Implementing a countdown timer in React with Hooks

Question

Im trying to render a count down timer on screen with react hooks, but I'm not sure the best way to render it.

I know i'm supposed to use the useEffect to compare current state to previous state, but I don't think I'm doing it correctly.

I would appreciate the help!

I've tried a couple of different ways, none of them work, like setting state whenever whenever it updates, but it just ends up flickering like crazy.

const Timer = ({ seconds }) => {
    const [timeLeft, setTimeLeft] = useState('');

    const now = Date.now();
    const then = now + seconds * 1000;

    const countDown = setInterval(() => {
        const secondsLeft = Math.round((then - Date.now()) / 1000);
        if(secondsLeft <= 0) {
            clearInterval(countDown);
            console.log('done!');
            return;
        }
        displayTimeLeft(secondsLeft);
    }, 1000);

    const displayTimeLeft = seconds => {
        let minutesLeft = Math.floor(seconds/60) ;
        let secondsLeft = seconds % 60;
        minutesLeft = minutesLeft.toString().length === 1 ? "0" + minutesLeft : minutesLeft;
        secondsLeft = secondsLeft.toString().length === 1 ? "0" + secondsLeft : secondsLeft;
        return `${minutesLeft}:${secondsLeft}`;
    }

    useEffect(() => {
        setInterval(() => {
            setTimeLeft(displayTimeLeft(seconds));
        }, 1000);
    }, [seconds])


    return (
        <div><h1>{timeLeft}</h1></div>
    )
}

export default Timer;```

Answer 1

const Timer = ({ seconds }) => {
  // initialize timeLeft with the seconds prop
  const [timeLeft, setTimeLeft] = useState(seconds);

  useEffect(() => {
    // exit early when we reach 0
    if (!timeLeft) return;

    // save intervalId to clear the interval when the
    // component re-renders
    const intervalId = setInterval(() => {
      setTimeLeft(timeLeft - 1);
    }, 1000);

    // clear interval on re-render to avoid memory leaks
    return () => clearInterval(intervalId);
    // add timeLeft as a dependency to re-rerun the effect
    // when we update it
  }, [timeLeft]);

  return (
    <div>
      <h1>{timeLeft}</h1>
    </div>
  );
};

Answer 2

You should use setInterval. I just wanted to add a slight improvement over @Asaf solution. You do not have to reset the interval every time you change the value. It's gonna remove the interval and add a new one every time (Might as well use a setTimeout in that case). So you can remove the dependencies of your useEffect (i.e. []):

function Countdown({ seconds }) {
  const [timeLeft, setTimeLeft] = useState(seconds);

  useEffect(() => {
    const intervalId = setInterval(() => {
      setTimeLeft((t) => t - 1);
    }, 1000);
    return () => clearInterval(intervalId);
  }, []);

  return <div>{timeLeft}s</div>;
}

Working example:

Note in the setter, we need to use this syntax (t) => t - 1 so that we get the latest value each time (see: https://reactjs.org/docs/hooks-reference.html#functional-updates).

---

Edit (22/10/2021)

If you want to use a setInterval and stop the counter at 0, here is what you can do:

function Countdown({ seconds }) {
  const [timeLeft, setTimeLeft] = useState(seconds);
  const intervalRef = useRef(); // Add a ref to store the interval id

  useEffect(() => {
    intervalRef.current = setInterval(() => {
      setTimeLeft((t) => t - 1);
    }, 1000);
    return () => clearInterval(intervalRef.current);
  }, []);

  // Add a listener to `timeLeft`
  useEffect(() => {
    if (timeLeft <= 0) {
      clearInterval(intervalRef.current);
    }
  }, [timeLeft]);

  return <div>{timeLeft}s</div>;
}