One of the most important considerations when writing any test, whether a granular unit test or a far-reaching E2E test, is the question of what to test. It's entirely possible to test the wrong thing; doing so can give us false confidence in our code. We may write a huge test suite and walk away grinning, thinking that our code now fulfills all expectations and is utterly fault-tolerant. But our test suite may not test the things we think it does. Perhaps it only tests a few narrow use cases, leaving us exposed to many possibilities of breakage. Or perhaps it conducts tests in a way that is never emulated in reality, leading to a situation where our tests don't protect us from failures in production. To protect us against these possibilities, we must understand what we truly wish to test.
Consider a function that we've written to extract...
