Unit Testing Basics — Series 2

Preview — 3 of 10 questions

What does expect.objectContaining add here?

javascript

await service.create({ name: 'Bombay' });

expect(repo.save).toHaveBeenCalledTimes(1);
expect(repo.save).toHaveBeenCalledWith(
  expect.objectContaining({ name: 'Bombay' }),
);

AIt converts the assertion into a snapshot comparison

BIt asserts that the saved object has exactly the listed properties and no others

CIt matches an argument that includes the listed properties, ignoring any extra ones — so the test does not break when an unrelated field such as createdAt or an id is added later

DIt deep-clones the argument before comparing, avoiding mutation issues

What does this do, and how does it differ from resetAllMocks()?

javascript

beforeEach(() => {
  jest.clearAllMocks();
});

AclearAllMocks wipes recorded calls and instances but keeps any implementation the mock was given; resetAllMocks also removes the implementation, so a mockResolvedValue set in the suite's setup would be lost

BThey are identical; clear is the older alias

CclearAllMocks removes the mocks entirely, so subsequent calls hit the real implementation

DclearAllMocks applies only to spies, and resetAllMocks only to jest.fn() mocks

What convention is this test following?

javascript

it('marks the order as paid once payment succeeds', async () => {
  const order = buildOrder({ status: 'pending' });
  repo.findOneBy.mockResolvedValue(order);
  payments.charge.mockResolvedValue({ ok: true });

  await service.pay(order.id);

  expect(repo.save).toHaveBeenCalledWith(expect.objectContaining({ status: 'paid' }));
});

ARed-green-refactor, applied within a single test body

BGiven-when-then, which requires a BDD framework such as Cucumber

CPage-object, adapted from browser testing

DArrange-act-assert: set up the state and doubles, perform exactly one action, then assert the outcome — which keeps each test about one behaviour and makes a failure immediately legible

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