compositionInheritance.md

Composition and inheritance

Composition and inheritance are two design patterns.

Inheritance is when you organise your classes based on what they are.

Composition is when you organise your classes based on what they do.

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Example: We need a dog and a cat:

Dog
  .bark()

Cat
  .meow()

Then, we realise both of them need to eat.

Dog
  .bark()
  .eat()

Cat
  .meow()
  .eat()

This creates duplication that we can extract in an Animal class.

Animal
  .eat()

  Dog
    .bark()

  Cat
    .meow()

After animals, we need a cleaning robot and a murder robot:

CleaningRobot
  .drive()
  .clean()

MurderRobot
  .drive()
  .kill()

Once again, there is duplication with drive, so we can extract that in a Robot class.

Robot
  .drive()

  CleaningRobot
    .clean()

  MurderRobot
    .kill()

If your application stayed like that, it would work fine. However, now we need a MurderRobotDog that can drive, bark, kill but can't eat.

We could create another class before Robot that would have our bark functionality and have MurderRobotDog inherit from MurderRobot like below but that would mean our objects would have a lot of functionality they don't need.

Game
  .bark()

  Robot
    .drive()

    CleaningRobot
      .clean()

    MurderRobot
      .kill()

      MurderRobotDog

This is where composition is a better approach than inheritance.

Composition helps you organise your code base on whay your objects do.

With composition, our app would look something more like this:

const barker = (state) => ({
  bark: () => console.log('Woof, I am ' + state.name)
})
const driver = (state) => ({
  drive: () => ({})
})

const murderRobotDog = (name)  => {
  let state = {
    name,
    speed: 100,
    position: 0
  }
  return Object.assign(
        {},
        barker(state),
        driver(state),
        killer(state)
    )
}
const bruno =  murderRobotDog('bruno')
bruno.bark() // "Woof, I am Bruno"

In the case above, we create a barker, driver and killer that we then merge into a new object and return it as part of our murderRobotDog object.

Based on this article