understand factory pattern in plain javascript

factory-pattern.js

class Animal {
	constructor(pet) {
		this.pet = pet;
	}
    
	speak() {
		console.log(this.pet);
	}

	join(animal) {
		this.pet = `${this.pet} joined to ${animal.pet}`;
		return this;
	}
}

class Cat extends Animal {
	constructor(number) {
		super('😎'.repeat(number));
	}
}

class Dog extends Animal {
	constructor(number) {
		super('🐱‍'.repeat(number));
	}
}

class Bird extends Animal {
	constructor(number) {
		super('🐤'.repeat(number));
	}
}

const ANIMAL = {
	BIRD: 'BIRD',
	CAT: 'CAT',
	DOG: 'DOG',
	DEFAULT: 'DEFAULT',
}

const Animals = {
    [ANIMAL.CAT]: (num) => new Cat(num),
    [ANIMAL.DOG]: (num) => new Dog(num),
    [ANIMAL.BIRD]: (num) => new Bird(num),
    [ANIMAL.DEFAULT]: () => { throw chalk.bgRed('Sorry, we don\'t have pet you wished') }
}

const AnimalFactory = (animal, num) => {
    return (Animals[animal] || Animal[ANIMAL.DEFAULT])(num);
}

const adoptPet = (pet, num = 0) => {
    return AnimalFactory(pet, num);
}

adoptPet(ANIMAL.BIRD, 3).speak();