Interfaces

An interface is a contract that defines the properties and what the object that implements it can do. For example, you could define what can do a Plumber and an Electrician:

interface Electrician {
  layWires(): void
}

interface Plumber {
  layPipes(): void
}

Then, you can consume the services of your interfaces:

function restoreHouse(e: Electrician, p: Plumber) {
  e.layWires()
  p.layPipes()
}

Notice that the way you have to implement an interface is free. You can do that by instantiating a class, or with a simple object:

let iAmAnElectrician = {
  layWires: () => { console.log("Work with wires…") }
}

An interface doesn’t exist at all at runtime, so it is not possible to make an introspection. It is the classic JavaScript way to deal with object programming, but with a good control at compile time of the defined contracts.

Abstract classes

A class is both a contract and the implementation of a factory. An abstract class is also an implementation but incomplete. Especially, an abstract class exists at runtime, even if it has only abstract methods (then instanceof can be used).

When you define an abstract class, you often try to control how a process has to be implemented. For example, you could write something like this:

abstract class HouseRestorer {
  protected abstract layWires(): void
  protected abstract layPipes(): void
  restoreHouse() {
    this.layWires()
    this.layPipes()
  }
}

This abstract class HouseRestorer defines how the methods layWires and layPipes will be used, but it is up to a child class to implement the specialized treatments before it can be used.

Abstract classes are a traditional OOP approach, which is not traditional in JavaScript.

Both approaches allow the same things to be done. They are two different ways of solving a problem.