Comment by goldenkek

He means you test pure functions and mutations. You dont stub data sources that your implementations use to see if they were called. tests need to be about enforcement of correct interface exposure. not exact implementations. its one of the most facepalm types of useless testing that ive seen littered around Amazon's Seattle teams due to these Indian managers wanting to make sure the data sources called in every method actually got called and that there is 100% coverage. serves no purpose except to basically make two copies of the code..the test as a doppleganger of the actual code but using stubs.