Mock vs stub vs spy vs fixture?

Definition:

Test doubles replace real collaborators so a test can control conditions or observe interactions. A stub supplies a response, a spy records calls, and a mock often combines replacement with expected interactions; terminology varies by library. Fixtures supply stable input data so cases are repeatable.

Key points:

• Stub: controlled behavior; spy: records calls; mock: replaces a dependency and may verify interactions.

• Fixture: repeatable test data.

• These terms overlap between tools; test observable behavior instead of every internal call.

• Reset changed mocks between tests and avoid mocking the exact logic the test is meant to verify.

Interview point:

• Prefer checking the resulting behavior; asserting every internal call makes harmless refactoring break tests.