Open/Closed Principle (OCP)

Make your code ready for change — without breaking old stuff

“A class should be open for extension but closed for modification.”
— Bertrand Meyer

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What Does That Even Mean?

Let’s break it down:

• Open for extension = You can add new things to it (new features, types, logic).

• Closed for modification = You don’t change old working code that’s already been tested.

So basically:

You should be able to add new features without editing old code.

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A Real-Life Example

Imagine you built a school prize system:

function getPrize(category) {
  if (category === 'sports') return 'Trophy';
  if (category === 'science') return 'Medal';
  if (category === 'music') return 'Certificate';
}

Now you want to add "Art" as a new category.

What do you have to do?

You’ll go inside the getPrize() function and change it — but that might accidentally break the others!

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Let’s Follow the Rule

We’ll make a system in which each category has its own logic:

class Prize {
  getPrize() {
    return 'No prize';
  }
}

class SportsPrize extends Prize {
  getPrize() {
    return 'Trophy';
  }
}

class SciencePrize extends Prize {
  getPrize() {
    return 'Medal';
  }
}

class MusicPrize extends Prize {
  getPrize() {
    return 'Certificate';
  }
}

Now to add Art:

class ArtPrize extends Prize {
  getPrize() {
    return 'Sketchbook';
  }
}

• No changes to old classes.

• We just extended the system by adding a new class.

That’s the Open/Closed Principle!

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Why Should You Care?

• Adding new feature is easier

• Easily maintainable when project will grow.

• You don’t need to worry about the old code

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Final Thoughts

The Open/Closed Principle is like building with LEGO:

• You don’t break the old code.

• You just add new code!

The Liskov Substitution Principle—what happens when classes don't behave as we expect them to—will be covered in the upcoming post.

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Let’s Keep Building Together

I hope you enjoyed this..! Feel free to share your thoughts, questions, or ideas for future topics in the comments.

Thanks for reading

Happy coding!

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