What is dependency injection and how does it facilitate testing?

Short answer: Dependency Injection (DI) is a design pattern where dependencies are provided rather than created inside a class. DI makes testing easier by allowing tests to inject mock or fake implementations of dependencies, isolating the unit under test.

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.