Comment by tail_exchange

  The problem is when IOService has edge cases. When building the mock, does it address the edge cases? When you want to find bugs by testing, the tests need to test the real world. So to work, the mock for IOService needs to model the edge cases of IOService. Does it? Does the developer know they need to model the edge cases, or the mock is not helping with finding bugs? Do you even know the edge cases of IOService? When IOService is a database service, does your mock work for records that do not exist? Or return more than one record?

It depends. Mocks are used to remove variables from the experiment you are running (the tests) and see if it behaves under very specific conditions. If you want to test how the code behaves when a specific row is returned by the database, but instead the dependency returns something else, then you are not testing that use case anymore. Reproducibility also has its values. But yes, you can definitely make your mocks return errors and fail in a myriad of ways.

Not to say you should mock everything. Of course having proper integration tests is also important, but articles like these will rarely tell you to have a good balance between them, and will instead tell you that something is correct and something else is wrong. You should do what makes sense for that specific case and exercise your abilities to make the right choice, and not blindly follow instructions you read in a blog post.