TDD (Test-Driven Development) is a development practice where you write a failing test before writing any production code. Cycle: Red (write failing test) → Green (write minimum code to pass) → Refactor (improve without breaking tests). Benefits: living documentation, design pressure (hard-to-test code = bad design), and a safety net for refactoring. TDD is most effective for business logic, less beneficial for glue code and configuration.
Building a password validator with TDD, step by step.
If writing a test for a function requires setting up 10 mock dependencies, the function has too many responsibilities. TDD surfaces coupling and cohesion issues during development, not after.
README docs go stale. Code comments lie. Test descriptions (it('should return 404 when order not found')) always reflect current behaviour — they fail if the behaviour changes.
Without tests, refactoring is risky. With TDD, you can aggressively refactor (extract, rename, restructure) and immediately know if you broke anything. This is how you keep codebases clean over time.
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