I’m quite new to react, and I have this problem that when trying to map an array to a list of React stateful component, all my components lose their state. My code:
const { useState, useEffect} = React;
const htmlRoot = document.getElementById('root');
const reactRoot = ReactDOM.createRoot(htmlRoot);
const App = () => {
const Item = (props) => {
const [count, setCount] = useState(0);
useEffect(() => {
const timer = setInterval(() => {
setCount((lastCount) => lastCount + 1);
}, 1000);
return () => clearInterval(timer);
}, []);
return <h1>{props.children + ' ' + count}</h1>;
};
const arr = [1, 2, 3];
const [arrState, setArrState] = useState(arr);
const clickHandler = () => {
setArrState((lastArrState) => [
...lastArrState,
lastArrState.at(-1) + 1,
]);
};
return (
// Unfortunately, Stack Snippets don't support the shorthand form
// of fragments
<React.Fragment>
{arrState.map((num, index) => (
<Item key={index}>{num}</Item>
))}
<button onClick={clickHandler}>Click Me</button>
</React.Fragment>
);
};
reactRoot.render(<App />);<div id="root"></div>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/18.1.0/umd/react.development.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/18.1.0/umd/react-dom.development.js"></script>Is this even possible for them to keep their state? If it is, how?
Update:
>Solution :
You’re creating a new Item component every time your App component is called to render. Since the subsequent renders are using different components that previous renders, React gets rid of the previous instances and creates new ones with fresh state.
Instead, define your Item component outside App and then use it within:
const { useState, useEffect } = React;
const htmlRoot = document.getElementById('root');
const reactRoot = ReactDOM.createRoot(htmlRoot);
const Item = (props) => {
const [count, setCount] = useState(0);
useEffect(() => {
const timer = setInterval(() => {
setCount((lastCount) => lastCount + 1);
}, 1000);
return () => clearInterval(timer);
}, []);
return <h1>{props.children + ' ' + count}</h1>;
};
const App = () => {
const arr = [1, 2, 3];
const [arrState, setArrState] = useState(arr);
const clickHandler = () => {
setArrState((lastArrState) => [
...lastArrState,
lastArrState.at(-1) + 1,
]);
};
return (
// (Sadly, Stack Snippets don't support the shorthand form
// of fragments.)
<React.Fragment>
{arrState.map((num, index) => (
<Item key={index}>{num}</Item>
))}
<button onClick={clickHandler}>Click Me</button>
</React.Fragment>
);
};
reactRoot.render(<App />);<div id="root"></div>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/18.1.0/umd/react.development.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/18.1.0/umd/react-dom.development.js"></script>Separately, don’t use index for key (unless the list is static or ever-growing and never reordered — yours fits that latter category, but I’m guessing at some point you’ll remove items from it). Details about why in the documentation and the article it links to. Instead, ensure the things being listed have some form of unique, reusable identifier.