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Why is the value on display not in sync with value logged to console?

Can anyone tell me why at button click, the value outputted to the console is always one unit smaller than displayed on the screen?
The values are not in sync as expected.
Example below in React
In Child:
import React, {useState } from "react";
export const ChildComp = ({getNumProps}) => {
const [num, setNum] = useState(0);
const onPlusClick = () => {
if (num< 12) {
setNum(num + 1);// num does not increase immediately after this line, except when focus reenters here on second method call
}
getNumProps(num);
}
return(
<div>
<button onClick={onPlusClick}>
Click to increment
</button>
{num}
</div>
);
}
In parent
import { ChildComp } from "./ChildComp"
export const ParentComp = () => {
const getNum= (num) => {
console.log(num);
}
return (<ChildComp getNumProps={getNum}/>)
}
The page initially shows 0
When I click once the number increments to 1, but console displays 0
When I click once the number increments to 2, but console displays 1
I should see in the console the same as the page display
Appreciate if you can leave a commen on how the question can be improved.
This is a child to parent communication example. Also, any objections about standards used, please let me know.
Thanks.
Update: I notice the values would be in sync if
instead of getNumProps(num);
I did getNumProps(num + 1); but that doesn't change the fact that previously on this line
setNum(num + 1);, as already pointed out in the comment, num does not increase immediately after this line, except when focus reenters here on second method call. Not sure why.

The prop function getNumProps is a side effect, and should be put into a hook, instead of inside of that onPlusClick function.
Instead, do this:
useEffect(() => {
getNumProp(num);
}, [num]);
Alternatively, to avoid the error: "React Hook useEffect has a missing dependency: 'getNumProps'. See this doc on using the useCallback hook
const callback = useCallback(() => {
getNumProp(num);
}, [num]);
function onPlusClick(...) {
...
callback();
}
The change to the state of num will cause a re-render of the child component, not the parent.

Is there a way to update the function in the eventlistener in a React Functional Component?

I need to find a way to use both a variable declared inside a functional component and part of the props of the functional component inside the callback function on a mousemove eventlistener.
So far I have the code below, but it seems the props don't get updated within the onHover function.
The full component is a bit more complex than this example code, but the example below is the most basic way of showing my problem. On initialisation the value in the props would be 1, and in the onHover function it stays 1 even if there's a change and re-render and elsewhere in the component it shows the correct value of 2.
Is there a way to make sure the onHover function references the most recent version of the props?
I've tried using a useEffect hook to remove and add the eventlistener, but then I can't seem to access the variableNeededInOnHover
function FunctionalComp(props){
let variableNeededInOnHover = null;
useEffect(()=>{
window.addEventListener("mousemove", onHover, true)
return () => {
window.removeEventListener("mousemove", onHover, true)
}
}, [])
function onHover(event) {
console.log(props.value) // stays 1
console.log(variableNeededInOnHover)
}
return ( <div>{props.value}</div> ) // Updates to show a value of 2
}
Is there a way to solve this problem?
EDIT:
I've tried Taxel's proposed solution and it seems that the variableNeededInOnHover keeps it's null value even though I set it's value in the useEffect hook, mimicing the componentDidMount() lifecycle method.
function FunctionalComp(props){
let variableNeededInOnHover = null;
useEffect(()=>{
variableNeededInOnHover = "Non Null Value";
}, [])
useEffect(()=>{
window.addEventListener("mousemove", onHover, true)
return () => {
window.removeEventListener("mousemove", onHover, true)
}
}, [props.value])
function onHover(event) {
console.log(props.value) // stays 1
console.log(variableNeededInOnHover)
}
return ( <div>{props.value}</div> ) // Updates to show a value of 2
}

I'm not 100% sure this solves your issue as you have no way that props.value is being updated in the example. Generally you need to use React state for any values that will get changed and you want to cause a rerender. React wont know about a let that gets updated somewhere.
I've added an example below that shows the value being updated.
function FunctionalComp(props){
const [value, setValue] = React.useState(props.value);
React.useEffect(()=>{
setValue(oldValue => oldValue + 1);
}, [])
React.useEffect(()=>{
window.addEventListener("mousemove", onHover, true)
return () => {
window.removeEventListener("mousemove", onHover, true)
}
}, [value])
function onHover(event) {
console.log(value) // stays 1
}
return ( <div>{value}</div> ) // Updates to show a value of 2
}
ReactDOM.render(
<FunctionalComp value={1} />,
document.getElementById("react")
);
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/17.0.1/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/17.0.1/umd/react-dom.production.min.js"></script>
<div id="react"></div>

Click handler, in react, doesn't return latest change in state when it executes a 2nd time

This takes place in a functional component:
import {useEffect} from 'react';
let [clickedOnPiece, setClickedOnPiece] = useState(false);
let [testRender, setTestRender] = useState(false);
useEffect(() => {
testRenderFunction();
}, [])
function testRenderFunction() {
let el = <div onClick={onClickHandler}>Click me</div>;
setTestRender(el);
}
function onClickHandler() {
if (clickedOnPiece) {
console.log("already clicked")
return
}
console.log(clickedOnPiece); //returns false the 1st & 2nd time.
setClickedOnPiece("clicked");
}
return (
<>
{testRender}
</>
)
When I click on div for the first time, I wait until setClickedOnPiece("clicked") successfully updates clickedOnPiece to "clicked". (I check this with React Developer Tools).
When I click div the 2nd time, it doesn't log the new change in state. It still logs clickedOnPiece as false. Why is this?

Okey this problem is because useState is asyncronus. u can read more about this useState set method not reflecting change immediately.
I think the solution is add useEffect like this.
useEffect( () => {
console.log(clickOnPiece);
}
, [clickOnPiece])

If you want to toggle the state, you could do something like this:
let [clickedOnPiece, setClickedOnPiece] = useState(false);
const onClickHandler = () => {
// set the value of clickedOnPiece to the opposite of what it was
// i.e. if it was 'true', set it to 'false'.
setClickedOnPiece(!clickedOnPiece);
console.log(clickedOnPiece);
}
// call the onClickHandler on click
<div onClick={()=>onClickHandler()}>Click me</div>

Looks like you are toggling
let [clickedOnPiece, setClickedOnPiece] = useState(false);
const onClickHandler = () => {
console.log(clickedOnPiece);
setClickedOnPiece(!clickedOnPiece);
}
console.log(clickedOnPiece);
<div onClick={onClickHandler}>Click me</div>
After setting state, don't console immediately because state is an asynchronous.

onClickHandler references the old, previous variable, clickedOnPiece. I believe this is because onClickHandler is not defined in the return statement part of the functional component which would have allowed it a new onClickHandler body to be created each time. Instead, we have the old onClickHandler continually referencing the old clickedOnPiece.
This problem is known as 'stale closures' - a concept I found discussed well at the bottom of this article

React useState hook event handler using initial state

I'm still getting my head around react hooks but struggling to see what I'm doing wrong here. I have a component for resizing panels, onmousedown of an edge I update a value on state then have an event handler for mousemove which uses this value however it doesn't seem to be updating after the value has changed.
Here is my code:
export default memo(() => {
const [activePoint, setActivePoint] = useState(null); // initial is null
const handleResize = () => {
console.log(activePoint); // is null but should be 'top|bottom|left|right'
};
const resizerMouseDown = (e, point) => {
setActivePoint(point); // setting state as 'top|bottom|left|right'
window.addEventListener('mousemove', handleResize);
window.addEventListener('mouseup', cleanup); // removed for clarity
};
return (
<div className="interfaceResizeHandler">
{resizePoints.map(point => (
<div
key={ point }
className={ `interfaceResizeHandler__resizer interfaceResizeHandler__resizer--${ point }` }
onMouseDown={ e => resizerMouseDown(e, point) }
/>
))}
</div>
);
});
The problem is with the handleResize function, this should be using the latest version of activePoint which would be a string top|left|bottom|right but instead is null.

How to Fix a Stale useState
Currently, your issue is that you're reading a value from the past. When you define handleResize it belongs to that render, therefore, when you rerender, nothing happens to the event listener so it still reads the old value from its render.
There are a several ways to solve this. First let's look at the most simple solution.
Create your function in scope
Your event listener for the mouse down event passes the point value to your resizerMouseDown function. That value is the same value that you set your activePoint to, so you can move the definition of your handleResize function into resizerMouseDown and console.log(point). Because this solution is so simple, it cannot account for situations where you need to access your state outside of resizerMouseDown in another context.
See the in-scope function solution live on CodeSandbox.
useRef to read a future value
A more versatile solution would be to create a useRef that you update whenever activePoint changes so that you can read the current value from any stale context.
const [activePoint, _setActivePoint] = React.useState(null);
// Create a ref
const activePointRef = React.useRef(activePoint);
// And create our custom function in place of the original setActivePoint
function setActivePoint(point) {
activePointRef.current = point; // Updates the ref
_setActivePoint(point);
}
function handleResize() {
// Now you'll have access to the up-to-date activePoint when you read from activePointRef.current in a stale context
console.log(activePointRef.current);
}
function resizerMouseDown(event, point) {
/* Truncated */
}
See the useRef solution live on CodeSandbox.
Addendum
It should be noted that these are not the only ways to solve this problem, but these are my preferred methods because the logic is more clear to me despite some of the solutions being longer than other solutions offered. Please use whichever solution you and your team best understand and find to best meet your specific needs; don't forget to document what your code does though.

You have access to current state from setter function, so you could make it:
const handleResize = () => {
setActivePoint(activePoint => {
console.log(activePoint);
return activePoint;
})
};

useRef for the callback
A similar approach to Andria's can be taken by using useRef to update the event listener's callback itself instead of the useState value. This allows you to use many up-to-date useState values inside one callback with only one useRef.
If you create a ref with useRef and update its value to the handleResize callback on every render, the callback stored in the ref will always have access to up-to-date useState values, and the handleResize callback will be accessible to any stale callbacks like event handlers.
function handleResize() {
console.log(activePoint);
}
// Create the ref,
const handleResizeRef = useRef(handleResize);
// and then update it on each re-render.
handleResizeRef.current = handleResize;
// After that, you can access it via handleResizeRef.current like so
window.addEventListener("mousemove", event => handleResizeRef.current());
With this in mind, we can also abstract away the creation and updating of the ref into a custom hook.
Example
See it live on CodeSandbox.
/**
* A custom hook that creates a ref for a function, and updates it on every render.
* The new value is always the same function, but the function's context changes on every render.
*/
function useRefEventListener(fn) {
const fnRef = useRef(fn);
fnRef.current = fn;
return fnRef;
}
export default memo(() => {
const [activePoint, setActivePoint] = useState(null);
// We can use the custom hook declared above
const handleResizeRef = useRefEventListener((event) => {
// The context of this function will be up-to-date on every re-render.
console.log(activePoint);
});
function resizerMouseDown(event, point) {
setActivePoint(point);
// Here we can use the handleResizeRef in our event listener.
function handleResize(event) {
handleResizeRef.current(event);
}
window.addEventListener("mousemove", handleResize);
// cleanup removed for clarity
window.addEventListener("mouseup", cleanup);
}
return (
<div className="interfaceResizeHandler">
{resizePoints.map((point) => (
<div
key={point}
className={`interfaceResizeHandler__resizer interfaceResizeHandler__resizer--${point}`}
onMouseDown={(event) => resizerMouseDown(event, point)}
/>
))}
</div>
);
});

const [activePoint, setActivePoint] = useState(null); // initial is null
const handleResize = () => {
setActivePoint(currentActivePoint => { // call set method to get the value
console.log(currentActivePoint);
return currentActivePoint; // set the same value, so nothing will change
// or a different value, depends on your use case
});
};

Just small addition to the awe ChrisBrownie55's advice.
A custom hook can be implemented to avoid duplicating this code and use this solution almost the same way as the standard useState:
// useReferredState.js
import React from "react";
export default function useReferredState(initialValue) {
const [state, setState] = React.useState(initialValue);
const reference = React.useRef(state);
const setReferredState = value => {
reference.current = value;
setState(value);
};
return [reference, setReferredState];
}
// SomeComponent.js
import React from "react";
const SomeComponent = () => {
const [someValueRef, setSomeValue] = useReferredState();
// console.log(someValueRef.current);
};

For those using typescript, you can use this function:
export const useReferredState = <T>(
initialValue: T = undefined
): [T, React.MutableRefObject<T>, React.Dispatch<T>] => {
const [state, setState] = useState<T>(initialValue);
const reference = useRef<T>(state);
const setReferredState = (value) => {
reference.current = value;
setState(value);
};
return [state, reference, setReferredState];
};
And call it like that:
const [
recordingState,
recordingStateRef,
setRecordingState,
] = useReferredState<{ test: true }>();
and when you call setRecordingState it will automatically update the ref and the state.

You can make use of the useEffect hook and initialise the event listeners every time activePoint changes. This way you can minimise the use of unnecessary refs in your code.

When you need to add event listener on component mount
Use, useEffect() hook
We need to use the useEffect to set event listener and cleanup the same.
The use effect dependency list need to have the state variables which are being used in event handler. This will make sure handler don't access any stale event.
See the following example. We have a simple count state which gets incremented when we click on given button. Keydown event listener prints the same state value. If we remove the count variable from the dependency list, our event listener will print the old value of state.
import { useEffect, useState } from 'react';
function App() {
const [count, setCount] = useState(0);
const clickHandler = () => {
console.log({ count });
setCount(c => c + 1);
}
useEffect(() => {
document.addEventListener('keydown', normalFunction);
//Cleanup function of this hook
return () => {
document.removeEventListener('keydown', normalFunction);
}
}, [count])
return (
<div className="App">
Learn
<button onClick={clickHandler}>Click me</button>
<div>{count}</div>
</div>
);
}
export default App;

Update state with setState ReactJs

I'm trying to update state variable when button click.but my issue is,it's update once with correct data then again it updated with constructor defined data.
constructor(props) {
super(props);
this.state = {
popupshow: [{ check: false, id: '' }]
}
}
componentDidUpdate(prevProps, prevState) {
console.log("this.state.popupshow",this.state.popupshow)
}
Details(type){
this.state.popupshow[i].id = type
this.state.popupshow[i].check = true;
this.setState({ popupshow: this.state.popupshow });
}
render() {
return (
<a onClick={() => this.Details("Tv Series")}>Update </>
)
}
my console.log is like bellow

You should not update React state directly. You should always update/set React state via setState method.
These lines are against React principal
this.state.popupshow[i].id = type
this.state.popupshow[i].check = true;
Update your Details as follows
Details(type){
let { popupshow } = this.state;
let i = 0;
popupshow[i].id = type
popupshow[i].check = true;
this.setState({ popupshow });
}
Note I dont have idea of variable i so assumed that as 0

I think you should rewrite details functions like :
Details(type, i){
const popupDetail = Object.assign([], this.state.popupshow);
popupDetail[i].id = type
popupDetail[i].check = true;
this.setState({ popupshow: popupDetail });
}
you are setting popupshow: this.state.popupshow this is causing forceupdate which re renders the component hence its value gets reset.

I totally agree with the other answers have given for the question, however there are few things worth noting is you might wanna add the function to the context.
The argument in favour of adding these lines to the constructor is so that the new bound functions are only created once per instance of the class. You could also use
onClick={this.Details.bind(this, "Tv Series")}
or (ES6):
onClick={() => this.Details("Tv Series")}
but either of these methods will create a new function every time the component is re-rendered.
Then change the function to arrow fucntion too like
Details(type, i){
const popupDetail = Object.assign([], this.state.popupshow);
popupDetail[i].id = type
popupDetail[i].check = true;
this.setState({ popupshow: popupDetail });
}