Inheritance vs Composition

Let us first look at an example of how we would achieve inheritance in ES5+

class Animal {
    constructor(name, energy) {
        this.name = name
        this.energy = energy
    }
    eat(amount) {
        console.log(`${this.name} is eating.`)
        this.energy += amount
    }
    sleep(length) {
        console.log(`${this.name} is sleeping.`)
        this.energy += length
    }
    play(length) {
        console.log(`${this.name} is playing.`)
        this.energy -= length
    }
}

class Dog extends Animal {
    constructor(name, energy, breed) {
        super(name, energy)
        this.breed = breed
    }
    bark() {
        console.log('Woof Woof!')
        this.energy -= .1
    }
}

class Cat extends Animal {
    constructor(name, energy, declawed) {
        super(name, energy)
        this.declawed = declawed
    }
    meow() {
        console.log('Meow!')
        this.energy -= .1
    }
}

We can visualize the hierarchy like so:

Animal
    name
    energy
    eat()
    sleep()
    play()

    Dog
        breed
        bark()

    Cat
        declawed
        meow()

Let's say later on, you are tasked with adding another entity to the system: User.

User
    email
    username
    pets
    friends
    adopt()
    befriend()

Animal
    name
    energy
    eat()
    sleep()
    play()

    Dog
        breed
        bark()

    Cat
        declawed
        meow()

Everything works fine & dandy till now. However, your project manager now tells you to add the ability of eat, sleep & play to User as well. How would you do it? Here's how we would tackle this in OOP:

Mammal
    name
    eat()
    sleep()
    play()

    User
        email
        username
        pets
        friends
        adopt()
        befriend()

    Animal
        energy

        Dog
            breed
            bark()

        Cat
            declawed
            meow()

This looks pretty fragile because another entity had to be introduced which will now have it's own significance as the program grows. This anti-pattern is popularly called God Object. Hence, the problem with OOP is that entities have a meaning when you write them which can be changed later as the requirements change. These changes can crumble the class hierarchy structure.

I think the lack of reusability comes in object-oriented languages, not in functional languages. Because the problem with object-oriented languages is they've got all this implicit environment that they carry around with them. You wanted a banana but what you got was a gorilla holding the banana and the entire jungle.
-- Joe Armstrong (Creator of Erlang)

Rather than thinking what things are, let's shift to what things do.

• A dog is a sleeper, eater, player & barker.
• A cat is a sleeper, eater, player & meower.

Instead of having these methods tightly coupled to a class, we can have have them as functions and compose them together whenever we need. Great! But how do we operate on a specific instance then? Well, we pass the instance directly to our functions. Closure lets the functions remember the state(instance) that was passed.

const eater = (state) => ({
    eat(amount) {
        console.log(`${state.name} is eating.`)
        state.energy += amount
    }
})
const sleeper = (state) => ({
    sleep(length) {
        console.log(`${state.name} is sleeping.`)
        state.energy += length
    }
})
const player = (state) => ({
    play(length) {
        console.log(`${state.name} is eating.`)
        state.energy -= length
    }
})
const barker = (state) => ({
    bark(length) {
        console.log(`Woof Woof!`)
        state.energy -= .1
    }
})
const meower = (state) => ({
    meow(length) {
        console.log(`Meow!`)
        state.energy -= .1
    }
})

Example of Dog being a sleeper, eater, player & barker:

function Dog(name, energy, breed) {
    let dog = {
        name,
        energy,
        breed
    }

    return Object.assign(
        dog,
        eater(dog),
        sleeper(dog),
        player(dog),
        barker(dog)
    )
}

const dog = Dog('Dog', 10, 'Bulldog')
dog.eat(10)
dog.bark()

Now users can also eat, sleep & play:

function User(email, username) {
    let user = {
        email,
        username,
        pets: [],
        friends: []
    }

    return Object.assign(
        user,
        eater(user),
        sleeper(user),
        player(user)
    )
}

Congratulations, you've freed yourself from tightly coupled inheritance structures.