When is it appropriate to use mocks in unit testing?

Fakes in unit tests are used when real dependencies (such as external services, databases) need to be replaced with simplified objects that mimic the behavior of these dependencies.

It is advisable to use fakes if:

• Real dependencies are difficult or expensive to connect in tests.
• There is a need to control the behavior of dependencies (for example, returning specific data).
• It is necessary to isolate the tested module from external factors.

Example: instead of connecting to a real database, a fake repository that stores data in memory can be used.

This speeds up tests and makes them more reliable.