Liskov Substitution Principle

The Liskov substitution principle is one of the most important principles to adhere to in object-oriented programming (OOP).

It was introduced by the computer scientist Barbara Liskov in 1987 in a paper she co-authored with Jeannette Wing.

The principle states that child classes or subclasses must be substitutable for their parent classes or super classes.

In other words, the child class must be able to replace the parent class. This has the advantage of letting you know what to expect from your code.

Here’s an example of a code that does not violate the Liskov substitution principle:

class Animal {
  constructor(name) {
    this.name = name;
  }

  makeSound() {
    console.log(`${this.name} makes a sound`);
  }
}

class Dog extends Animal {
  makeSound() {
    console.log(`${this.name} barks`);
  }
}

class Cat extends Animal {
  makeSound() {
    console.log(`${this.name} meows`);
  }
}

function makeAnimalSound(animal) {
  animal.makeSound();
}

const cheetah = new Animal('Cheetah');
makeAnimalSound(cheetah); // Cheetah makes a sound

const dog = new Dog('Jack');
makeAnimalSound(dog); // Jack barks

const cat = new Cat('Khloe');
makeAnimalSound(cat); // Khloe meows

The Dog and Cat classes can successfully replace the parent Animal class.

On the other hand, let’s look at how the code below does violate the Liskov substitution principle:

class Bird extends Animal {
  fly() {
    console.log(`${this.name} flaps wings`);
  }
}

const parrot = new Bird('Titi the Parrot');
makeAnimalSound(parrot); // Titi the Parrot makes a sound
parrot.fly(); // Titi the Parrot flaps wings

The Bird class violates the Liskov substitution principle because it’s not implementing its own makeSound from the parent Animal class. Instead, it’s inheriting the generic sound.

To fix this, you have to make it use the makeSound method too:

class Bird extends Animal {
  makeSound() {
    console.log(`${this.name} chirps`);
  }

  fly() {
    console.log(`${this.name} flaps wings`);
  }
}

const parrot = new Bird('Titi the Parrot');
makeAnimalSound(parrot); // Titi the Parrot chirps
parrot.fly(); // Titi the Parrot flaps wings