Today, we’ll briefly discuss why it’s important to write automated tests for any software project, and shed light on some of the common types of automated testing. We’ll build a to-do list app by following the Test-Driven Development (TDD) approach. I’ll show you how to write both unit and functional tests, and in the process, explain what code mocks are by mocking a few libraries. I’ll be using a combination of RTL and Jest — both of which come pre-installed in any new project created with Create-React-App (CRA).

https://www.smashingmagazine.com/2020/07/react-apps-testing-library/