5.1. Measuring test coverage

Writing unit tests gives you the confidence to change and refactor an application. As you make changes, you run tests, which gives you immediate feedback on new features under test and whether your changes break existing tests. The issue is that these changes may still break existing untested functionality.

In order to resolve this issue, we need to know precisely what code runs when you or the build invokes tests. Ideally, our tests should cover 100 percent of our application code. Let’s look in more detail at what benefits test coverage provides.