I'm trying to add a key-value pair to an object using useState inside map function. However, the spread operation in setContainer({...container, [data.pk]: data.name}); seems to be ineffective.
The code snippet below exactly illustrates the problem that I'm trying to solve. In fact, in my original code container is declared at another ContextProvider component and referenced using useContext, but I replaced it with useState for simpliticy.
I guess it has to do with the asynchronous nature of setState hook, but I don't fully understand what's going on under the hood.
Please let me know what is causing this issue and how to yield the outcome I expected.
Thank you.
const {useState, useEffect} = React;
const myData = [{pk: 1, name:'apple'},
{pk: 2, name:'banana'},
{pk: 3, name:'citrus'},
]
const Example = () => {
const [container, setContainer] = useState({});
useEffect(()=>{
myData.map(data=>{
console.log(`During this iteration, a key-value pair (key ${data.pk} and value '${data.name}') should be added to container`);
setContainer({...container, [data.pk]: data.name});
})
}, [])
return (
<div>
<div>result I expected: {{'1': 'apple', '2': 'banana','3': 'citrus'}}</div>
<div>result: {JSON.stringify(container)}</div>
</div>
);
};
// Render it
ReactDOM.render(
<Example />,
document.getElementById(react)
);<script src=https://cdnjs.cloudflare.com/ajax/libs/react/16.8.4/umd/react.production.min.js></script>
<script src=https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.4/umd/react-dom.production.min.js></script>
<div id=react></div>Update
The main reason I wanted to use useState in loops was to use the state as a container of refs of components. However, I found it inefficient to do so, because it caused too many rerenders and was hard to determine when the loop finished. After a lot of searching, I ended up declaring useRef({}) in the Context Provider and use it as a refs container (which seems to fit the actual purpose it was created I guess). The refs container can be used as following:
const refsContainer = useRef({});
import React from 'react';
const Notes = () => {
const setRef = (noteId) => (element) => {
refsContainer.current[noteId] = element
};
return (
<Masonry>
{pubData?.map(publication => {
return <PublicationCard innerRef={setRef(publication.noteId)}/>
})}
</Masonry>
);
};
export default Notes;
const pubCardRef = refsContainer.current[rank.note.noteId];
window.scrollTo({top: pubCardRef?.offsetTop - 80, behavior: 'smooth'})
