Why am I getting the \'Invalid hook call\' error in React and how can I fix it?

Invalid Hook Call in React: Understanding the Issue and Resolution

The error "Invalid hook call. Hooks can only be called inside of the body of a function component" arises when hooks are invoked improperly within a React application. This error typically occurs for one of the following reasons:

• Mismatching React and Renderer Versions: Ensure the versions of React and the renderer (such as React DOM) are aligned.
• Breaking the Rules of Hooks: Adhere to the guidelines for using hooks in function components, such as calling hooks only inside functional components and avoiding calling them more than once per component.
• Multiple Copies of React: Avoid having multiple instances of React in a single application, which can lead to conflicts.

In the provided code snippet, the error occurs because hooks (specifically the useState hook) are being employed within a class component. This is incorrect as hooks are exclusively designed for use in functional components.

To resolve the issue, the code should be rewritten as a function component, as seen below:

<code class="javascript">import React, { useState } from 'react';
import { makeStyles } from '@material-ui/core/styles';
import Table from '@material-ui/core/Table';
import TableBody from '@material-ui/core/TableBody';
import TableCell from '@material-ui/core/TableCell';
import TableHead from '@material-ui/core/TableHead';
import TableRow from '@material-ui/core/TableRow';
import Paper from '@material-ui/core/Paper';

const useStyles = makeStyles(theme => ({
  root: {
    width: '100%',
    marginTop: theme.spacing(3),
    overflowX: 'auto'
  },
  table: {
    minWidth: 650
  }
}));

const Allowance = () => {
  const classes = useStyles();
  const [allowances, setAllowances] = useState([]);

  const fetchAllowances = async () => {
    const res = await fetch('http://127.0.0.1:8000/allowances');
    const data = await res.json();
    setAllowances(data);
  };

  useEffect(() => {
    fetchAllowances();
  }, []);

  return (
    <Paper className={classes.root}>
      <Table className={classes.table}>
        <TableHead>
          <TableRow>
            <TableCell>Allow ID</TableCell>
            <TableCell align="right">Description</TableCell>
            <TableCell align="right">Allow Amount</TableCell>
            <TableCell align="right">AllowType</TableCell>
          </TableRow>
        </TableHead>
        <TableBody>
          {allowances.map(row => (
            <TableRow key={row.id}>
              <TableCell component="th" scope="row">
                {row.AllowID}
              </TableCell>
              <TableCell align="right">{row.AllowDesc}</TableCell>
              <TableCell align="right">{row.AllowAmt}</TableCell>
              <TableCell align="right">{row.AllowType}</TableCell>
            </TableRow>
          ))}
        </TableBody>
      </Table>
    </Paper>
  );
};

export default Allowance;</code>

By converting the component to a functional component, the hook can be correctly utilized, resolving the "Invalid hook call" error.

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