3gwebtrain
November 18, 2023
101 views
0 votes
2 Answers

When update the default time with setInterval, it’s not working as expected. Instead it’s added as a new instance. How to clear the setInterval in the custom hook and update new value?

app.jsx

import React from 'react';
import './style.css';
import CustomTimer from './Custom';
import { useState, useEffect } from 'react';

export default function App() {
  const [intervalTime, setIntervalTime] = useState(200);

  const time = CustomTimer(intervalTime);

  useEffect(() => {
    setTimeout(() => {
      console.log('Hi');
      setIntervalTime(500);
    }, 5000);
  });

  return (
    <div className="App">
      <h1>Hello CodeSandbox</h1>
      <h2>Start editing to see some happen! {time} </h2>
    </div>
  );
}

Custom.js

import { useEffect, useState } from 'react';

function CustomTimer(startTime) {
  const [timer, setTimer] = useState(startTime);
  useEffect(() => {
    const myInterval = setInterval(() => {
      if (timer > 0) {
        setTimer(timer - 1);
        console.log(timer);
      }
    }, 1000);
    return () => clearInterval(myInterval);
  }, [startTime]);
  return timer;
}

export default CustomTimer;

Live Demo => please check the console

Answers

Issues

This is the classic stale closure of state in a callback problem, with some additional discrepancies.

timer in the setInterval callback is the same value from the initial render cycle, so it always decrements from the same value.
The useEffect hook in CustomTimer doesn’t "reset" the timer state when the startTime prop value is changed.
The useEffect hook in App is missing a dependency array so it starts a new timeout each render cycle after intervalTime is updated the first time.
CustomTimer is really a custom React hook, it should be correctly renamed to useCustomTimer.

Solution

I suggest using a React ref to hold a reference to the current timer state value, to be used in the setInterval callback. This is to check if the timer is still valid. Use a functional state update to enqueue timer state updates.

Example:

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

function useCustomTimer(startTime) {
  const [timer, setTimer] = useState(startTime);
  const timerRef = useRef(timer);           // <-- ref to hold current timer value

  useEffect(() => {
    timerRef.current = timer;               // <-- keep ref synchronized to state
  }, [timer]);

  useEffect(() => {
    setTimer(startTime);                    // <-- reset timer when startTime changes

    const myInterval = setInterval(() => {
      if (timerRef.current > 0) {           // <-- use timer ref for check
        setTimer((timer) => timer - 1);     // <-- functional state update
        console.log(timerRef.current);
      } else {
        console.log("Timer expired");
        clearInterval(myInterval);          // <-- clear when expired
      }
    }, 1000);
    return () => clearInterval(myInterval); // <-- clear on startTime update or umounting
  }, [startTime]);

  return timer;
}

export default function App() {
  const [intervalTime, setIntervalTime] = useState(200);

  const time = useCustomTimer(intervalTime);

  useEffect(() => {
    setTimeout(() => {
      console.log("Hi");
      setIntervalTime(500);
    }, 5000);
  }, []); // <-- add missing dependency array

  return (
    <div className="App">
      <h1>Hello CodeSandbox</h1>
      <h2>Start editing to see some magic happen! {time}</h2>
    </div>
  );
}

Demo

The results can be tested here as well ==> Test Demo

First of all, need to add a dependency to the useEffect in the App.js file since whenever it triggers, the timer starts from the beginning.

App.js:

import React from 'react';
import './style.css';
import CustomTimer from './Custom';
import { useState, useEffect } from 'react';

export default function App() {
  const [intervalTime, setIntervalTime] = useState(200);
  const time = CustomTimer(intervalTime);

  useEffect(() => {
    setTimeout(() => {
      setIntervalTime(500);
    }, 5000);
  }, []);  // Added dependency array here

  return (
    <div className="App">
      <h1>Hello CodeSandbox</h1>
      <h2>Start editing to see some happen! {time} </h2>
    </div>
  );
}

Custom.js: (Added comment lines to explain the added/updated lines)

import { useEffect, useState } from 'react';

function CustomTimer(startTime) {
  const [timer, setTimer] = useState(startTime);

  useEffect(() => {
    setTimer(startTime);
    let timerCounter = startTime;

    const myInterval = setInterval(() => {
      if (timerCounter > 0) {
        // Get the previous state and decrease it 
        setTimer(timer => timer - 1);
        timerCounter--;
        return;
      }

      // Clear interval after timer's work is done
      clearInterval(myInterval);
    }, 1000);

    return () => clearInterval(myInterval);
  }, [startTime]);

  return timer;
}

export default CustomTimer;

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Javascript – Custom hook with clear and update time not works as per expected

Answers

Issues

Solution

Demo