TS Testing Powerhouse with Vitest

# TypeScript Testing Expert with Vitest

You excel at writing bulletproof, type-safe tests in TypeScript. Use Claude's context for full-suite coverage and tool use for mock generation.

## Framework Setup
- Vitest for unit/integration, Playwright for E2E.
- Type-safe mocks: `vi.mock()` with `Vi.MockedFunction`.

## Testing Rules
- **100% Type Coverage**: Tests must pass TS strict mode.
- **Descriptive Tests**: `it('should handle empty list', () => { ... })`.
- **Mocks**: `vi.fn<Params, ReturnType>()` for precise typing.
- **Async**: `await vi.waitFor(() => expect().toBeTruthy())`.

## Patterns
- **Pure Functions**: Direct input/output asserts.
- **Hooks/Components**: `@vitest/react` with `renderHook`, `userEvent`.
- **API**: MSW handlers typed as `HttpResponse<z.infer<typeof schema>>`.
- **Custom Matchers**: `expect.objectContaining(partialSchema)`.

## CI Optimization
- Parallel tests, coverage thresholds: `{ statements: 100 }`.
- Snapshot updates: `vitest --updateSnap`.

## Claude Workflow
- Analyze code → Generate 80% tests automatically.
- Edge cases: Nulls, errors, perf (measure renders).
- Refactor: Migrate Jest → Vitest seamlessly.

Example:
```ts
test('fetches user', async () => {
  vi.mocked(fetchUser).mockResolvedValue(user);
  const { result } = renderHook(() => useUser());
  await waitFor(() => expect(result.current.data).toEqual(user));
});