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Test-first yields analysis, it helps us determine what is clear, what is unclear or missing, and ferrets out misunderstandings. Unit tests are unforgiving, they don’t let you get away with anything.
But Test-Driven Development also creates better design. Bad design is hard to test, and so moving tests into a primary position reveals the pain of a bad design very early, before much commitment has been made to it.
Write your tests first, but learn how to listen to what they tell you about your product design.