Liskov Substitution Principle

If we have a function which accepts a Class. The function should also work for all the Sub Class.

Here makeBirdFly function works for Bird Class. For Duck Sub Class it passes. But for Penguin Sub Class it fails.

class Bird {
  fly() {
    console.log("I can fly");
  }
}

class Duck extends Bird {
  quark() {
    console.log("I can quark");
  }
}

class Penguin extends Bird {
  fly() {
    throw new Error("I can't fly");
  }

  swim() {
    console.log("I can swim");
  }
}

const duck = new Duck();
const penguin = new Penguin();

function makeBirdFly(bird) {
  bird.fly();
}

makeBirdFly(duck); // I can fly
makeBirdFly(penguin); // ERROR! throw new Error("I can't fly")

We can fix this by changing what we are inheriting from.

class FlyingBird {
  fly() {
    console.log("I can fly");
  }
}

class SwimmingBird {
  swim() {
    console.log("I can swim");
  }
}

class Duck extends FlyingBird {
  quark() {
    console.log("I can quark");
  }
}

class Penguin extends SwimmingBird {
  fly() {
    throw new Error("I can't fly");
  }
}

const duck = new Duck();
const penguin = new Penguin();

function makeFlyingBirdFly(bird) {
  bird.fly();
}

function makeSwimmingBirdSwim(bird) {
  bird.swim();
}

makeFlyingBirdFly(duck); // I can fly
makeSwimmingBirdSwim(penguin); // I can swim

One possible issue with Object Oriented Programming & Inheritence is that, here in this example Duck can Swim and Fly. But it cannot inherit from two Classes FlyingBird & SwimmingBird.

One way to fix this is by Composition. Just add swimming and flying functionality in Duck without extending from any class.