Develop Reference

React: Handling focus using array of useRef

I have created a basic Todo app but having a hard time in implementing proper focus into it. Following are the requirements for focus:
Focus should be on the newly created input field when "Add" is clicked so the user can begin typing right away.
On deleting an item "Button X", focus will move to the input field in the row which replaced the deleted row, nowhere if there are no fields left, or on the new last row if the last row was deleted.
On moving an item up, focus should be placed on the newly moved field, and all associated buttons should move alongside the element. If a field is already at the top of a list, no reordering should occur, but focus should be transferred to the topmost field nonetheless.
On moving an item down, same principle should apply here (focus should be placed on the field that is moved down). If its the last field, focus it nonetheless.
Here is my implementation.
App.js:
import React, { useState, useRef } from "https://cdn.skypack.dev/react#17.0.1";
import ReactDOM from "https://cdn.skypack.dev/react-dom#17.0.1";
const App = () => {
const [myRows, setMyRows] = useState([]);
const focusInput = useRef([]);
const onAddRow = () => {
setMyRows((prevRows) => {
return [
...prevRows,
{ id: prevRows.length, text: "", up: "↑", down: "↓", delete: "X" },
];
});
focusInput.current[myRows.length - 1].focus();
};
const onMoveUp = (index) => (event) => {
const currentState = [...myRows];
if (index !== 0) {
const prevObject = currentState[index - 1];
const nextObject = currentState[index];
currentState[index - 1] = nextObject;
currentState[index] = prevObject;
setMyRows(currentState);
}
};
const onMoveDown = (index) => (event) => {
const currentState = [...myRows];
if (index !== myRows.length - 1) {
const currObject = currentState[index];
const nextObject = currentState[index + 1];
currentState[index] = nextObject;
currentState[index + 1] = currObject;
setMyRows(currentState);
}
};
const onDelete = (index) => (event) => {
const currentState = [...myRows];
currentState.splice(index, 1);
setMyRows(currentState);
};
const onTextUpdate = (id) => (event) => {
setMyRows((prevState) => {
const data = [...prevState];
data[id] = {
...data[id],
text: event.target.value,
};
return data;
});
};
return (
<div className="container">
<button onClick={onAddRow}>Add</button>
<br />
{myRows?.map((row, index) => {
return (
<div key={row.id}>
<input
ref={(el) => (focusInput.current[index] = el)}
onChange={onTextUpdate(index)}
value={row.text}
type="text"></input>
<button onClick={onMoveUp(index)}>{row.up}</button>
<button onClick={onMoveDown(index)}>{row.down}</button>
<button onClick={onDelete(index)}>{row.delete}</button>
</div>
);
})}
</div>
);
}
ReactDOM.render(<App />,
document.getElementById("root"))
In my implementation when I click Add, focus changes but in unexpected way (First click doesn't do anything and subsequent click moves the focus but with the lag, i.e. focus should be on the next item, but it is at the prior item.
Would really appreciate if someone can assist me in this. Thanks!

The strange behaviour that you observe is caused by the fact that state updates are asynchronous. When you, in onAddRow, do this:
focusInput.current[myRows.length - 1].focus()
then myRows.length - 1 is still the last index of the previous set of rows, corresponding to the penultimate row of what you're actually seeing. This explains exactly the behaviour you're describing - the new focus is always "one behind" where it should be, if all you're doing is adding rows.
Given that description, you might think you could fix this by just replacing myRows.length - 1 with myRows.length in the above statement. But it isn't so simple. Doing this will work even less well, because at the point this code runs, right when the Add button is clicked, focusInput hasn't yet been adjusted to the new length, and nor in fact has the new row even been rendered in the DOM yet. That all happens a little bit later (although appears instantaneous to the human eye), after React has realised there has been a state change and done its thing.
Given that you are manipulating the focus in a number of different ways as described in your requirements, I believe the easiest way to fix this is to make the index you want to focus its own piece of state. That makes it quite easy to manage focus in any way you want, just by calling the appropriate state-updating function.
This is implemented in the code below, which I got working by testing it out on your Codepen link. I've tried to make it a snippet here on Stack Overflow, but for some reason couldn't get it to run without errors, despite including React and enabling Babel to transform the JSX - but if you paste the below into the JS of your Codepen, I think you'll find it working to your satisfaction. (Or, if I've misinterpreted some requirements, hopefully it gets you at least a lot closer than you were.)
Rather than just leaving you to study the code yourself though, I'll explain the key parts, which are:
the introduction of that new state variable I just mentioned, which I've called focusIndex
as mentioned, the calling of setFocusIndex with an appropriate value whenever rows are added, removed or moved. (I've been trying to follow your requirements here and it seems to work well to me, but as I said, I may have misunderstood.)
the key is the useEffect which runs whenever focusIndex updates, and does the actual focusing in the DOM. Without this, of course, the focus will never be updated on calling setFocusIndex, but with it, calling that function will "always" have the desired effect.
one last subtlety is that the "always" I put above is not strictly true. The useEffect only runs when focusIndex actually changes, but when moving rows there are some situations where it is set to the same value it had before, but where you still want to move focus. I found this happening when clicking outside the inputs, then moving the first field up or the last one down - nothing happened, when we want the first/last input to be focused. This was happening because focusIndex was being set to the value it already had, so the useEffect didn't run, but we still wanted it to in order to set the focus. The solution I came up with was to add an onBlur handler to each input to ensure that the focus index is set to some "impossible" value (I chose -1, but something like null or undefined would have worked fine as well) when focus is lost - this may seem artificial but actually better represents the fact that when the focus is on no inputs, you don't want to have a "sensible" focusIndex, otherwise the React state is saying one of the inputs is focused, when none are. Note that I also used -1 for the initial state, for much the same reason - if it starts at 0 then adding the first row doesn't cause focus to change.
I hope this helps and my explanations are clear enough - if you're confused by anything, or notice anything going wrong with this implementation (I confess I have not exactly tested it to destruction), please let me know!
import React, { useState, useRef, useEffect } from "https://cdn.skypack.dev/react#17.0.1";
import ReactDOM from "https://cdn.skypack.dev/react-dom#17.0.1";
const App = () => {
const [myRows, setMyRows] = useState([]);
const focusInput = useRef([]);
const [focusIndex, setFocusIndex] = useState(-1);
const onAddRow = () => {
setMyRows((prevRows) => {
return [
...prevRows,
{ id: prevRows.length, text: "", up: "↑", down: "↓", delete: "X" },
];
});
setFocusIndex(myRows.length);
};
useEffect(() => {
console.log(`focusing index ${focusIndex} in`, focusInput.current);
focusInput.current[focusIndex]?.focus();
}, [focusIndex]);
const onMoveUp = (index) => (event) => {
const currentState = [...myRows];
if (index !== 0) {
const prevObject = currentState[index - 1];
const nextObject = currentState[index];
currentState[index - 1] = nextObject;
currentState[index] = prevObject;
setMyRows(currentState);
setFocusIndex(index - 1);
} else {
setFocusIndex(0);
}
};
const onMoveDown = (index) => (event) => {
const currentState = [...myRows];
if (index !== myRows.length - 1) {
const currObject = currentState[index];
const nextObject = currentState[index + 1];
currentState[index] = nextObject;
currentState[index + 1] = currObject;
setMyRows(currentState);
setFocusIndex(index + 1);
} else {
setFocusIndex(myRows.length - 1);
}
};
const onDelete = (index) => (event) => {
const currentState = [...myRows];
currentState.splice(index, 1);
setMyRows(currentState);
const newFocusIndex = index < currentState.length
? index
: currentState.length - 1;
setFocusIndex(newFocusIndex);
};
const onTextUpdate = (id) => (event) => {
setMyRows((prevState) => {
const data = [...prevState];
data[id] = {
...data[id],
text: event.target.value,
};
return data;
});
};
return (
<div className="container">
<button onClick={onAddRow}>Add</button>
<br />
{myRows?.map((row, index) => {
return (
<div key={row.id}>
<input
ref={(el) => (focusInput.current[index] = el)}
onChange={onTextUpdate(index)}
onBlur={() => setFocusIndex(-1)}
value={row.text}
type="text"></input>
<button onClick={onMoveUp(index)}>{row.up}</button>
<button onClick={onMoveDown(index)}>{row.down}</button>
<button onClick={onDelete(index)}>{row.delete}</button>
</div>
);
})}
</div>
);
}
ReactDOM.render(<App />,
document.getElementById("root"))

Update DOM elements when state is updated using setTimeout() React

I have the following code in React. This code uses a time expensive function that performs some coputations over a large number of data. in the begining the state is filled with an array that says no computations. When the button is pressed, I want that array of the state to be filled with waiting, and after each iteration over i is complete, the element on potition i to be 'Done'. The problem is that by using set timeout, the DOM elements are not updating untill the whole function is over. Ignore the computations in the for, they are meaningless in this example. To have an ideea the function performs polynomial fit over the given data. My problem is how can I update the Dom elements while the function is running, So i can knwow which data is done and which is computing in real time
const Component = ( {data1, data2} ) => {
const [state, setState] = useState(Array(data1.length).fill('No Computations'))
const timeExpensiveFunction = () => {
// data1.length = 10
let variable0
let variable1
let variable2
let variable3
for(let i = 0; i< data1.length; i++){
//data1[i].length = 40 000
for(let j = 0; j< data1[i].length; j++){
variable1 += data1[i][j]
variable2 += variable1*data2[i][j]**3
variable3 += data2[i][j]**2
variable1 += data1[i][j]*data2[i][j]
}
setTimeout(() => {
setState(state => {
return[
...state.slice(0,i),
'Done',
...state.slice(i+1),
]
})
},1000)
}
return([variable1,variable2,variable3,variable4])
}
const randomFunc = (e) => {
setTimeout((e) => setState(Array(data1.length).fill('Waiting ...')),1000)
timeExpensiveFunction() //Duration depends on the number of data, it is usually higher that 10-15 seconds
}
return (
<div>
<div>
{state.map((element) => (
{element} //This does not update with the function setTimeout. It sets after the expensive computation is over
))}
</div>
<div>
<button onClick = {(e) => randomFunc(e)}>press</button>
</div>
</div>
)
}

First of all, You must use useState and useEffect to set a state and re-render the DOM.
You can use like below:
import { useState, useEffect } from 'react'
const Component = () => {
const [state, setState] = useState([1,2,3,4,5,6,7,8,9,10])
const [isPress, setPress] = useState(false); // use it to not call a method on first render
const timeExpensiveFunction = () => {
// lot of work here
}
const randomFunc = (e) => {
setState([10,9,8,7,6,5,4,3,2,1])
setPress(true)
}
useEffect({
if (isPress) setTimeout(() => timeExpensiveFunction(),1000)
}, [state])
return (
<div>
<div>
{state.map((element) => (
{element}
))}
</div>
<div>
<button onClick = {(e) => randomFunc(e)}>press</button>
</div>
</div>
)
}

How to change showing data?

I take datas from json file and there is so much data inside because of this I don't want to show all. In the page I want to show just 20 data and when I click NEXT button i want to see another 20 data. It's working but in the filter function it update filteredDatas but don't update showingDatas
import data from './data.json'
var i = 0
var j = 20
function App() {
const [showingDatas, setShowingDatas] = useState([])
const [filteredDatas, setFilteredDatas] = useState([])
const getDatas = () =>{
setFilteredDatas(data)
setShowingDatas(data.slice(0,20))
}
useEffect(()=>{
getDatas()
},[])
const next = () => {
i += 20;
j += 20;
setShowingDatas(filteredDatas.slice(i,j))
}
const filter = () => {
setFilteredDatas(data.filter(item => item.productCompany === "Mavi"))
i = 0
j = 20
setShowingDatas(filteredDatas.slice(0,20))
}

You can not do this like that, because of React working principles. But if you want to do that I suggest you use useEffect to keep filteredData changes, then in useEffect set it. Like:
useEffect(() => {
setShowingDatas(filteredDatas)
}, [filteredDatas]);
If you really want to do with useState I suggest you search useState with callback.

React useState not updating because of useRef

I am experiencing a very odd issue with my react code : useState isn't updating the view and after literally trying everything the issue is still there. I made a simple code to explain the issue :
function(){
const [enterJob, setEnterJob] = useState(false);
const [jobSelection, setJobSelection] = useState(Array(someList.length).fill(false));
const jobRef = useRef();
const handleJobClick = i => {
const n = parseInt(i.target.id.charAt(0)); // the list is small enough to allow this
let c = jobSelection;
c[n] = !c[n];
setJobSelection(c);
};
const handleMouse = (e) =>{
if (!jobRef.current.contains(e.target)){
setEnterJob(false);
};
};
useEffect(() => {
window.addEventListener("mousedown", handleMouse);
return () => window.removeEventListener("mousedown", handleMouse);
});
return(
<div ref={jobRef}>
<input onFocus={()=> setEnterJob(true)} />
<div style={{display: `${enterJob ? 'flex' : 'none'}`}} >
<ul>
{ someList.map((item,index)=>
<li id={`${index}`} onClick={handleJobClick}> {jobSelection[index] ? item : "you clicked on the button"} </li> )}
</ul>
</div>
</div>
)
}
Some explanations: I am using UseEffect and useRef to create a dropDown menu that disappears when you clic outside the container. Now when I want to clic on a value of this drop-down menu it doesn't update the DOM while I am using useState to update the value of the string responsible for the change.
Thank you in advance,
Charbel

The problem is that you are mutatiing your jobSelection instead of creating a new object. And react will skip the rerender if the the objects has the same reference as before:
const handleJobClick = i => {
const n = parseInt(i.target.id.charAt(0)); // the list is small enough to allow this
let c = [...jobSelection]; // Create a new array
c[n] = !c[n];
setJobSelection(c);
};

Issues
If I understand your issue then I believe it is because you are directly mutating your state.
const handleJobClick = i => {
const n = parseInt(i.target.id.charAt(0)); // the list is small enough to allow this
let c = jobSelection;
c[n] = !c[n]; // <-- mutation!
setJobSelection(c);
};
You are also missing react keys on the mapped list items.
Solution
Since the next state depends on the previous state you should use a functional state update to copy your state first, then update it.
I suggest:
converting handleJobClick to consume the index directly, a curried function handles this cleanly
Add a react key to the mapped list items
Code
const handleJobClick = index => () => {
setJobSelection(jobSelection => jobSelection.map(
(selection, i) => index === i ? !selection : selection // <-- toggle selection at matched index
);
};
...
<ul>
{someList.map((item, index)=> (
<li
key={index} // <-- index as react key, ok since not adding/removing/sorting jobs
onClick={handleJobClick(index)} // <-- pass index to handler
>
{jobSelection[index] ? item : "you clicked on the button"}
</li>
))}
</ul>

Getting Blank Array on First Click?

I am stuck with a Reactjs problem. The problem is mentioned in App.js file. Please look into this. Initially all buttons are White and not clickable, how can I change there color after some time. Should I use useEffect hook or some other way. I am not getting the index of button randomly to change their color and isDisable.
Three Type of Button
White: initially all buttons are white and not clickable.
Red: Randomly from the white button, One button will be red and clickable.
Blue: Clicked Button. After clicking on the red button color will change to blue and not clickable.
When all white buttons become blue, give custom popup in the center of the screen with the icon "You won game…!!!"
I am trying below code to change button color randomly, but fails.
const changeColorHandle = () => {
const rand = Math.floor(Math.random() * 4);
btnArray.map((i, k) => {
if (i.index === rand) {
return ([...btnArray, { color: 'red', isDisable: false }])
}
return [...btnArray]
})
console.log(rand)
};
useEffect(() => {
if (btnArray.length > 0) {
setTimeout(() => {
changeColorHandle()
}, 2000);
}
}, [btnArray])
Can anybody please solve this problem Here. And show me where I am doing it wrong.
Thanks.

One thing you don't want to really mess with the new react hooks is that they are not really applying changes synchronously, that way sometimes you can have a little delay.
I don't think it's the issue here but you can have a more efficient code that also solve the problem :)
Code Sandbox is here
Raw snippet with lines changed is here:
import React, { useState } from "react";
import Button from "./Button";
const Game = () => {
const [buttonValue, setButtonValue] = useState(0);
const [btnArray, setBtnArray] = useState([]);
const handleChange = e => {
setButtonValue(parseInt(e.target.value, 10));
};
const onClickHandle = () => {
const tmpArray = [];
for (let i = 0; i < buttonValue; i++) {
tmpArray.push({ color: "white", isDisable: true });
}
setBtnArray(tmpArray);
};
const changeColorHandle = () => {
alert("clicked");
};
(rest is ok 👍)
So as you can see, buttonValue is removed, and the job of the onClick is just to generate a new array according to the actual value of the input.
You don't get any issue with the user playing with the input value as the btnArray will only be rendered once he has clicked the button :)
(rendering triggered by the state change done with setBtnArray()
Edit: One nice sweet addition could be getting rid of buttonValue as a string as you may use math operations on it rather than using string conversion back and forth

Essentially, you're setting multiple states very quickly in succession. By the time you get to your loop, your buttonCount state is behind and thus the loop where you're pushing values onto btnArray doesn't really run (ends immediately).
Quick aside: You're attempting to mutate btnArray directly inside the loop, don't do that if possible. Create a new Array, push values onto that and then setBtnArray to the new Array.
This works:
const onClickHandle = () => {
if (btnArray.length > 0) {
setBtnArray([]);
}
let newBtnArray = [];
for (let i = 0; i < parseInt(buttonValue, 10); i++) {
newBtnArray.push({ color: "blue", isDisable: true });
}
setBtnArray(newBtnArray); };
I threw out btnCount here, but if you need it for something else, you can set it seperately.

I have figured out your issue, you are setting the button count after clicking so it is running one step behind the array values, and button color change function is also added, check this: CodeSandBox: https://codesandbox.io/s/dreamy-ganguly-e805g?file=/src/components/Game.js
import React, { useState, useEffect } from "react";
import Button from "./Button";
const Game = () => {
const [buttonValue, setButtonValue] = useState("");
const [buttonCount, setButtonCount] = useState(0);
const [btnArray, setBtnArray] = useState([]);
const handleChange = e => {
//you are not using button value any where
//setButtonValue(e.target.value);
setButtonCount(parseInt(e.target.value, 10));
};
const onClickHandle = () => {
if (btnArray.length > 0) {
setBtnArray([]);
}
// setButtonCount(parseInt(buttonValue, 10)); //actually you were setting the count very late so it is one step behind
setButtonValue("");
console.log(buttonCount);
let tempArr = [];
for (let i = 0; i < buttonCount; i++) {
tempArr.push({ index: i, color: "white", isDisable: false });
setBtnArray(tempArr);
}
console.log(btnArray);
};
const changeColorHandle = btnIndex => {
console.log(btnIndex);
//change that button color and save btn array again
let newBtnArray = btnArray.map(btn => {
if (btn.index === btnIndex) {
btn["color"] = "blue";
} else {
//else change all buttons color back to white
btn["color"] = "white";
}
return btn;
});
setBtnArray(newBtnArray); // set new array with modified colors
};
return (
<>
<div className="container">
<div className="row">
<div className="col">
<input type="text" onChange={handleChange} />
<button onClick={onClickHandle}>Enter</button>
</div>
</div>
<br />
<div className="row">
{btnArray.map((i, k) => (
<Button
key={k}
color={i.color}
isDisable={i.isDisable}
onClick={() => changeColorHandle(i.index)}
/>
))}
</div>
</div>
</>
);
};
export default Game;