> Firstly if the dependency isn't doing any io, you can test your code as a whole along with its dependency. No need to mock.

This the thing. People have a tendency to overuse mocks. The point of automated testing (whether it's unit tests or something else), is to enable refactoring. That's really the only reason. Code that you don't touch doesn't suddenly change its behaviour one day.

In a previous jobs the developers started to go crazy with mocking and it reached a kind of singularity where essentially if a function called anything, that thing was mocked. It definitely tests each function in complete isolation, but what's the point? It makes refactoring impossible, which is the entire point of the tests in the first place!

This excellent talk completely changed the way I approached testing. Every developer who writes tests needs to watch this now! https://www.youtube.com/watch?v=EZ05e7EMOLM