What is mocking vs spying in unit testing?

Short answer: Mocking involves creating objects that simulate behavior and expectations to verify interactions. Spying records information about how real or partial objects are used, focusing on what happened rather than controlling behavior.

Real-world example (ShopNest)

When testing ShopNest’s order service, mock IPaymentGateway so tests do not call Razorpay.

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.