.tip

The rule "one assert per test" produces twenty tests that share setup and fail together. The useful rule is one behaviour: arrange, act once, then assert everything that behaviour implies.

it("cancelling a shipped order refunds nothing and keeps the shipment", () => {
  const result = cancel(shippedOrder);
  expect(result.refund).toBe(0);
  expect(result.shipment.status).toBe("in_transit");
});

When it fails, the name says which behaviour broke and the assertions say how.