Полиморфизм

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

Animal.prototype.greet = function () {
  return "Не ожидал, что я разговариваю?!";
};

let a = new Animal();
console.log(a.greet());

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

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

function makeSound(animal) {
  return animal.speak();
}

const dog = new Dog();
const cat = new Cat();

console.log(makeSound(dog)); // Bark
console.log(makeSound(cat)); // Meow