TIL Open-Closed Principle in OOP

POSTED ON: Sep 19, 2022

TAGS: oop example

In object-oriented programming, the open–closed principle (OCP) states "software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification";

To use an example:

// The base class
class Animal {
  constructor(name, species) {
    this.name = name;
    this.species = species;
  }

  makeSound() {
    throw new Error('Method not implemented');
  }
}

If you want to wanted to make more methods, you can extend it.
But you cannot modify the base class!

// Everything else Extends off of the base class
class Dog extends Animal {
  constructor(name) {
    super(name, 'Dog');
  }

  makeSound() {
    return 'Woof!';
  }
}

class Cat extends Animal {
  constructor(name) {
    super(name, 'Cat');
  }

  makeSound() {
    return 'Meow!';
  }
}

class Cow extends Animal {
  constructor(name) {
    super(name, 'Cow');
  }

  makeSound() {
    return 'Moo!';
  }
}

Sweet! Now to see it in action:

// Test
function makeAnimalSound(animal) {
  console.log(`${animal.name} the ${animal.species} says ${animal.makeSound()}`);
}


// Run your code
const cow = new Cow('Betsy');
makeAnimalSound(cow); // Betsy the Cow says Moo!

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