I think that a well covered codebase should include mostly unit tests, plenty of integration tests, and few E2E tests, i.e. model the test pyramid[1]. Mocking

I think that a well covered codebase should include mostly unit tests, plenty of integration tests, and few E2E tests, i.e. model the test pyramid[1]. Mocking in unit tests is generally acceptable to avoid also testing other systems. E.g. if the function does OS or network calls it's a good idea to mock that and keep the test isolated. But you should also have integration tests for the feature which do make external calls without mocking (or mocking at a higher level), as well as E2E tests that test the system as a whole. This way you can generally quickly pinpoint where something breaks, which may or may not break higher level tests. [1]: https://martinfowler.com/bliki/TestPyramid.html https://martinfowler.com/bliki/TestPyramid.html