CODE EXAMPLE FOR JAVASCRIPT

react clearinterval outside of useefect

/*
If you need to save variables across re-renders use useRef which in this case acts
like a class instance field, also note that mutations to refs does not trigger a
re-render.

This will give you the ability to clear the interval from outside of useEffect
*/

function Child(props) {
  let [timerCount, setTimer] = useState(0)
  const intervalRef = useRef(null)

  useEffect(() => {
    intervalRef.current = setInterval(() => {
      setTimer(prevState => prevState + 1)
    }, 1000)

    return () => clearInterval(intervalRef.current)
  }, [])

  function clearTimer() {
    clearInterval(intervalRef.current)
    intervalRef.current = null
  }

  return (
    <div>
      <div>Timer {timerCount}</div>
      <button onClick={clearTimer}>ClearTimer</button>
    </div>
  )
}