useEffect_EffectCleanup.js

const EffectCleanup = () => {
  useEffect(() => {
    const clicked = () => console.log('window clicked')
    window.addEventListener('click', clicked)

    // return a clean-up function
    return () => {
      window.removeEventListener('click', clicked)
    }
  }, [])

  return <div>
    When you click the window you'll 
    find a message logged to the console
  </div>
}