Why Is Test-Driven Development Useful?

Test-driven development (TDD) starts with a special integration or end-to-end test that defines system expectations before any code is written. This test ensures the basic flow works. After it passes, developers use the Red/Green/Refactor cycle for each unit: write a failing test, make it pass with the simplest code, then refactor. Never refactor or add tests while a test is failing. Writing tests after code can cause missed functionality, false positives, and over-engineering. TDD is not useful for IO layers, test support, or declarative languages like HTML/CSS. The Transformation Priority Premise guides the green phase by prioritizing simple transformations. TDD takes more time initially but leads to more reliable software.