midUnit Testing
What is a spy in testing?
Updated May 6, 2026
Short answer
A spy monitors function calls without modifying behavior.
Deep explanation
Spies track how functions are used (calls, arguments) while still executing original logic.
Real-world example
Verifying logging behavior in services.
Common mistakes
- Confusing spies with mocks.
Follow-up questions
- Spy vs mock difference?
- When to use spies?