How do you decide what to test in your application?

Short answer: Focus on testing: Critical business logic. Edge cases and boundary conditions. Public methods and APIs. Error handling and exception paths. Code that is prone to bugs or complex.

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.