No 'Access-Control-Allow-Origin' what is socket io or express problem?

Question

Socket.io

I am creating a simple chat. And almost at the very beginning I ran into a problem that I can’t solve for 4 days! I hope that kind people will pout here who can help with this error:

I tried a lot of options I found on the Internet (including this site).
I rewrote the code, watched the video, enabled CORS , passed 'Access-Control-Allow-Origin': '*' to cors as options, and did a lot of other things. all to no avail!
Here is the server:

const express = require('express');
const app = express();
const server = require('http').createServer(app);
const io = require('socket.io')(server);
const cors = require('cors');

const chat = {
  rooms: {
    name: 'test',
    message: 'test test'
  };
};

app.use(cors());

app.get('/', (req, res) => {
  res.json(chat)
});

io.on('connection', (socket) => {
  console.log('User connected', socket.id);
});

app.listen(2000, (err) => {
  if (err) {
    throw Error(err);
  }
  console.log('Server run');
});

and this is from the client side:

import React from "react";
import io from "socket.io-client";


function App() {

  const connectChat = () => {
    io('http://localhost:2000');
  }

  return (
    <div className="App">
      <h2>Hello</h2>
      <button onClick={connectChat}>connect</button>
    </div>
  );
}

export default App;

PS - I reviewed a bunch of video tutorials on express socket io, they don't even install CORS and everything works! maybe I'm doing something wrong?