https://medium.com/javascript-scene/mocking-is-a-code-smell-944a70c90a6a

One of the biggest complaints I hear about TDD and unit tests is that people struggle with all of the mocking required to isolate units. Some people struggle to understand how their unit tests are even meaningful. In fact, I’ve seen developers get so lost in mocks, fakes, and stubs that they wrote entire files of unit tests where no actual implementation code was exercised at all. Oops.

On the other end of the spectrum, it’s common to see developers get so sucked into the dogma of TDD that they think they absolutely must achieve 100% code coverage, by any means necessary, even if that means they have to make their codebase more complex to pull it off.