Glossary

Mocking (Test Doubles)

A mock is a fake object that simulates the behaviour of a real dependency in tests. Instead of a real email service, database, or external API — a mock returns preset data and verifies that the code behaves correctly.

Types of test doubles

Stub — returns preset values. No call verification
Mock — additionally verifies that certain methods were (or were not) called
Spy — records calls but delegates to the real implementation
Fake — a simplified real implementation (in-memory repository)
Dummy — a placeholder object passed but never used

When not to mock

Mock external dependencies (email, S3, payment systems), not your own code. If you find yourself mocking your own classes, that is a signal the architecture needs refactoring.