test-driven-development

Test-driven development enforces writing a minimal failing test before production code. The red-green-refactor workflow involves writing a failing test, implementing the smallest code change to pass it, then refactoring while keeping tests green.

To write unit tests for bug fixes and features using TDD, write one minimal failing test demonstrating desired behavior, run the suite to confirm the expected failure, implement minimal code to pass, and maintain green tests during refactoring.

Unit tests fail to validate behavior when they are mock-heavy or use test-only production APIs. TDD mandates watching tests fail for the expected reason to ensure they validate deterministic behavior rather than implementation details.

Common test-driven development anti-patterns to avoid during refactoring include mock-heavy tests, test-only production APIs, and incomplete mocks. Avoiding these ensures repeatable validation and maintains green tests through behavior changes.

You should not use test-driven development for behavior changes requiring non-deterministic validation. TDD applies strictly to features, bug fixes, and refactors where deterministic, repeatable unit and integration tests validate APIs and edge cases.

Test-driven development works without external dependencies by enforcing behavior-driven development through a verification checklist. It requires only writing failing tests, verifying failures, and implementing minimal code to pass.