Why are functional tests not enough? What do unit tests offer?

Question

I just had a conversation with my lead developer who disagreed that unit tests are all that necessary or important. In his view, functional tests with a high enough code cov

Answer 1

In TDD/BDD, unit tests are necessary to write the program. The process goes

failing test -> code -> passing test -> refactor -> repeat

The article linked also mentions the benefits of TDD/BDD. In summary:

• Comes very close to eliminating the use of a debugger (I only use it in tests now and very rarely for those)
• Code can't stay messy for longer than a few minutes
• Documentation examples for an API built-in
• Forces loose coupling

The link also has a (silly) walk-through example of TDD/BDD, but it's in PowerPoint (ew), so here's an html version.