Is this an implementation detail? Or is it a feature that should be tested with Jest?

I have a greeting component that updates the time of day. Depending on the time of day the greeting will print

Good Morning, Jane
Good Afternoon, Jane
Good Evening, Jane

I have function called getTimeOfDay and test that this function is working. If you're interested view my tests here.

In my component I have a timer that checks every minute to see if the time of day message should update.

const [date, setDate] = useState(new Date())
const timeOfDay = getTimeOfDay(date)

useEffect(() => {
  const timer = setInterval(() => {
    setDate(new Date())
  }, 60000)

  return () => {
    clearInterval(timer)
  }
}, [])

I have been going back and forth on whether to test that this message changes correctly as the time passes. I know that testing implementation details is bad practice and was not sure if this was an implementation detail or a feature that should be tested.

If this is something I should test, I can't seem to easily implement a timer in jest that checks the message and then speeds up the timer 8 hours. What are your thoughts?

Solution

My final tests are as followed:

describe('Greeting', () => {
  const name = 'Jack'
  
  it('renders component as expected', () => {
    const wrapper = mount(<Greeting name={name} /> )
    expect(wrapper.text().includes(name)).toBe(true)
  })

  it('Should update message after time', () => {
    jest.useFakeTimers()
    setMockDate(new Date('Feb 22, 2021 11:59:00'))
    const wrapper = mount(<Greeting name={name} />)

    const greetingText = wrapper.text()
    setMockDate(new Date('Feb 22, 2021 12:00:00'))
    jest.advanceTimersByTime(60000)
    expect(wrapper.text()).not.toBe(greetingText)
  })

  it('Should clear interval on unmount', () => {
    const spyOnSetInterval = jest.spyOn(window, 'setInterval')
    const spyOnClearInterval = jest.spyOn(window, 'clearInterval')
    spyOnSetInterval.mockReturnValueOnce((33 as unknown) as NodeJS.Timeout)
    const wrapper = mount(<Greeting name={name} />)

    wrapper.unmount()
    expect(spyOnClearInterval).toHaveBeenCalledWith(33)
  })
})

I created a helper function to mock the date. The final version is:

/**
 * @param {Date} expected
 * @returns {Function} Call to remove Date mocking
 */
const setMockDate = (expected: Date): AnyObject => {
  const RealDate = Date

  function MockDate(mockOverride?: Date | number) {
    return new RealDate(mockOverride || expected)
  }

  MockDate.now = () => expected.getTime()
  MockDate.prototype = RealDate.prototype

  // eslint-disable-next-line @typescript-eslint/no-explicit-any
  global.Date = MockDate as any

  return () => {
    global.Date = RealDate
  }
}

export default setMockDate