test-driven-development

Unit testing before implementation requires writing a failing test that describes expected behavior, then writing minimal code to pass it. This systematic approach prevents bugs and regressions by ensuring production code is always preceded by a verifiable test.

The Red-Green-Refactor cycle is a test-driven development workflow where you write a failing test (Red), implement minimal code to pass the test (Green), then clean up and refactor the code while maintaining test coverage.

Writing tests first prevents regressions by mandating a systematic approach to verifiable, behavior-driven tests. This process clarifies requirements upfront, ensuring every piece of production code is validated against expected behavior before introduction.

To fix a bug using test-first development, write a test that describes the expected behavior and fails due to the bug. Then, implement the minimal code changes required to make that test pass, ensuring the regression is permanently resolved.

You should refactor code during the final phase of the Red-Green-Refactor cycle. After writing minimal code to pass the failing test, clean up and improve the code structure while ensuring the tests remain green.

Yes, you can use this approach for both new features and bug fixes. Whether starting a new feature or addressing a bug, the methodology guides you to write a test describing expected behavior, then implement the code to make it pass.