Simple demo of using mocha to unit test `npm install && npm test` I have provided 2 implementations of UserService. You will find that usage of UserService is the same regardless of which implementation is chosen. Therefore, testing classes that depend on the UserService is also agnostic to the chosen implementation. However, testing the UserService implementations will likely be implementation specific. One of the UserService implementations has a bug. You should not be able to discover the bug by unit testing upstream classes. If you do, that means you have not mocked out the UserService, and are doing integration tests instead of unit tests. You should only be able to discover the bug by unit testing the implementation class itself.