test-driven-development

Test-first development requires writing a failing test that defines expected behavior before implementing production code. You confirm the test fails for the right reason, then write the minimal implementation to make it pass, ensuring safer iterative design changes and regression prevention.

The red green refactor cycle is a test-driven discipline where you write a failing test (red), implement the smallest code to pass it (green), and safely improve code structure (refactor). This workflow verifies real behavior and prevents regressions during iterative design changes.

To safely fix bugs using unit tests, you first write a test that reproduces the exact bug and confirm it fails. Then you implement the minimal code correction to make the test pass, protecting existing behavior and verifying the fix without regressions.

Behavior verification tests should use limited mocking to ensure real-behavior coverage. By minimizing mocks, you verify actual system interactions and outcomes rather than isolated implementation details, maintaining higher confidence in regression prevention.

You should apply test-driven development for refactoring when you need to improve code structure without losing confidence in existing behavior. By adding tests that lock in current outcomes before changing code, you protect against regressions and ensure safe iterative design modifications.