According to the docs:
componentDidUpdate() is invoked immediately after updating occurs. This method is not called for the initial render.
We can use the new useEffect() hook to simulate componentDidUpdate(), but it seems like useEffect() is being ran after every render, even the first time. How do I get it to not run on initial render?
As you can see in the example below, componentDidUpdateFunction is printed during the initial render but componentDidUpdateClass was not printed during the initial render.
function ComponentDidUpdateFunction() {
const [count, setCount] = React.useState(0);
React.useEffect(() => {
console.log("componentDidUpdateFunction");
});
return (
<div>
<p>componentDidUpdateFunction: {count} times</p>
<button
onClick={() => {
setCount(count + 1);
}}
>
Click Me
</button>
</div>
);
}
class ComponentDidUpdateClass extends React.Component {
constructor(props) {
super(props);
this.state = {
count: 0,
};
}
componentDidUpdate() {
console.log("componentDidUpdateClass");
}
render() {
return (
<div>
<p>componentDidUpdateClass: {this.state.count} times</p>
<button
onClick={() => {
this.setState({ count: this.state.count + 1 });
}}
>
Click Me
</button>
</div>
);
}
}
ReactDOM.render(
<div>
<ComponentDidUpdateFunction />
<ComponentDidUpdateClass />
</div>,
document.querySelector("#app")
);
<script src="https://unpkg.com/react#16.7.0-alpha.0/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom#16.7.0-alpha.0/umd/react-dom.development.js"></script>
<div id="app"></div>
We can use the useRef hook to store any mutable value we like, so we could use that to keep track of if it's the first time the useEffect function is being run.
If we want the effect to run in the same phase that componentDidUpdate does, we can use useLayoutEffect instead.
Example
const { useState, useRef, useLayoutEffect } = React;
function ComponentDidUpdateFunction() {
const [count, setCount] = useState(0);
const firstUpdate = useRef(true);
useLayoutEffect(() => {
if (firstUpdate.current) {
firstUpdate.current = false;
return;
}
console.log("componentDidUpdateFunction");
});
return (
<div>
<p>componentDidUpdateFunction: {count} times</p>
<button
onClick={() => {
setCount(count + 1);
}}
>
Click Me
</button>
</div>
);
}
ReactDOM.render(
<ComponentDidUpdateFunction />,
document.getElementById("app")
);
<script src="https://unpkg.com/react#16.7.0-alpha.0/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom#16.7.0-alpha.0/umd/react-dom.development.js"></script>
<div id="app"></div>
You can turn it into custom hooks, like so:
import React, { useEffect, useRef } from 'react';
const useDidMountEffect = (func, deps) => {
const didMount = useRef(false);
useEffect(() => {
if (didMount.current) func();
else didMount.current = true;
}, deps);
}
export default useDidMountEffect;
Usage example:
import React, { useState, useEffect } from 'react';
import useDidMountEffect from '../path/to/useDidMountEffect';
const MyComponent = (props) => {
const [state, setState] = useState({
key: false
});
useEffect(() => {
// you know what is this, don't you?
}, []);
useDidMountEffect(() => {
// react please run me if 'key' changes, but not on initial render
}, [state.key]);
return (
<div>
...
</div>
);
}
// ...
I made a simple useFirstRender hook to handle cases like focussing a form input:
import { useRef, useEffect } from 'react';
export function useFirstRender() {
const firstRender = useRef(true);
useEffect(() => {
firstRender.current = false;
}, []);
return firstRender.current;
}
It starts out as true, then switches to false in the useEffect, which only runs once, and never again.
In your component, use it:
const firstRender = useFirstRender();
const phoneNumberRef = useRef(null);
useEffect(() => {
if (firstRender || errors.phoneNumber) {
phoneNumberRef.current.focus();
}
}, [firstRender, errors.phoneNumber]);
For your case, you would just use if (!firstRender) { ....
Same approach as Tholle's answer, but using useState instead of useRef.
const [skipCount, setSkipCount] = useState(true);
...
useEffect(() => {
if (skipCount) setSkipCount(false);
if (!skipCount) runYourFunction();
}, [dependencies])
EDIT
While this also works, it involves updating state which will cause your component to re-render. If all your component's useEffect calls (and also all of its children's) have a dependency array, this doesn't matter. But keep in mind that any useEffect without a dependency array (useEffect(() => {...}) will be run again.
Using and updating useRef will not cause any re-renders.
#ravi, yours doesn't call the passed-in unmount function. Here's a version that's a little more complete:
/**
* Identical to React.useEffect, except that it never runs on mount. This is
* the equivalent of the componentDidUpdate lifecycle function.
*
* #param {function:function} effect - A useEffect effect.
* #param {array} [dependencies] - useEffect dependency list.
*/
export const useEffectExceptOnMount = (effect, dependencies) => {
const mounted = React.useRef(false);
React.useEffect(() => {
if (mounted.current) {
const unmount = effect();
return () => unmount && unmount();
} else {
mounted.current = true;
}
}, dependencies);
// Reset on unmount for the next mount.
React.useEffect(() => {
return () => mounted.current = false;
}, []);
};
a simple way is to create a let, out of your component and set in to true.
then say if its true set it to false then return (stop) the useEffect function
like that:
import { useEffect} from 'react';
//your let must be out of component to avoid re-evaluation
let isFirst = true
function App() {
useEffect(() => {
if(isFirst){
isFirst = false
return
}
//your code that don't want to execute at first time
},[])
return (
<div>
<p>its simple huh...</p>
</div>
);
}
its Similar to #Carmine Tambasciabs solution but without using state :)
function useEffectAfterMount(effect, deps) {
const isMounted = useRef(false);
useEffect(() => {
if (isMounted.current) return effect();
else isMounted.current = true;
}, deps);
// reset on unmount; in React 18, components can mount again
useEffect(() => {
isMounted.current = false;
});
}
We need to return what comes back from effect(), because it might be a cleanup function. But we don't need to determine if it is or not. Just pass it on and let useEffect figure it out.
In an earlier version of this post I said resetting the ref (isMounted.current = false) wasn't necessary. But in React 18 it is, because components can remount with their previous state (thanks #Whatabrain).
I thought creating a custom hook would be overkill and I didn't want to muddle my component's readability by using the useLayoutEffect hook for something unrelated to layouts, so, in my case, I simply checked to see if the value of my stateful variable selectedItem that triggers the useEffect callback is its original value in order to determine if it's the initial render:
export default function MyComponent(props) {
const [selectedItem, setSelectedItem] = useState(null);
useEffect(() => {
if(!selectedItem) return; // If selected item is its initial value (null), don't continue
//... This will not happen on initial render
}, [selectedItem]);
// ...
}
This is the best implementation I've created so far using typescript. Basically, the idea is the same, using the Ref but I'm also considering the callback returned by useEffect to perform cleanup on component unmount.
import {
useRef,
EffectCallback,
DependencyList,
useEffect
} from 'react';
/**
* #param effect
* #param dependencies
*
*/
export default function useNoInitialEffect(
effect: EffectCallback,
dependencies?: DependencyList
) {
//Preserving the true by default as initial render cycle
const initialRender = useRef(true);
useEffect(() => {
let effectReturns: void | (() => void) = () => {};
// Updating the ref to false on the first render, causing
// subsequent render to execute the effect
if (initialRender.current) {
initialRender.current = false;
} else {
effectReturns = effect();
}
// Preserving and allowing the Destructor returned by the effect
// to execute on component unmount and perform cleanup if
// required.
if (effectReturns && typeof effectReturns === 'function') {
return effectReturns;
}
return undefined;
}, dependencies);
}
You can simply use it, as usual as you use the useEffect hook but this time, it won't run on the initial render. Here is how you can use this hook.
useNoInitialEffect(() => {
// perform something, returning callback is supported
}, [a, b]);
If you use ESLint and want to use the react-hooks/exhaustive-deps rule for this custom hook:
{
"rules": {
// ...
"react-hooks/exhaustive-deps": ["warn", {
"additionalHooks": "useNoInitialEffect"
}]
}
}
#MehdiDehghani, your solution work perfectly fine, one addition you have to do is on unmount, reset the didMount.current value to false. When to try to use this custom hook somewhere else, you don't get cache value.
import React, { useEffect, useRef } from 'react';
const useDidMountEffect = (func, deps) => {
const didMount = useRef(false);
useEffect(() => {
let unmount;
if (didMount.current) unmount = func();
else didMount.current = true;
return () => {
didMount.current = false;
unmount && unmount();
}
}, deps);
}
export default useDidMountEffect;
Simplified implementation
import { useRef, useEffect } from 'react';
function MyComp(props) {
const firstRender = useRef(true);
useEffect(() => {
if (firstRender.current) {
firstRender.current = false;
} else {
myProp = 'some val';
};
}, [props.myProp])
return (
<div>
...
</div>
)
}
You can use custom hook to run use effect after mount.
const useEffectAfterMount = (cb, dependencies) => {
const mounted = useRef(true);
useEffect(() => {
if (!mounted.current) {
return cb();
}
mounted.current = false;
}, dependencies); // eslint-disable-line react-hooks/exhaustive-deps
};
Here is the typescript version:
const useEffectAfterMount = (cb: EffectCallback, dependencies: DependencyList | undefined) => {
const mounted = useRef(true);
useEffect(() => {
if (!mounted.current) {
return cb();
}
mounted.current = false;
}, dependencies); // eslint-disable-line react-hooks/exhaustive-deps
};
For people who are having trouble with React 18 strict mode calling the useeffect on the initial render twice, try this:
// The init variable is necessary if your state is an object/array, because the == operator compares the references, not the actual values.
const init = [];
const [state, setState] = useState(init);
const dummyState = useRef(init);
useEffect(() => {
// Compare the old state with the new state
if (dummyState.current == state) {
// This means that the component is mounting
} else {
// This means that the component updated.
dummyState.current = state;
}
}, [state]);
Works in development mode...
function App() {
const init = [];
const [state, setState] = React.useState(init);
const dummyState = React.useRef(init);
React.useEffect(() => {
if (dummyState.current == state) {
console.log('mount');
} else {
console.log('update');
dummyState.current = state;
}
}, [state]);
return (
<button onClick={() => setState([...state, Math.random()])}>Update state </button>
);
}
ReactDOM.createRoot(document.getElementById("app")).render(
<React.StrictMode>
<App />
</React.StrictMode>
);
<script crossorigin src="https://unpkg.com/react#18/umd/react.development.js"></script>
<script crossorigin src="https://unpkg.com/react-dom#18/umd/react-dom.development.js"></script>
<div id="app"></div>
And in production.
function App() {
const init = [];
const [state, setState] = React.useState(init);
const dummyState = React.useRef(init);
React.useEffect(() => {
if (dummyState.current == state) {
console.log('mount');
} else {
console.log('update');
dummyState.current = state;
}
}, [state]);
return (
<button onClick={() => setState([...state, Math.random()])}>Update state </button>
);
}
ReactDOM.createRoot(document.getElementById("app")).render(
<React.StrictMode>
<App />
</React.StrictMode>
);
<script crossorigin src="https://unpkg.com/react#18/umd/react.production.min.js"></script>
<script crossorigin src="https://unpkg.com/react-dom#18/umd/react-dom.production.min.js"></script>
<div id="app"></div>
If you want to skip the first render, you can create a state "firstRenderDone" and set it to true in the useEffect with empty dependecy list (that works like a didMount). Then, in your other useEffect, you can check if the first render was already done before doing something.
const [firstRenderDone, setFirstRenderDone] = useState(false);
//useEffect with empty dependecy list (that works like a componentDidMount)
useEffect(() => {
setFirstRenderDone(true);
}, []);
// your other useEffect (that works as componetDidUpdate)
useEffect(() => {
if(firstRenderDone){
console.log("componentDidUpdateFunction");
}
}, [firstRenderDone]);
All previous are good, but this can be achieved in a simplier way considering that the action in useEffect can be "skipped" placing an if condition(or any other ) that is basically not run first time, and still with the dependency.
For example I had the case of :
Load data from an API but my title has to be "Loading" till the date were not there, so I have an array, tours that is empty at beginning and show the text "Showing"
Have a component rendered with different information from those API.
The user can delete one by one those info, even all making the tour array empty again as the beginning but this time the API fetch is been already done
Once the tour list is empty by deleting then show another title.
so my "solution" was to create another useState to create a boolean value that change only after the data fetch making another condition in useEffect true in order to run another function that also depend on the tour length.
useEffect(() => {
if (isTitle) {
changeTitle(newTitle)
}else{
isSetTitle(true)
}
}, [tours])
here my App.js
import React, { useState, useEffect } from 'react'
import Loading from './Loading'
import Tours from './Tours'
const url = 'API url'
let newTours
function App() {
const [loading, setLoading ] = useState(true)
const [tours, setTours] = useState([])
const [isTitle, isSetTitle] = useState(false)
const [title, setTitle] = useState("Our Tours")
const newTitle = "Tours are empty"
const removeTours = (id) => {
newTours = tours.filter(tour => ( tour.id !== id))
return setTours(newTours)
}
const changeTitle = (title) =>{
if(tours.length === 0 && loading === false){
setTitle(title)
}
}
const fetchTours = async () => {
setLoading(true)
try {
const response = await fetch(url)
const tours = await response.json()
setLoading(false)
setTours(tours)
}catch(error) {
setLoading(false)
console.log(error)
}
}
useEffect(()=>{
fetchTours()
},[])
useEffect(() => {
if (isTitle) {
changeTitle(newTitle)
}else{
isSetTitle(true)
}
}, [tours])
if(loading){
return (
<main>
<Loading />
</main>
)
}else{
return (
<main>
<Tours tours={tours} title={title} changeTitle={changeTitle}
removeTours={removeTours} />
</main>
)
}
}
export default App
const [dojob, setDojob] = useState(false);
yourfunction(){
setDojob(true);
}
useEffect(()=>{
if(dojob){
yourfunction();
setDojob(false);
}
},[dojob]);
I wanted to build a timer application in React using functional component and below are the requirements.
The component will display a number initialized to 0 know as counter.
The component will display a Start button below the counter number.
On clicking the Start button the counter will start running. This means the counter number will start incrementing by 1 for every one second.
When the counter is running(incrementing), the Start button will become the Pause button.
On clicking the Pause button, the counter will preserve its value (number) but stops running(incrementing).
The component will also display a Reset button.
On clicking the Reset button, the counter will go to its initial value(which is 0 in our case) and stops running(incrementing).
Below is the code that I have implemented, but clearInterval doesn't seems to be working, Also how do i implement Reset Button?
Code:
import React, { useState, useEffect } from "react";
export default function Counter() {
const [counter, setCounter] = useState(0);
const [flag, setFlag] = useState(false);
const [isClicked, setClicked] = useState(false);
var myInterval;
function incrementCounter() {
setClicked(!isClicked);
if (flag) {
myInterval = setInterval(
() => setCounter((counter) => counter + 1),
1000
);
setFlag(false);
} else {
console.log("sasdsad");
clearInterval(myInterval);
}
}
function resetCounter() {
clearInterval(myInterval);
setCounter(0);
}
useEffect(() => {
setFlag(true);
}, []);
return (
<div>
<p>{counter}</p>
<button onClick={incrementCounter}>
{isClicked ? "Pause" : "Start"}
</button>
<button onClick={resetCounter}>Reset</button>
</div>
);
}
Codesandbox link:
CodeSandbox
I did a slightly different version that use an extra useEffect that runs on isRunning (changed name from flag) change:
import React, { useState, useEffect, useRef } from "react";
export default function Counter() {
const [counter, setCounter] = useState(0);
// Change initial value to `false` if you don't want
// to have timer running on load
// Changed `flag` name to more significant name
const [isRunning, setIsRunning] = useState(false);
// You don't need 2 variable for this
//const [isClicked, setClicked] = useState(false);
// Using `useRef` to store a reference to the interval
const myInterval = useRef();
useEffect(() => {
// You had this line to start timer on load
// but you can just set the initial state to `true`
//setFlag(true);
// Clear time on component dismount
return () => clearInterval(myInterval.current);
}, []);
// useEffect that start/stop interval on flag change
useEffect(() => {
if (isRunning) {
myInterval.current = setInterval(
() => setCounter((counter) => counter + 1),
1000
);
} else {
clearInterval(myInterval.current);
myInterval.current = null;
}
}, [isRunning]);
// Now on click you only change the flag
function toggleTimer() {
setIsRunning((isRunning) => !isRunning);
}
function resetCounter() {
clearInterval(myInterval.current);
myInterval.current = null;
setCounter(0);
setIsRunning(false);
}
return (
<div>
<p>{counter}</p>
<button onClick={toggleTimer}>{isRunning ? "Pause" : "Start"}</button>
<button onClick={resetCounter}>Reset</button>
</div>
);
}
Demo: https://codesandbox.io/s/dank-night-wwxqz3?file=/src/Counter.js
As a little extra i've made a version that uses a custom hook useTimer. In this way the component code is way cleaner:
https://codesandbox.io/s/agitated-curie-nkjf62?file=/src/Counter.js
Use useRef to make the interval as a ref. Then use resetCounter() to clean the interval ref.
const intervalRef = useRef(null)
const incrementCounter = () => {
intervalRef.current = setInterval(() => {
setCounter(prevState => prevState + 1)
}, 1000);
};
const resetCounter = () => {
clearInterval(intervalRef.current);
intervalRef.current = null;
};
Between each rendering your variable myInterval value doesn't survive. That's why you need to use the [useRef][1] hook that save the reference of this variable across each rendering.
Besides, you don't need an flag function, as you have all information with the myClicked variable
Here is a modification of your code with those modifications. Don't hesitate if you have any question.
import React, { useState, useEffect, useRef } from "react";
export default function Counter() {
const [counter, setCounter] = useState(0);
const [isStarted, setIsStarted] = useState(false);
const myInterval = useRef();
function start() {
setIsStarted(true);
myInterval.current = setInterval(() => setCounter((counter) => counter + 1), 100);
100;
}
function pause() {
setIsStarted(false);
clearInterval(myInterval.current);
}
function resetCounter() {
clearInterval(myInterval.current);
setCounter(0);
}
return (
<div>
<p>{counter}</p>
{!isStarted ?
<button onClick={start}>
Start
</button>
:
<button onClick={pause}>
Pause
</button>
}
<button onClick={resetCounter}>Reset</button>
</div>
);
}
\\\
[1]: https://reactjs.org/docs/hooks-reference.html#useref
I'll just leave this here for anyone having the same problem.
in my case, the issue was node setInterval was used instead of window.setInterval.
this is a problem since this returns a type of Node.Timer which is an object instead of number (setInterval ID) for the clearInterval() to work as it needs an argument type of number. so to fix this,
React.useEffect(() => {
let timeoutId;
timeoutId = window.setInterval(callback, 100);
return = () => {
if(timeoutId) clearInterval(timeoutId)
}
}, [])
or in class components use componentWillMount()
You have to store myInterval in state. After that when button is clicked and flag is false, you can clear interval (myInterval in state).
I'm so confused about useState in React hooks.
I do not know why console.log in setTimeout function calls more than one time when I use useState.
If I remove useState it normally calls only once.
And If I use Class state instead hooks, it normally calls only once as well.
Why is it happened that ?
And how can I handle it ?
(here is my code)
import React, { useState, useEffect } from "react";
import "./App.css";
const usePassword = () => {
const [passwordValue, setPasswordValue] = useState({
password: "",
passwordHidden: "",
});
let timer = null;
const trigger = () => {
clearTimeout(timer);
timer = setTimeout(() => console.log("end"), 1000);
};
const onPasswordChanged = (name, value) => {
setPasswordValue((prev) => ({ ...passwordValue, passwordHidden: value }));
trigger();
};
return { passwordValue, onPasswordChanged };
};
function App() {
const { passwordValue, onPasswordChanged } = usePassword();
const onChanged = (event) => {
const { name, value } = event.target;
onPasswordChanged(name, value);
};
const onSubmit = () => {
console.log("submitted!", passwordValue);
};
return (
<div className="App">
<header className="App-header">
<input name="password" onKeyUp={onChanged} />
<button onClick={onSubmit}>Submit</button>
</header>
</div>
);
}
export default App;
Whenever you set the state using useState you get a new timer variable, as the function is called again. This is why your clearTimeout does not work.
You can use a ref to hold on to the value between render cycles:
const timer = useRef(null);
const trigger = () => {
clearTimeout(timer.current);
timer.current = setTimeout(() => console.log("end"), 1000);
};
I'm learning state and hooks in React. I'm doing an exercise to deal cards from a deck using the deckofcardsAPI, not the most important code on the interwebs, but it does bring about a question. In fact, it's not even part of the exercise, I just really want to do it.
I have a button that is stored in a useRef object. I really would like the onClick function to change, but I'm not sure how to do it.
import React, { useState, useEffect, useRef } from "react";
import axios from 'axios';
// auto dealer, 1 card/s turned on or off
function CardTable2() {
const [src, setSrc] = useState('');
const deckId = useRef();
const remaining = useRef(52);
const isDealing = useRef(false);
const timerId = useRef();
const button = useRef();
useEffect(() => {
timerId.current = setInterval(() => {
if (isDealing.current) {
dealCard();
};
}, 1000);
async function createDeck() {
const res = await axios.get(`http://deckofcardsapi.com/api/deck/new/shuffle/?deck_count=1`);
deckId.current = res.data.deck_id;
};
createDeck();
return () => { clearInterval(timerId.current) }
}, []);
async function dealCard() {
if (remaining.current === 0) {
alert('You are out of cards');
isDealing.current = false;
button.current.innerText = "Shuffle deck";
button.current.onClick = shuffleDeck;
return;
}
console.log(button);
const res = await axios.get(`http://deckofcardsapi.com/api/deck/${deckId.current}/draw/?count=1`);
remaining.current = res.data.remaining;
setSrc(res.data.cards[0].image);
};
async function shuffleDeck() {
const res = await axios.get(`http://deckofcardsapi.com/api/deck/${deckId.current}/shuffle/`);
setSrc('');
button.current.innerText = "Start dealing cards";
button.current.onClick = toggleDealing;
}
function toggleDealing() {
isDealing.current = !isDealing.current;
button.current.innerText = "Start dealing cards";
};
return (
<>
<button onClick={toggleDealing} ref={button}>{isDealing.current ? "Stop dealing cards" : "Start dealing cards"}</button>
<div>
<img className="CardBox" src={src} alt="a card" />
</div>
</>
)
};
export default CardTable2;
I've tried setting the button.current.onClick to the function, as seen above, but it doesn't actually seem to have an effect. Am I missing something?
I think what's happening is the return is just putting back {toggleDealing} every time.
Why not get rid of the ref and just have a generic function handle both cases?
const [buttonMode, setButtonMode] = useState("shuffle")
function handleClick() {
if (buttonMode === "deal") {
deal()
} else if (buttonMode === "shuffle") {
shuffle()
}
}
const [timer,setTimer] = useState()
const [number, setNumber] = useState()
const [list, setlist] = useState([])
const numberChange = (number)=>{
setNumber(number)
if (!(list.find(item=>item===number))){
setlist([...list,number])}
}
const randomNumber=()=> 1+Math.floor(Math.random()*90)
const randNumberChange=()=>{
let randNumber = randomNumber()
if (list.find(item=>item===randNumber))
randNumberChange()
else
numberChange(randNumber)
}
const startTimer = () => {
setTimer(setInterval(()=>{
randNumberChange()
}, 5000))
}
const stopTimer=()=>{
clearInterval(timer)
}
The list is always rendering only one item and not appending it.
When randNumberChange is called separately then the list gets appended but not with setInterval.
When startTimer funcion is executed is stopped with stopTimer and then started again it appends second item then stop and it repeats
Change setlist([...list,number])} to setlist((prevState) => [...prevState, number]). React set state is async in nature. So to get the correct list value from the state you would need to get the value from previous state. Doc
Suggestion: that instead of setting timer in state, you can start the interval in useEffect.
Also in numberChange function, you should get the list from previous state and then append the new number in that. This will make sure that the list value is updated before adding new number.
import React, { Component, useState } from "react";
import { render } from "react-dom";
import Hello from "./Hello";
import "./style.css";
const Test = () => {
const [number, setNumber] = useState(null);
const [list, setlist] = useState([]);
const numberChange = number => {
setNumber(number);
if (!list.find(item => item === number)) {
setlist((prevState) => [...prevState, number]);// instead of directly using list value, get it from previous state
}
};
const randomNumber = () => 1 + Math.floor(Math.random() * 90);
const randNumberChange = () => {
console.log("here");
let randNumber = randomNumber();
if (list.find(item => item === randNumber)) randNumberChange();
else numberChange(randNumber);
};
const startTimer = () => {
return setInterval(() => { randNumberChange(); }, 5000);
}
const stopTimer = (timer) => {
clearInterval(timer)
}
React.useEffect(() => {
const timer = startTimer();
return ()=> stopTimer(timer);
}, []);
console.log(list);
return <div>{number}</div>;
};
You'll need to use useEffect hook:
useEffect(() => {
// You don't need timer state, we'll clear this later
const interval = setInterval(() => {
randNumberChange()
},5000)
return () => { // clear up
clearInterval(interval)
}
},[])
use useEffect with setTimeout it is work as setInterval
useEffect(() => {
setTimeout(() => setList([...list, newValue]), 2000)
}, [list])