react use .map() with useState

index.js

const [questionRepliesAll, setQuestionRepliesAll] = useState([])

const [questionReplies, setQuestionReplies] = useState({
    replies: [
        {
            desc: 'hello word 1'
        },
        {
            desc: 'hello word 2'
        }
    ],
    reply_users: [
        {
            user: 1
        },
        {
            user: 2
        }
    ]
})

questionReplies.replies.map((reply, i) => {
    setQuestionRepliesAll([...questionRepliesAll, {'reply': desc, user_id: questionReplies.reply_users[i].user])})

results

[{reply: 'hello word 1', user_id: 1}, {reply: 'hello word 2', user_id: 2}] // expected

[] // actual result

first implementation

const combined = questionReplies.replies.map((reply, i) => {
                    return {'reply': desc,
                    user_id: questionReplies.reply_users[i].id}
                })
setQuestionRepliesAll(combined)

Basically, I want to combine 2 arrays into 1 and add it to questionRepliesAll, i tried adding it with just assigning the combined array to a variable ahnd then setting it setQuestionRepliesAll(combined) but that threw errors when i tried using .map() which was Objects are not valid as a React child, go down below to see what I tried doing at first, any help is appreciated!

Answer

I would recommend using a for loop for this functionality.

let arr = {
    replies: [
        {
            desc: 'hello word 1'
        },
        {
            desc: 'hello word 2'
        }
    ],
    reply_users: [
        {
            user: 1
        },
        {
            user: 2
        }
    ]
}
let formattedArray = [];
for(i=0;i<arr.replies.length;i++){
  formattedArray.push({reply:arr.replies[i].desc,user:arr.reply_users[i].user})
}
console.log(formattedArray)

Implementation in your case :

import {useEffect, useState} from 'react';

export default function App() {
  const [questionRepliesAll, setQuestionRepliesAll] = useState([])
    
  const [questionReplies, setQuestionReplies] = useState({
      replies: [
          {
              desc: 'hello word 1'
          },
          {
              desc: 'hello word 2'
          }
      ],
      reply_users: [
          {
              user: 1
          },
          {
              user: 2
          }
      ]
  });

  useEffect(()=>{
    let formattedArray =[]
    for(var i=0;i<questionReplies.replies.length;i++){   
      formattedArray.push({reply:questionReplies.replies[i].desc,user:questionReplies.reply_users[i].user})
    }
    setQuestionRepliesAll(formattedArray);  
  },[questionReplies])


  return (
    <div>
      <ul>{questionRepliesAll.map((item,key)=><li>user {item.user} ,  reply : {item.reply}</li>)}</ul>
    </div>
  );
}