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Access React state of function enclosed inside useRef doesn't update

I'm trying to access React state inside a function enclosed inside of useRef. However, even with a helper function bound to App to access the state, the state never updates inside of the useRef function.
getCount outside 0
// after clicking, getCount inside is still 0, even though count now equals 1
getCount outside 1
getCount inside 0
import React, { useState, useRef } from 'react'
import ReactDOM from 'react-dom'
const App = function () {
const [count, setCount] = useState(0)
const getCount = function () {
return count
}.bind(App)
console.log('getCount outside', getCount())
const onClick = useRef(() => {
console.log('getCount inside', getCount())
})
return (
<>
<div onClick={() => setCount(count + 1)}>
increment count
</div>
<div onClick={onClick.current}>
{count}
</div>
</>
)
}
const wrapper = document.getElementById('root')
ReactDOM.render(<App />, wrapper)

The argument passed to useRef is only considered when the component mounts. Only at that time is the value assigned to the ref; it won't update when the component re-renders.
When the component mounts, the count variable that the ref's function closes over is the initial state value, which is 0. No matter how many times the component re-renders, the ref's function, if not reassigned, will still close over that original state value.
If you want the ref's function to result in an up-to-date value, assign it anew each time there's a re-render.
// assign nothing notable on mount, just create a ref
const onClickRef = useRef();
// on mount and on re-render, assign a function
// with an up-to-date reference to the state variable
onClickRef.current = () => {
console.log(count);
};
Though, in React, usually it'd be better to pass the state itself down and use it, rather than a ref - refs are generally for things that you can't accomplish using the more functional tools provided by React.

Using callback with previousValue in setter solves this problem:
const onClick = useRef(() => {
console.log(count); <----- always 0 (initial state)
setCount((previousValue)=> {
console.log(previousValue); <---- correct current value each time
return previousValue+1;
}
})
Source: idea was from #bogdanoff's first comment from the question. Upvoted.

How to prevent useState variable to get reset every time my react components handles an event hook set with useEffect?

I would like a react component to handle keyboard inputs. I have attached a simplified version of my component (that shows the problem I am having) below.
The problem is: The state variable inputText does not get updated. It appears that it gets set back to the initial value "" every time I press a key. My assumption is that my component gets re-rendered every time I press a key. But why?
When using a "traditional" input element, this approach works perfectly fine. I have found many examples where the value of an input element is set to a state variable that is updated when the user types text into the input element (and the onChange event fires).
What I am doing wrong?
import './TestInput.css'
const { useState, useEffect } = require('react');
const TestInput = ( props ) => {
const [ inputText, setInputText ] = useState("");
useEffect(() => {
window.addEventListener('keypress', handleKeyPress);
console.log('Event listener added')
return () => {
window.removeEventListener('keypress', handleKeyPress);
console.log('Event listener removed');
}
}, []);
function handleKeyPress(e) {
setInputText(inputText + e.key);
}
return (
<div id="container">
<div id="input"></div>
</div>
)
}
export default TestInput;

Because you are using useEffect with empty array dependencies so window.addEventListener only call once time with handleKeyPress in the first render. And in this function, you are using directly state inputText and it still has value is the initial value '' and doesn't update to new state when event keypress was called.
The easiest way to fix is you don't use directly state inputText and pass the function to setInputText:
function handleKeyPress(e) {
setInputText(preState => preState + e.key);
}

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;

React hooks vs eventListener

So... I was trying useEffect but I found a strange behavior.
I have a state in a dumb component.
I call useEffect and inside of it I add a new eventListener.
This event listener has to change the state given a condition.
Problem is the state never changes...
Ideas?
const componentToRender=()=>{
const [renderStatus, changeRenderStatus]=useState(false);
const [transitionStatus, changeTransitionStatus]=useState(false);
if(!renderStatus){
useEffect(()=>{
window.addEventListener("transitionend",(event)=>{
if(event.propertyName==="width"){
changeTransitionStatus(transitionStatus?false:true);
}
})
})
changeRenderStatus(true)
}
return (transitionStatus)?<div> First case </div>:<div> Second case</div>
}
there's another function with some DOM manipulation onMouseOver.
This function should change the state from the event listener but it doesn't.

you can't use hooks inside a if statement, see hooks-rules
you should return a clean up function from your useEffect hooks to remove the event listener and avoid memory leaks
you probably want the effect to run only once, so provide an empty array as second argument to useEffect (I don't think you need renderStatus)
inside the useEffect, when calling a state setter, prefer the functional form so that you always have a fresh state value.
example
const componentToRender = () => {
//const [renderStatus, changeRenderStatus] = useState(false);
const [transitionStatus, changeTransitionStatus] = useState(false);
// No condition
useEffect(() => {
const handler = (event) => {
if (event.propertyName === "width") {
//passe a function to state setter to get fresh state value
changeTransitionStatus(transitionStatus => transitionStatus ? false : true);
}
};
window.addEventListener("transitionend", handler);
// clean up
return () => window.removeEventListener("transitionend", handler);
}, []); // empty array => run only once
return (transitionStatus) ? <div> First case </div> : <div> Second case</div>
}