How do you ensure tests are fast and reliable in TDD?

Short answer: Mock external dependencies. Keep tests focused and independent. Avoid I/O operations in unit tests. Run tests frequently during development.

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.