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Value of variable outside of useEffect hook has old data

What the code does: It's performing a DOM search based on what's typed in an input (it's searching elements by text). All this is happening in a React component.
import { useEffect, useReducer } from "react";
let elements: any[] = [];
const App = () => {
const initialState = { keyEvent: {}, value: "Initial state" };
const [state, updateState] = useReducer(
(state: any, updates: any) => ({ ...state, ...updates }),
initialState
);
function handleInputChange(event: any) {
updateState({ value: event.target.value });
}
function isCommand(event: KeyboardEvent) {
return event.ctrlKey;
}
function handleDocumentKeyDown(event: any) {
if (isCommand(event)) {
updateState({ keyEvent: event });
}
}
useEffect(() => {
document.addEventListener("keydown", handleDocumentKeyDown);
return () => {
document.removeEventListener("keydown", handleDocumentKeyDown);
};
}, []);
useEffect(() => {
const selectors = "button";
const pattern = new RegExp(state.value === "" ? "^$" : state.value);
elements = Array.from(document.querySelectorAll(selectors)).filter(
(element) => {
if (element.childNodes) {
const nodeWithText = Array.from(element.childNodes).find(
(childNode) => childNode.nodeType === Node.TEXT_NODE
);
if (nodeWithText) {
// The delay won't happenn if you comment out this conditional statement:
if (nodeWithText.textContent?.match(pattern)) {
return element;
}
}
}
}
);
console.log('elements 1:', elements)
}, [state]);
console.log('elemets 2:', elements)
return (
<div>
<input
id="input"
type="text"
onChange={handleInputChange}
value={state.value}
/>
<div id="count">{elements.length}</div>
<button>a</button>
<button>b</button>
<button>c</button>
</div>
);
};
export default App;
The problem: The value of elements outside of useEffect is the old data. For example, if you type a in the input, console.log('elements 1:', elements) will log 1, and console.log('elements 2:', elements) will log 0. Note: there are 3 buttons, and one of them has the text a.
The strange thing is that the problem doesn't happen if you comment out this if-statement:
// The delay won't happenn if you comment out this conditional statement:
if (nodeWithText.textContent?.match(pattern)) {
return element;
}
In this case, if you type anything (since the pattern matching has been commented out), console.log('elements 1:', elements) and console.log('elements 2:', elements) will log 3. Note: there are 3 buttons.
Question: What could be the problem, and how to fix it? I want to render the current length of elements.
Live code:

It's happening because of the elements variable is not a state, so it's not reactive.
Create a state for the elements:
const [elements, setElements] = useState<HTMLButtonElement[]>([])
And use this state to handle the elements.
import { useEffect, useReducer, useState } from "react";
const App = () => {
const initialState = { keyEvent: {}, value: "Initial state" };
const [state, updateState] = useReducer(
(state: any, updates: any) => ({ ...state, ...updates }),
initialState
);
const [elements, setElements] = useState<HTMLButtonElement[]>([])
function handleInputChange(event: any) {
updateState({ value: event.target.value });
}
function isCommand(event: KeyboardEvent) {
return event.ctrlKey;
}
function handleDocumentKeyDown(event: any) {
if (isCommand(event)) {
updateState({ keyEvent: event });
}
}
useEffect(() => {
document.addEventListener("keydown", handleDocumentKeyDown);
return () => {
document.removeEventListener("keydown", handleDocumentKeyDown);
};
}, []);
useEffect(() => {
const selectors = "button";
const pattern = new RegExp(state.value === "" ? "^$" : state.value);
let newElements = Array.from(document.querySelectorAll(selectors)).filter(
(element) => {
if (element.childNodes) {
const nodeWithText = Array.from(element.childNodes).find(
(childNode) => childNode.nodeType === Node.TEXT_NODE
);
if (nodeWithText) {
// The delay won't happenn if you comment out this conditional statement:
if (nodeWithText.textContent?.match(pattern)) {
return element;
}
}
}
}
);
setElements(newElements)
console.log("elements 1:", elements?.length);
}, [state]);
console.log("elemets 2:", elements?.length);
return (
<div>
<input
id="input"
type="text"
onChange={handleInputChange}
value={state.value}
/>
<div id="count">{elements?.length}</div>
<button>a</button>
<button>b</button>
<button>c</button>
</div>
);
};
export default App;

Your useEffect() runs after your component has rendendered. So the sequence is:
You type something into input, that triggers handleInputChange
handleInputChange then updates your state using updateState()
The state update causes a rerender, so App is called App()
console.log('elemets 2:', elements.length) runs and logs elements as 0 as it's still empty
App returns the new JSX
Your useEffect() callback runs, updating elements
Notice how we're only updating the elements after you've rerendered and App has been called.
The state of your React app should be used to describe your UI in React. Since elements isn't React state, it has a chance of becoming out of sync with the UI (as you've seen), whereas using state doesn't have this issue as state updates always trigger a UI update. Consider making elements part of your state. If it needs to be accessible throughout your entire App, you can pass it down as props to children components, or use context to make it accessible throughout all your components.
With that being said, I would make the following updates:
Add elements to your state
Remove your useEffect() with the dependency of [state]. If we were to update the elements state within this effect, then that would trigger another rerender directly after the one we just did for the state update. This isn't efficient, and instead, we can tie the update directly to your event handler. See You Might Not Need an Effect for more details and dealing with other types of scenarios:
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/18.2.0/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/18.2.0/umd/react-dom.production.min.js"></script>
<script src="https://unpkg.com/#babel/standalone/babel.min.js"></script>
<script type="text/babel">
const { useEffect, useReducer} = React;
const App = () => {
const initialState = {keyEvent: {}, value: "Initial state", elements: []};
const [state, updateState] = useReducer(
(state: any, updates: any) => ({ ...state, ...updates}),
initialState
);
function searchDOM(value) {
const selectors = "button";
const pattern = new RegExp(value === "" ? "^$" : value);
return Array.from(document.querySelectorAll(selectors)).filter(
(element) => {
if (element.childNodes) {
const nodeWithText = Array.from(element.childNodes).find(
(childNode) => childNode.nodeType === Node.TEXT_NODE
);
return nodeWithText?.textContent?.match(pattern);
}
return false;
}
);
}
function handleInputChange(event) {
updateState({
value: event.target.value,
elements: searchDOM(event.target.value)
});
}
function isCommand(event) {
return event.ctrlKey;
}
function handleDocumentKeyDown(event) {
if (isCommand(event)) {
updateState({
keyEvent: event
});
}
}
useEffect(() => {
document.addEventListener("keydown", handleDocumentKeyDown);
return () => {
document.removeEventListener("keydown", handleDocumentKeyDown);
};
}, []);
console.log("elements:", state.elements.length);
return (
<div>
<input id="input" type="text" onChange={handleInputChange} value={state.value} />
<div id="count">{state.elements.length}</div>
<button>a</button>
<button>b</button>
<button>c</button>
</div>
);
};
ReactDOM.createRoot(document.body).render(<App />);
</script>

useEffect triggered after react completed its render phase & flush the new changes to the DOM.
In your case you have two useEffects. The first one register your event lister which will then update your component state when input field change. This triggers a state update.( because of the setState )
So React will start render the component again & finish the cycle. And now you have 2nd useEffect which has state in dependency array. Since the state was updated & the new changes are committed to the DOM, react will execute 2nd useEffect logic.
Since your 2nd useEffect just assign some values to a normal variable React will not go re render your component again.
Based on your requirement you don't need a 2nd useEffect. You can use a useMemo,
let elements = useMemo(() => {
const selectors = "button";
const pattern = new RegExp(state.value === "" ? "^$" : state.value);
return Array.from(document.querySelectorAll(selectors)).filter(
(element) => {
if (element.childNodes) {
const nodeWithText = Array.from(element.childNodes).find(
(childNode) => childNode.nodeType === Node.TEXT_NODE
);
if (nodeWithText) {
// The delay won't happenn if you comment out this conditional statement:
if (nodeWithText.textContent?.match(pattern)) {
return element;
}
}
}
})
}, [state])
Note: You don't need to assign your elements into another state. It just create another unwanted re render in cycle. Since you are just doing a calculation to find out the element array you can do it with the useMemo

How to map an array of objects in React useReducer function

I have an array of objects in my React state. I want to be able to map through them, find the one I need to update and update its value field. The body of my request being sent to the server should look like:
{ name: "nameOfInput", value:"theUserSetValue" type: "typeOfInput" }
What I thought would be simple is causing me some heartache. My reducer function calls, and I hit the "I AM RUNNING" log where it then jumps over my map and simply returns my state (which is empty). Please note that I NEVER see the "I SHOULD RETURN SOMETHING BUT I DONT" log.
NOTE: I have learned that I could be simply handingling this with useState
function Form(props) {
const title = props.title;
const paragraph = props.paragraph;
const formBlocks = props.blocks.formBlocks
const submitEndpoint = props.blocks.submitEndpoint || "";
const action = props.blocks.action || "POST";
const formReducer = (state, e) => {
console.log("I AM RUNNING")
state.map((obj) => {
console.log("I SHOULD RETURN SOMETHING BUT I DONT")
if (obj.name === e.target.name) {
console.log("OBJ EXISTS", obj)
return {...obj, [e.target.name]:obj.value}
} else {
console.log("NO MATCH", obj)
return obj
}
});
return state
}
const [formData, setFormData] = useReducer(formReducer, []);
const [isSubmitting, setIsSubmitting] = useState(false);
=====================================================================
Where I am calling my reducer from:
<div className="form-block-wrapper">
{formBlocks.map((block, i) => {
return <FormBlock
key={block.title + i}
title={block.title}
paragraph={block.paragraph}
inputs={block.inputs}
buttons={block.buttonRow}
changeHandler={setFormData}
/>
})}
</div>

Issues
When using the useReducer hook you should dispatch actions to effect changes to the state. The reducer function should handle the different cases. From what I see of the code snippet it's not clear if you even need to use the useReducer hook.
When mapping an array not only do you need to return a value for each iterated element, but you also need to return the new array.
Solution
Using useReducer
const formReducer = (state, action) => {
switch(action.type) {
case "UPDATE":
const { name, value } = action.payload;
return state.map((obj) => obj.name === name
? { ...obj, [name]: value }
: obj
);
default:
return state;
}
};
...
const [formData, dispatch] = useReducer(formReducer, []);
...
{formBlocks.map((block, i) => {
return (
<FormBlock
key={block.title + i}
title={block.title}
paragraph={block.paragraph}
inputs={block.inputs}
buttons={block.buttonRow}
changeHandler={e => dispatch({
type: "UPDATE",
payload: {...e.target}
})}
/>
);
})}
Using useState
const [formData, setFormData] = useState([]);
...
const changeHandler = e => {
const { name, value } = e.target;
setFormData(data => data.map(obj => obj.name === name
? { ...obj, [name]: value }
: obj
));
};
...
{formBlocks.map((block, i) => {
return (
<FormBlock
key={block.title + i}
title={block.title}
paragraph={block.paragraph}
inputs={block.inputs}
buttons={block.buttonRow}
changeHandler={changeHandler}
/>
);
})}

I have come to understand my problem much better now and I'll update my question to reflect this.
As the user interacted with an input I needed to figure out if they had interacted with it before
If they did interact with it before, I needed to find that interaction in the state[] and update the value as required
If they didn't I needed to add an entirely new object to my forms state[]
I wrote two new functions, an AddObjectToArray function and an UpdateObjectInArray function to serve these purposes.
const handleFormInputChange = (e) => {
const { name, value, type } = e.target;
const addObjectToArray = (obj) => {
console.log("OBJECT TO BE ADDED TO ARRAY:", obj)
setFormData(currentArray => ([...currentArray, obj]))
}
const updateObjectInArray = () => {
const updatedObject = formData.map(obj => {
if (obj.name === name) {
//If the name matches, Update the value of the input
return ({...obj, value:value})
}
else {
//if no match just return the object as is
return obj
}
})
setFormData(updatedObject)
}
//Check if the user has already interacted with this input
if (formData.find(input => input.name === name)) {
updateObjectInArray()
}
else {
addObjectToArray({name, value, type})
}
}
I could get more complicated with this now and begin to write custom hooks that take a setState function as a callback and the data to be handled.

Component keeps re-rendering after state change

I use handleMouseOver and handleMouseOut functions where I change the value of a state count. However, every time the state is changed the component re-renders instead of just the state. What am I missing here? Thanks.
function foo() {
const [state, setState] = useState({ count: 0, data: {}});
useEffect(() => {
const getData = async () => {
const response = await fetch(url);
const data = await response.json();
setState(prevState => ({
...prevState,
data: data,
}));
};
return ()=>{
getData();
}
}, []);
function handleMouseOver(e) {
setState(prevState => ({
...prevState,
count: e,
}));
};
function handleMouseLeave() {
setState(prevState => ({
...prevState,
count: null,
}));
};
const { count, data } = state;
const BlockComponent = () => {
const data = data.arr;
return (
<Wrapper >
{data.map((value, index) =>
value.map((value, index) => {
return (
<Block
key={index}
val={value}
onMouseEnter={e => handleMouseOver(value)}
onMouseOut={handleMouseLeave}
></Block>
);
})
)}
</Wrapper>
);
};
return (
<Wrapper>
<BlockComponent />
</Wrapper>
);
}
export default foo;

The Issue is with your handleMouseOver function. It is getting executed everytime there is a state Update and the same value is assigned to "count".
All you have to do is place setState inside the condition that will compare the value of event received by the function and the current value of sate.
It should be something like this.
function handleMouseOver(e) {
if (count !== e) {
setState((prevState) => ({
...prevState,
count: e,
}));
}
}

React updates component if component state is changed. That's correct behaviour.
I recommend you to learn react documentation, because component state is a basic concept.

That's one of the main points of state -> component is rerendered when you change state.

Input element losing it's focus after key press when it's controlled from outside the Modal (which uses Portal)

[Solved] My input component is losing focus as soon as I press any key only when its value is controlled from outside the portal
NOTE: I am sorry. While writing this, I found the problem in my code, but I decided to post this anyway
[Reason] I was inlining the close function, so the useEffect hook got triggered every time close changed when the component was rendered again due to state changes and thus calling the activeElement.blur() on each keystroke.
Portal
const root = document.getElementById('root')
const modalRoot = document.getElementById('modal-root')
const Portal = ({ children, className, drawer = false }) => {
const element = React.useMemo(() => document.createElement('div'), [])
React.useEffect(() => {
element.className = clsx('modal', className)
modalRoot.appendChild(element)
return () => {
modalRoot.removeChild(element)
}
}, [element, className])
return ReactDOM.createPortal(children, element)
}
Modal
const Modal = (props) => {
const { children, show = false, close, className } = props
const backdrop = React.useRef(null)
const handleTransitionEnd = React.useCallback(() => setActive(show), [show])
const handleBackdropClick = React.useCallback(
({ target }) => target === backdrop.current && close(),
[]
)
const handleKeyUp = React.useCallback(
({ key }) => ['Escape'].includes(key) && close(),
[]
)
React.useEffect(() => {
if (backdrop.current) {
window.addEventListener('keyup', handleKeyUp)
}
if (show) {
root.setAttribute('inert', 'true')
document.body.style.overflow = 'hidden'
document.activeElement.blur?.() // ! CULPRIT
}
return () => {
root.removeAttribute('inert')
document.body.style.overflow = 'auto'
window.removeEventListener('keyup', handleKeyUp)
}
}, [show, close])
return (
<>
{show && (
<Portal className={className}>
<div
ref={backdrop}
onClick={handleBackdropClick}
onTransitionEnd={handleTransitionEnd}
className={clsx('backdrop', show && 'active')}>
<div className="content">{children}</div>
</div>
</Portal>
)}
</>
)
}
Custom Textfield
const TextField = React.forwardRef(
({ label, className, ...props }, ref) => {
return (
<div className={clsx('textfield', className)}>
{label && <label>{label}</label>}
<input ref={ref} {...props} />
</div>
)
}
)

I was inlining the close function, so the useEffect hook got triggered every time close changed when the component was rendered again due to state changes and thus calling the activeElement.blur() on each keystroke.
In Modal.jsx
...
React.useEffect(() => {
...
if (show) {
root.setAttribute('inert', 'true')
document.body.style.overflow = 'hidden'
document.activeElement.blur?.() // ! CULPRIT
}
...
}, [show, close]) // as dependency
...
<Modal
show={show}
close={() => setShow(false)} // this was inlined
className="some-modal"
>
...
</Modal>
TAKEAWAY
Do not inline functions
Usually there is no reason to pass a function (pointer) as dependency

State of parent in button in child component always null

I have table with multiple columns. Each column can have modal for filtering.
I want only one modal to be visible at a time.
I pass state of component and setter of state to child component.
I can open it on click on button and I want to be able to close it by clicking same button.
const Parent = () => {
const [search, setSearch] = React.useState(null)
return
{columns.map(x => {
<...>
<Child showSearch={search} setSearch={setSearch} column={x} />
</...>
})}
}
const Child = ({ showSearch, setSearch, column }) => {
const isCurrentShowed = showSearch === column.id
const escFunction = useCallback((event) => {
if (event.keyCode === 27) {
setSearch(null);
}
}, [setSearch]);
const toggleSearch = () => {
console.log("Search variable is always null)
setSearch(showSearch === column.id ? null : column.id)
}
useEffect(() => {
document.addEventListener('keydown', escFunction, false);
return () => {
document.removeEventListener('keydown', escFunction, false);
};
}, [escFunction]);
useEffect(() => {
if (isCurrentShowed && inputRef && inputRef.current) {
inputRef.current.focus();
}
}, [isCurrentShowed]);
return (
<div>
<button onClick={toggleSearch}>
button
</button>
{isCurrentShowed && (
<div>
{column.render('Filter', { inputRef: inputRef})}
</div>
)}
</div>
);
}
In child component variable showSearch is available and I see changed value.
In toggleSearch funtion variable showSearch is always null and I'm not sure why.

You are losing this reference
Swap this line:
<button onClick={toggleSearch}>
With this:
<button onClick={toggleSearch.bind(this)}>
More about context: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this

I think column value is not properly passed to child.
const Parent = () => {
const [search, setSearch] = React.useState(null)
return
{columns.map(x => {
<...>
<Child showSearch={search} setSearch={setSearch} column={x} />
</...>
})}
}

setSearch(showSearch === column.id ? null : column.id)
Care this part. Are you sure of that column.id is not null?
If it is null then always provides the equality of null === null so ternary returns null again.