Puppy extends Dog {
  constructor(name) {
    super(name);
    this.age = 1;
  }
}
console.log(new Puppy("Bruno").age);

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Private fields

class Account {
  #balance = 0;     // private
  deposit(x) { this.#balance += x; }
  get balance() { return this.#balance; }
}

const a = new Account();
a.deposit(100);
console.log(a.balance);

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Summary

• class with a constructor and methods.
• extends and super for inheritance.
• static for class-level methods and #name for private fields.
• Next: JavaScript ES6 Features.

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