Countdown timer with react hooks

Question

I'm trying to implement countdown timer on my own just to know hooks more. I know there are libraries out there but don't want to use it. the problem with my code is, I cannot get updated state inside "timer" function which is updated in start timer function I'm trying to implement timer that will have triggers to start, stop, & resume & can be manually trigger. by other component that is using the countdown component

import React, { useState } from 'react';

const Countdown = ({ countDownTimerOpt }) => {
  const [getObj, setObj] = useState({
    formatTimer: null,
    countDownTimer: 0,
    intervalObj: null,
  });

  const { formatTimer, countDownTimer, intervalObj } = getObj;

  if (countDownTimerOpt > 0 && intervalObj === null) {
    startTimer();
  }

  function startTimer() {
    const x = setInterval(() => {
      timer();
    }, 1000);

    setObj((prev) => ({
      ...prev,
      countDownTimer: countDownTimerOpt,
      intervalObj: x,
    }));
  }

  function timer() {
    var days = Math.floor(countDownTimer / 24 / 60 / 60);
    var hoursLeft = Math.floor(countDownTimer - days * 86400);
    var hours = Math.floor(hoursLeft / 3600);
    var minutesLeft = Math.floor(hoursLeft - hours * 3600);
    var minutes = Math.floor(minutesLeft / 60);
    var remainingSeconds = countDownTimer % 60;

    const formatTimer1 =
      pad(days) +
      ':' +
      pad(hours) +
      ':' +
      pad(minutes) +
      ':' +
      pad(remainingSeconds);

    if (countDownTimer === 0) {
      clearInterval(intervalObj);
    } else {
      setObj((prev) => ({
        ...prev,
        formatTimer: formatTimer1,
        countDownTimer: prev['countDownTimer'] - 1,
      }));
    }
  }
  function pad(n) {
    return n < 10 ? '0' + n : n;
  }

  return <div>{formatTimer ? formatTimer : Math.random()}</div>;
};
export default Countdown;

import React, { useState, useEffect } from 'react';
import Timer from '../../components/countdown-timer/countdown.component';

const Training = () => {
  const [getValue, setValue] = useState(0);

  useEffect(() => {
    const x = setTimeout(() => {
      console.log('setTimeout');
      setValue(10000);
    }, 5000);

    return () => clearInterval(x);
  }, []);

  return <Timer countDownTimerOpt={getValue} />;

don't want to use any set interval inside training page as the countdown component will also be used in exam page

Answer 1

Usually with hooks I would combine your functionality into a custom hook and use it in different places.

const useTimer = (startTime) => {
    const [time, setTime] = useState(startTime)
    const [intervalID, setIntervalID] = useState(null)
    const hasTimerEnded = time <= 0
    const isTimerRunning = intervalID != null

    const update = () => {
        setTime(time => time - 1)
    }
    const startTimer = () => {
        if (!hasTimerEnded && !isTimerRunning) {
            setIntervalID(setInterval(update, 1000))
        }
    }
    const stopTimer = () => {
        clearInterval(intervalID)
        setIntervalID(null)
    }
    // clear interval when the timer ends
    useEffect(() => {
        if (hasTimerEnded) {
            clearInterval(intervalID)
            setIntervalID(null)
        }
    }, [hasTimerEnded])
    // clear interval when component unmounts
    useEffect(() => () => {
        clearInterval(intervalID)
    }, [])
    return {
        time,
        startTimer,
        stopTimer,
    }
}

You can of course add a reset function or do other changes but use could look like this:

const Training = () => {
    const { time, startTimer, stopTimer } = useTimer(20)
    return <>
        <div>{time}</div>
        <button onClick={startTimer}>start</button>
        <button onClick={stopTimer}>stop</button>
    </>
}

Answer 2

You can create a useCountDown Hook as follow (In Typescript) :

Gist

import { useEffect, useRef, useState } from 'react';

export const useCountDown: (
  total: number,
  ms?: number,
) => [number, () => void, () => void, () => void] = (
  total: number,
  ms: number = 1000,
) => {
  const [counter, setCountDown] = useState(total);
  const [startCountDown, setStartCountDown] = useState(false);
  // Store the created interval
  const intervalId = useRef<number>();
  const start: () => void = () => setStartCountDown(true);
  const pause: () => void = () => setStartCountDown(false);
  const reset: () => void = () => {
    clearInterval(intervalId.current);
    setStartCountDown(false);
    setCountDown(total);
  };

  useEffect(() => {
    intervalId.current = setInterval(() => {
      startCountDown && counter > 0 && setCountDown(counter => counter - 1);
    }, ms);
    // Clear interval when count to zero
    if (counter === 0) clearInterval(intervalId.current);
    // Clear interval when unmount
    return () => clearInterval(intervalId.current);
  }, [startCountDown, counter, ms]);

  return [counter, start, pause, reset];
};

Usage Demo: https://codesandbox.io/s/usecountdown-hook-56lqv