React Hooks - Timer example using setInterval

useTimer.js

import React from 'react';

function Timer() {
  const [time, setTime] = React.useState(0);
  const [timerRunning, setTimerRunning] = React.useState(0);
  const intervalRef = React.useRef();

  React.useEffect(() => {
    if (timerRunning) {
      intervalRef.current = setInterval(() => {
        setTime(prevTime => prevTime + 100);
      }, 100);
    }
    return () => clearInterval(intervalRef.current);
  }, [timerRunning]);

  function play() {
    setTimerRunning(true);
  }

  function pause() {
    setTimerRunning(false);
  }

  function clear() {
    setTime(0);
  }

  return {time, play, pause, clear};
}

export default Timer;