Mock

A test double that stands in for a real dependency and lets you verify how it was called — e.g. that a specific method ran with the expected arguments.

Mental model: Like a training dummy standing in for a real person during CPR practice.

Example: A test that swaps a real EmailService for a mock and asserts sendEmail() was called with the right recipient, without actually sending anything.

In practice: Overusing mocks can make a test pass even when the real integration is broken, since you're only verifying the fake's behavior — integration tests exist to catch what mocks miss.

Debated: Heavily contested — the 'London school' of TDD builds tests almost entirely around mocks, while the 'classicist' school treats heavy mocking as a sign your design has too many seams.

• often confused with Stub