What are the best practices for a healthy Jest suite?

A test suite is production infrastructure. Its value is not the number of assertions but the confidence it gives when changing code. That confidence depends on tests that fail for real regressions and stay quiet for refactors. Speed and determinism are part of the design, not a later optimisation.

Principles that hold up

• Assert on observable behaviour, not internal state or hook call counts
• Mock at system boundaries: network, clock, filesystem and third-party SDKs
• Create per-test state and clean up mocks, timers and globals
• Use the lowest level that still covers the risk: unit, integration or end-to-end
• Keep snapshots small and reviewed, or use explicit assertions instead
• Make failures informative by using specific matchers and clear test names

test("returns a validation error instead of writing", async () => {
  const save = jest.fn();
  const result = await submitOrder({ save, items: [] });

  expect(result).toEqual({ error: "cart is empty" });
  expect(save).not.toHaveBeenCalled();
});

Not every behaviour deserves a test at every level. Push detail into unit tests and let a smaller number of integration tests cover wiring. This keeps the suite fast and reduces duplication. A test that duplicates a lower-level assertion adds maintenance cost without new signal.