TDD

Test-Driven Development isn't about testing — it's about design. Writing the test first forces you to think about the interface before the implementation. The result is code that's testable by definition, with clear boundaries and minimal coupling.

The red-green-refactor cycle keeps you honest. Write a failing test, make it pass with the simplest code possible, then refactor. No speculative abstractions, no premature optimisation, no "I'll add tests later." The discipline pays compound interest over the lifetime of a codebase.