GOOS-Style TDD for TanStack Start

Philosophy

Core principle: Tests verify behavior through rendered UI, not implementation details. Components can change entirely; tests shouldn't.

Good tests exercise real components in real browsers (Vitest Browser Mode) or full application flows (Playwright). They describe what the user experiences, not how the code works. A good test reads like a specification — "user can login with valid credentials" tells you exactly what capability exists. These tests survive refactors because they don't care about internal structure.

Bad tests mock hooks, spy on query cache internals, or assert on DOM class names. The warning sign: your test breaks when you refactor, but behavior hasn't changed.

See tests.md for examples and mocking.md for mocking guidelines.

Anti-Pattern: Horizontal Slices

DO NOT write all tests first, then all implementation. This is "horizontal slicing."

WRONG (horizontal):
  RED:   test1, test2, test3, test4, test5