Jon introduced Test Driven Development to the team and led by example. I've never seen anyone refactor the large code-base that we had and contribute to new features at the same time.
A robust suite of unit tests acts as a safety harness, giving you courage to make bold changes. But there’s an art to making tests that give you useful information, while keeping the tests themselves readable & maintainable.
Unit tests written after production code are hard to write, and may only confirm your biases. Test Driven Development shapes production code in response to tests, in an evolutionary manner. The secret sauce? Continuous Refactoring.
We can’t just bang code together and expect it to be testable. Testability must be designed in. That design brings other benefits, making it easier to modify the code. The ultimate goal? Drive down your cost of change.