What are test doubles and types of test doubles?

Short answer: Test doubles are objects used in place of real components for testing. Types include: Dummy: Passed but never used. Stub: Provides canned responses. Mock: Verifies interactions. Fake: Working but simplified implementation. Spy: Records information about calls.

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.