Mocks, Spies & Integration — Series 2

Preview — 3 of 10 questions

What does mockReturnValue({ success: true }) configure, and what does this test verify?

javascript

function processOrder(paymentGateway, amount) {
  const result = paymentGateway.charge(amount);
  return result.success ? 'Order confirmed' : 'Payment failed';
}

test('confirms order on successful payment', () => {
  const paymentGateway = { charge: jest.fn() };
  paymentGateway.charge.mockReturnValue({ success: true });

  expect(processOrder(paymentGateway, 100)).toBe('Order confirmed');
  expect(paymentGateway.charge).toHaveBeenCalledWith(100);
});

AmockReturnValue only works for the first call to the mock; every subsequent call returns undefined.

BmockReturnValue replaces the mock function entirely with a real network request to a payment gateway.

CmockReturnValue configures what the mock function returns whenever it's called; the test passes, confirming processOrder correctly interprets a successful charge and that charge was called with 100.

DThe test fails, since jest.fn() cannot be combined with .mockReturnValue().

What does mockResolvedValue({...}) do differently from mockReturnValue({...})?

javascript

async function getUserName(api, id) {
  const user = await api.fetchUser(id);
  return user.name;
}

test('returns the user name from the API', async () => {
  const api = { fetchUser: jest.fn() };
  api.fetchUser.mockResolvedValue({ id: 1, name: 'Alice' });

  const name = await getUserName(api, 1);
  expect(name).toBe('Alice');
});

AmockResolvedValue makes the mock function synchronous, removing the need for await in the test.

BmockResolvedValue makes the mock return a Promise that resolves to the given value — the correct choice for mocking an async function, since calling code will await the result expecting a Promise-like value.

CmockResolvedValue and mockReturnValue are exactly identical; there is no difference.

DmockResolvedValue can only be used with jest.spyOn, never with jest.fn().

What is logged?

javascript

function logEvent(logger, eventName, payload) {
  logger.record(eventName, payload);
}

test('records call arguments correctly', () => {
  const logger = { record: jest.fn() };

  logEvent(logger, 'signup', { userId: 42 });
  logEvent(logger, 'login', { userId: 42 });

  console.log(logger.record.mock.calls.length);
  console.log(logger.record.mock.calls[0]);
  console.log(logger.record.mock.calls[1][0]);
});

A2, 'signup', { userId: 42 }

B1, ['signup', { userId: 42 }], 'login'

C2, ['signup', { userId: 42 }], { userId: 42 }

D2, ['signup', { userId: 42 }], 'login'

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