void-testing

Assert observable behavior at the boundary instead of implementation details like private methods or internal call sequences. For UI, query by accessible role or label rather than class or data-testid, so refactors that preserve behavior keep tests green.

Mock only at infrastructure boundaries: HTTP via MSW, databases via pglite, filesystem via memfs, time via fake timers, and randomness via seeded RNG. Mocking pure functions or business services is a coupling smell; run the real code instead.

Use factory functions with overrides for test data and reserve beforeEach for environment setup like mounting the DOM or resetting the database. Shared mutable state in beforeEach leaks between tests and hides each test's actual preconditions.

Snapshot testing is banned by default because snapshots devolve into update-on-green, passing regardless of correctness. It is allowed only for visual regression in __visual__ directories or compiler output in __generated__ with documented review.

A common heuristic is roughly 70% unit, 20% integration, and 10% E2E tests, but it is a guide rather than a quota. An inverted pyramid where E2E tests carry the proof usually signals design coupling that makes unit tests expensive.

Pristine output is a passing condition: no console.log from production code, no React act warnings, no unhandled rejections, and no leaked timers or handles. Configure Vitest with onConsoleLog set to fail so noisy output fails the test.