What is the difference between a mock, stub, fake, and spy?

These are forms of test doubles used to replace real dependencies during testing.

A stub returns predefined data needed by a test. A mock is commonly configured with expectations and can verify that specific interactions occurred. A fake provides a working but simplified implementation, such as an in-memory repository. A spy records calls so tests can inspect how a dependency was used.

Terminology varies between testing communities and libraries. The important interview point is to choose the simplest test double that supports the behavior being tested and avoid over-coupling tests to implementation details.