Introduction
Testing allows you to ensure your application works the way you think it does, especially as your codebase changes over time. If you have good tests, you can refactor and rewrite code with confidence. Tests are also the most concrete form of documentation of expected behavior, since other developers can figure out how to use your code by reading the tests.
Automated testing is critical because it allows you to run a far greater set of tests much more often than you could manually,