test-driven-development

Write a minimal failing test first, verify it fails for the right reason, then implement only the code needed to pass. Use Jest for isolated logic and Supertest for HTTP contract and NestJS module integration tests.

Red-green-refactor means writing a failing test (red), implementing the minimum code to pass it (green), then improving the design while keeping tests passing (refactor). The initial failure must be verified before implementation begins.

Use Playwright for E2E flows when behavior crosses multiple layers, such as full user journeys through frontend and backend. Use Jest unit or component tests for isolated logic where they provide real clarity.

Asserting on mocked elements verifies the mock exists, not that the component works, so the test proves nothing about real behavior. Test the real component or unmock it, and only mock to isolate slow or external dependencies.

No, test-only methods pollute production code and can be dangerous if called accidentally. Move test-only cleanup or setup logic into dedicated test utility modules instead of the production class.

Avoid dogmatic TDD when the test layer is wrong for the behavior, when snapshots replace real assertions, or when design is inflated before tests demand it. Escalate if project constraints prevent validating critical behavior with reasonable tests.