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For tests, prefer deterministic, intention-revealing assertions and keep runtime tests and type-only tests strictly separated.

Apply: 1) Use the right matcher for the claim

• Identity: use toBe instead of deep equality when referential identity matters.
• Async rejections: prefer matcher-based assertions over try/catch.

2) Make timer-based tests fast and stable

• If you must use timers, prefer fake timers and advance/run them deterministically.
• Keep real timer durations extremely short.

3) Don’t mix runtime and type-only tests

• Put compile-time checks into *.test-d.tsx and use expectTypeOf(...).toEqualTypeOf(...).
• In *.test-d.tsx, don’t rely on runtime behavior inside it blocks.

4) Ensure the test can prove which code path executed

• When a hook can call different queryFns (e.g., prefetch vs render), use distinct spies and assert call counts. Clear mocks after setup calls.

Example (async rejection + timers + type-only):

// runtime test
await expect(promise).rejects.toBe(testError)

// timer test (prefer fake timers)
vi.useFakeTimers()
vi.runAllTimersAsync()
expect(spy).toHaveBeenCalledTimes(0)

// type-only test in *.test-d.tsx
expectTypeOf(result4[0]).toEqualTypeOf<UseQueryResult<string, Error>>()

This reduces brittle/flaky tests, improves signal-to-noise, and guarantees type coverage lives where it’s actually executed by the tooling.

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