how do i use reducer to update a global variable

I need to pass a variable between two files, this variable will store data(userId) gotten from file one(fetched from an api) so that I can use the variable as a parameter in an endpoint in file two. I have used redux toolkit to create a global state but I can not figure out how to use reducers(from a third file slice.js) to update the global state to the data gotten from file one . I actually do not know how to go about this at all.

file one : inside this file ,there are buttons rendered dynamically from an api that leads to another page when any of the buttons is clicked. On this other page , there is gonna be some information displayed depending on the id gotten.

const [usersData, setUsersData] = userState([]);

useEffect({
fetchData()
});

export default function FileOne(){
return(
<>
  {usersData.map(userData => (
    <Link to='/fileTwo'>
       <button>{userData.id}</button>
    </Link>
  )}
</>
)
}

so i have used redux toolkit to create a global variable

slice.js:

import { createSlice } from '@reduxjs/toolkit';

const initialState = {
    userId:0
    // Your global state properties here (e.g., count: 0, data: [])
};

 export const idSlice = createSlice({
    name: 'id',
    initialState,
    reducers: {
        getUserId(state){
          
        },
        // Reducer functions to update the state (e.g., increment(state) { ... })
    },
});
export const { getUserId} = idSlice.actions;
export default idSlice.reducer;

what i need help with is how i'm gonna update userId(the global state)to each user's Id depending on the button clicked inside file one

Solution

If you are wanting to set a selected user id into the state then these are the changes/additions I'd suggest.

Add a new setUserId action to set the currently selected userId
Dispatch the setUserId action in the button's onClick handler
Read the current userId state value in the target component

Example:

const initialState = {
  userId: null
  // Your global state properties here (e.g., count: 0, data: [])
};

export const idSlice = createSlice({
  name: 'id',
  initialState,
  reducers: {
    ....,
    setUserId: (state, action) => {
      state.userId = action.payload;
    },
  },
});

export const {
  ....,
  setUserId,
} = idSlice.actions;

import { useDispatch } from 'react-redux';
import { setUserId } from '../path/to/idSlice;'

export default function FileOne() {
  const dispatch = useDispatch();

  return usersData.map(userData => (
    <Link key={userData.id} to='/fileTwo'>
      <button onClick={() => dispatch(setUserId(userData.id))}>
        {userData.id}
      </button>
    </Link>
  );
}

import { useSelector } from 'react-redux';

const FileTwo = () => {
  const selectedUserId = useSelector(state => state.id.userId);

  ...
};

For what it's worth this seems like rather transient state and likely doesn't need to be anything stored in the Redux store. Since you are using a Link component (assumed React-Router library usage here) and effecting a navigation action you could likely just include the user id in the navigation action, either in the URL path, i.e. "/fileTwo/:userId", or in the route state.

Passing userData.id in route state:

import { Link } from 'react-router-dom';

export default function FileOne() {
  const dispatch = useDispatch();

  return usersData.map(userData => (
    <Link key={userData.id} to='/fileTwo' state={{ userId: userData.id }}>
      {userData.id}
    </Link>
  );
}

import { useLocation } from 'react-router-dom';

const FileTwo = () => {
  const { state } = useLocation();
  const { userId } = state ?? {};

  ...
};

Passing userData.id in URL path segment:

<Route
  path="/fileTwo/:userId?"
  element={<FileTwo />}
</Route>

import { Link } from 'react-router-dom';

export default function FileOne() {
  const dispatch = useDispatch();

  return usersData.map(userData => (
    <Link key={userData.id} to={`/fileTwo/${userData.id}`}>
      {userData.id}
    </Link>
  );
}

import { useParams } from 'react-router-dom';

const FileTwo = () => {
  const { userId } = useParams();

  ...
};