Jest 26 fakeTimers and promises

test.ts

// figured out thanks to:
// https://stackoverflow.com/questions/52177631/jest-timer-and-promise-dont-work-well-settimeout-and-async-function/52196951
// https://github.com/facebook/jest/issues/7151#issuecomment-622134853
// https://stackoverflow.com/questions/51126786/jest-fake-timers-with-promises/51132058#51132058
// https://github.com/facebook/jest/issues/2157

// additional info
// https://github.com/facebook/jest/issues/10221#issuecomment-654687396
// https://stackoverflow.com/questions/66391541/jest-advancetimersbytime-doesnt-work-when-i-try-to-test-my-retry-util-function
// https://gist.github.com/dwiyatci/740a52a08eb6147baa1f0be9b4f38785

const flushPromises = () => new Promise(jest.requireActual("timers").setImmediate);

it('should throw an error in case the message sending fails', async () => {
  // given
  const expectedError = new Error('Send raw email failed')
  const emailService = ({
    // using `async-retry` package to retry 3 times
    sendRawEmail: jest.fn().mockRejectedValue(expectedError),
  } as unknown) as EmailService;
  const smsService = { sendSms: jest.fn() };

  const notificationService = notificationServiceFactory({
    emailService,
    smsService,
  });

  jest.useFakeTimers();

  // when
  const sendingNotifications = notificationService.sendNotification({
    recipient,
    subject,
    message,
    channels,
  });

  await flushPromises().then(() => jest.runAllTimers()); // Progress 1st retry
  await flushPromises().then(() => jest.runAllTimers()); // Progress 2nd retry
  await flushPromises().then(() => jest.runAllTimers()); // Progress 2nd retry

  // then
  await expect(sendingNotifications).rejects.toEqual(expectedError);
});