Mocks, Spies, and Stubs: How to Use?

Mocks, spies, and stubs are essential tools in software testing, particularly in unit and API testing, for isolating components from external dependencies and ensuring accurate functionality testing. Mocks simulate complex real objects by mimicking their behavior in a controlled way, allowing developers to test specific responses and scenarios without relying on actual software components, thus offering benefits like isolation, control, efficiency, and simplicity. In contrast, spies observe the behavior of functions during tests without altering them, providing insights into function calls and ensuring non-intrusive verification of code execution. Meanwhile, stubs replace complex or unavailable components with simplified versions that return fixed responses, creating a controlled testing environment that is predictable and efficient, useful in simulating external services, managing non-deterministic outputs, and handling edge cases. The use of these test doubles enables developers to create more stable and reliable software by allowing clear, specific tests that ensure the application functions correctly under various conditions.