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Why only first time React function component getting props

React throw an error when we try to update the state on an unmounted component.So When I test react component for that I am getting errors on the first render only.
I made a component that enable child component based on click. And child component have button which updates state after some settimeout which throw react warning
Warning: Can't perform a React state update on an unmounted component. This is a no-op, but it indicates a memory leak in your application. To fix, cancel all subscriptions and asynchronous tasks in a useEffect cleanup function.
Which is perfectly valid. But to overcome this I am passing enable props from the parent component based on that there is a condition just before setTimeout in the child component. So why does it throw an error the first time only?
To reproduce
Click on the child component button which is false and click on parent component button enable which unmount the child component.
**My question is why react throw an error on the first time only ? And why it is working fine on second time **
Parent component
import { useState } from "react";
import "./styles.css";
import { Test } from "./Test";
export default function App() {
const [state, setstate] = useState(true);
const changeState = () => {
setstate(!state);
};
return (
<div className="App">
<button onClick={changeState}>enable </button>
{state && <Test enable={state} />}
</div>
);
}
Child Component
import React, { useState } from "react";
export const Test = (props) => {
const [state, setstate] = useState(false);
const fetchData = () => {
setstate(!state);
if (props.enable) {
setTimeout(() => {
setstate(false);
}, 1000);
}
};
return (
<>
<button onClick={fetchData}> {`${state}`}</button>
</>
);
};
Codesandbox link to test

Nice track, Just you are missing a minor point, when you write a state thats needed some time to execute and the same time we can visit the flow again and again base on any action, then we need to clear old subscription before go to new one...
For example, in your code here, you update state flow, but the state flow is register a new subscription every time we visit a component with valid props and click on button, so that, prev execution may still work when you trigger new event, so simply, what we need to do unmounted old subscription and we can do that by this for your case:
import React, { useState, useEffect } from "react";
export const Test = (props) => {
const [state, setstate] = useState(false);
useEffect(() => {
if (props.enable) {
const timer = setTimeout(() => {
setstate(false);
}, 1000);
return () => clearTimeout(timer);
}
}, [state, props.enable]);
const fetchData = () => {
setstate((prev) => !prev);
};
return (
<>
<button onClick={fetchData}> {`${state}`}</button>
</>
);
};
Look at code above, simply we add code need to cleanup in effect which its look to my state and prop, now when I click on button, the effect will trigger, if we do that again, the clearTime will work for prev subscription and then add new one and so on...
Notes:
In your case we can remove function and use setState direct on your button.
Prefer to use useCallBack in your function like const fetchData = useCAllback...
You can use setstate((prev) => !prev); its will be work as snapshot, and its usefull when you depends on old value..but may it not needed in some cases too, but just to know about this feature.
Update 1:
What is Subscription:
You can say the subscription represents a disposable resource, such as the execution of an Observable. A Subscription has one important method, unsubscribe, that takes no argument and just disposes the resource held by the subscription, in another word, you can say yes, any async task or any job will be invoke to react life-cycle state and its needed to observe changes, then you talk about subscribe, like API or time out or time interval and so on, any of these action thats need to clear prev subscribe (stop observer - unsubscribe) to prevent any memory leek and clear memory to keep state flow safe and prevent unneeded reredner.

React & Firebase - Appending component with Firebase onSnapshot

Currently, I'm using a button with an onClick function to append a JSX component to the UI every time it's been pressed. I'm also using a useEffect with some firebase database logic which is an onSnapshot to get live reads on the data to display the information onto the UI.
The problem is when a user refreshes the page the onClick function that appends the JSX is deleted from the state and doesn't display the firebase data onto the UI. How would I go about sorting this problem out?
Code:
import React, { useEffect, useState } from "react";
import DashboardBody from "./DashboardComponents/DashboardBody";
import db from "../../firebase";
function Test() {
const [exercise, setExercise] = useState([]);
const [count, setCount] = useState(0);
useEffect(() => {
db.collection("users").onSnapshot((snapshot) => {
setExercise(
snapshot.docs.map((doc) => ({
id: doc.id,
exercise: doc.data().exercise,
}))
);
});
return;
}, []);
return (
<div>
{[...Array(count)].map((count, index) => (
<DashboardBody key={index} exercise={exercise} />
))}
<div>
<button onClick={() => setCount(count + 1)}>Click</button>
</div>
</div>
);
}
export default Test;

The count variable is stored in the component's state and is incremented whenever the button is clicked. When the page reloads (or the component unmounts and mounts again), the state variable is lost. To preserve the state, you would have to store it in the database (or in the browser's storage) so that you can fetch it when the component mounts again.

react memo is not getting props

React memo isn't capturing the props neither the prevProps nor the nextProps and the component render well. The react docs say
If your function component renders the same result given the same props, you can wrap it in a call to React.memo for a performance boost.
my problem is to stop twice rendering using react memo, but memo seems to be not working and the component renders twice with the same props.
The component renders when the Create New Event is clicked on /events
here is the live sandbox.
Child Component located at /components/Event/CreateEvent/CreateEvent.js
the parent component is located at /Pages/Event/Event.js line number 999' from where the child component is being triggered
Here is the Code:
import React from "react";
import AuthContext from "../../context/global-context";
import CreateEvent from "../../components/Event/CreateEvent/CreateEvent";
function Events({ location }) {
// Sate Managing
const [allEvents, setAllEvents] = React.useState([]);
const [creating, setCreating] = React.useState(false);
// Context As State
const { token, email } = React.useContext(AuthContext);
// Creating Event Showing
const modelBoxHandler = () => {
// works on when the ViewEvent is open
if (eventSelected) {
setEventSelected(null);
return;
}
setCreating(!creating);
};
return (
<div className="events">
{/* New Event Creating */}
{creating && (
<CreateEvent onHidder={modelBoxHandler} allEvents={allEvents} />
)}
{console.log("Event Rendered.js =>")}
</div>
);
}
export default React.memo(Events, () => true);
Child Component where the Rect memo doesn't have props:
import React from "react";
import AuthContext from "../../../context/global-context";
function CreateEvent({ onHidder, allEvents }) {
// Context
const { token } = React.useContext(AuthContext);
console.log("CreatedEvent.js REnder");
return (
... Some code here
);
}
export default React.memo(CreateEvent, (prevProps, nextProps) => {
console.log("Hello", prevProps, nextProps);
});
Thanks in advance for your valuable answer and times!

The problem is that on basis of creating variable you are actually remounting and not rendering the CreateEvent component. What it means is that if creating variable changes, the component is unmounted and re-mounted when creating is true, so its not a re-render
Also you must note that modelBoxHandler function reference also changes on each re-render so even if your CreateEvent component is in rendered state and the parent re-rendered due to some reason , the CreateEvent component too will re-render
There are 2 changes that you need to make to make it work better
Define modelBoxHandler with a useCallback hook
perform conditional rendering in createEvent based on creating prop
// Creating Event Showing
const modelBoxHandler = useCallback(() => {
// works on when the ViewEvent is open
if (eventSelected) {
setEventSelected(null);
return;
}
setCreating(prevCreating => !prevCreating);
}, [eventSelected]);
...
return (
<div className="events">
{/* New Event Creating */}
<CreateEvent creating={creating} onHidder={modelBoxHandler} allEvents={allEvents} />
{console.log("Event Rendered.js =>")}
</div>
);
and in createEvent
function CreateEvent({ onHidder, allEvents, creating }) {
// Context
const { token } = React.useContext(AuthContext);
console.log("CreatedEvent.js REnder");
if(!creating) {
return null;
}
return (
... Some code here
);
}
export default React.memo(CreateEvent);

In your example, you don't have an additional render for React.memo to work.
According to your render logic, there aren't any nextProps, you unmount the component with conditional rendering (creating).
// You toggle with `creating` value, there is only single render each time
creating && <CreateEvent onHidder={modelBoxHandler} allEvents={allEvents}/>
// Works, because there will be multiple renders (nextProps)
true && <CreateEvent onHidder={modelBoxHandler} allEvents={allEvents} />
In this case, you might not need React.memo.

Conditional rendering in React Native with global variable

I know this is horrible convention, but I'm trying to quickly conditionally render screens in my React Native app with global variables (so no redux):
App.js:
if (global.clickStatus !== 'clicked') {
return <Screen1 />;
}
return <Screen2 />;
The app begins on Screen1, where there is a button that makes global.clickStatus = 'clicked'. When this is clicked, I want Screen2 to render. The problem is, the global.clickStatus doesn't seem to update on my App.js (even though global.clickStatus is changed, it still renders Screen1.
How can I get it to update?

I believe in <App /> component because it is a function component you can introduce a state if your button is clicked. Then with clicked state you can manipulate which component to show.
Similarly like the following - obviously this is a simplified example:
const App = () => {
const [clicked, setClicked] = useState(false);
return <>
<div onClick={() => setClicked(true)}>Click me</div>
{ clicked ? <Screen2 /> : <Screen1 /> }
</>
}
Suggested read is Using the State Hook.

The app begins on Screen1, where there is a button that makes global.clickStatus = 'clicked'
When you click the button, you did not set any state for App.js component => no re-render action is made.
I just assume the button is in Screen 1. Try code below:
import React from "react";
import "./styles.css";
export default function App() {
// Create a state
const [renderIndex, setRenderIndex] = useState(new Date().getTime())
if (global.clickStatus !== 'clicked') {
// Assume you have a button in Screen1
// Pass a callback function from this component to Screen1
// When button in Screen1 is clicked, call this callback function to update renderIndex => App component will re-render
return <Screen1 callBack={() => setRenderIndex(new Date().getTime())}/>;
}
return <Screen2 />;
}

ReactJs, Losing child component local state on global state change

I'm not understanding some ReactJs behavior and would need some help.
I have a Root Functional Component ("Index"), that contains another functional Component ("Preview").
That Preview component contains several other Functional Components ("InlineField").
The app is a simple form, where InlineField is component that renders an input and also contains a state to know if the field is "opened" or "closed" (when close it is displayed as a text, when open it is displayed as an input).
The global state is defined using hooks ad the "Index" level and moved down to the field through props (I've tried the same using Context). This state contains all form values.
The InlineField Component uses hook to maintain its local state only (is open / is closed).
When a an input is changed it updates the state (Index level) which triggers a re-render of the Index as well as its children.
This translate into the currently edited field (InlineField Component with local state = open) to refresh and lose its state value.
My question:
How can I make sure these InlineField Components retain their state even after updating global state?
I could simply move that InlineField Component state to the global state too, but I don't think it makes much sense.
I must be getting something wrong...
Thanks!
Edit: added code sample
Index Component:
import React, { useState, useEffect } from "react"
import Layout from "../components/layout"
const IndexPage = () => {
const [formValues, setFormValues] = useState({
name: 'Myname',
email: 'myemail#mail.com',
})
const onFormValueChange = (key, value) => {
setFormValues({...formValues, [key]: value})
}
return (
<Layout>
<Preview
key="previewyaknow"
formValues={formValues}
onFieldChange={setFormValues}
/>
</Layout>
)
}
export default IndexPage
Preview Component:
import React from 'react'
import { Box, TextField } from "#material-ui/core"
import { InlineField } from './inlineField'
export const Preview = ({formValues, onFieldChange}) => {
return (
<>
<Box display="flex" alignItems="center">
<InlineField
value={formValues.email}
onChange={onFormValueChange}
id="email"
field={<TextField value={formValues.email}/>>>}
/>
</>
)
}
InlineEdit Component
import React, { useState, useEffect } from "react"
export const InlineField = ({onChange, value, id, field}) => {
const [isEdit, setIsEdit] = useState(false)
const onBlur = (e) => {
setIsEdit(false)
}
let view = (<div>{value}</div>);
if (isEdit) {
view = (
<FieldContainer className={classes.fieldContainer}>
{React.cloneElement(field, {
'onBlur': onBlur,
'autoFocus': true,
'onChange': (e) => {
onChange(id, e.target.value)
}
})
}
</FieldContainer>
)
}
return (
<div onClick={()=>setIsEdit(!isEdit)}>
{view}
</div>
)
}