Summary
Test-Driven Development (TDD) is a development discipline in which a failing test is written first, the minimum code to pass it is then implemented, and the code is subsequently refactored—following the Red/Green/Refactor cycle.
What is TDD?
TDD inverts the traditional development workflow. Instead of writing code and then testing it, the developer first writes a failing test that defines the desired behaviour. Only then is the production code written—just enough to make the test pass. Finally, the code is refactored for clarity and quality while keeping tests green.
This cycle—Red (failing test), Green (passing test), Refactor (clean code)—is typically iterated in very short loops of a few minutes. The result is code that is highly testable by design, with near-complete test coverage emerging organically.
TDD is particularly effective for unit testing but the same discipline can be applied at higher levels (ATDD—Acceptance Test-Driven Development). Long-term benefits include reduced debugging time and lower defect rates.
Why is TDD relevant?
- Defect reduction: Studies consistently show TDD reduces defect rates by 40–90% at a modest increase in development time
- Design improvement: Writing tests first forces clear interfaces and single-responsibility code
- Living documentation: The test suite documents expected behaviour that stays in sync with the code