[AnimalMuppet]

> Two important questions to ask are: What aids refactoring? What stops refactoring? The answers to these questions are also different for large codebases as opposed to small ones.

Part of the point of thorough unit tests that really test all the external behaviors of all your classes (or functions, if you're doing FP) is that it can give you the courage to refactor. If all the tests run for this class, then I didn't change any external behavior of the class. If the whole project's unit tests all pass, then I can be highly confident that my change did not introduce any bugs.

Note well the condition, though: if the tests really test all the external behaviors of all the classes. For that to be true, you almost have start with TDD from the beginning of the project.