What are the challenges of writing unit tests?

Short answer: Challenges include: Managing external dependencies and state. Writing tests for legacy or tightly coupled code. Maintaining tests as code evolves. Ensuring tests are meaningful and not brittle. Balancing test coverage and development speed.

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.