What is unit testing and why is it important?

Short answer: Unit testing involves testing the smallest parts of an application (units) independently to ensure they work correctly. It is important because it helps detect bugs early, improves code quality, supports refactoring, and provides documentation for expected behavior.

Real-world example (ShopNest)

ShopNest unit tests cover pricing and discount rules so a bad coupon change fails in CI before customers see it.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.