Tests Doubles And Isolation

Goal

Use test doubles to control dependencies while keeping tests readable, trustworthy, and resistant to harmless refactors.

For concise mocking examples, see MOCKING.md.

Vocabulary

• Fake: generic replacement that looks like a real collaborator.
• Stub: fake used to feed controlled input or errors into the unit. A stub should not decide whether the test passes.
• Mock/spy: fake used to verify an interaction. It can fail the test.
• Isolation framework: library that creates and configures fakes dynamically.

If the project or framework uses different names, keep the conceptual distinction clear in variable names.

Decision Flow