What is the difference between unit testing, integration testing, and functional testing?

Short answer: Unit Testing: Tests individual units in isolation. Integration Testing: Tests interaction between multiple components or systems. Functional Testing: Tests end-to-end functionality from the user's perspective.

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.