Building a robust suite of unit tests that catches real bugs and doesn't flag false positives as we refactor our code is one of the hardest tasks we do as software developers. Jest is a great testing tool that helps us meet this challenge, as we'll find out in this chapter.
Perhaps the easiest bits of an app to unit test are pure functions, because there are no side effects to deal with. We'll revisit the validator functions we built in Chapter 7, Working with Form, and implement some unit tests against them in order to learn how to unit test pure functions.
Unit testing components is the most common type of unit test we'll be carrying out while building our apps. We'll learn about it in detail, and leverage a library to help us implement tests that don't unnecessarily break when we refactor our code.
We'll learn what...