The router in my react app throws an error - reactjs

Question

This is the error:
throw new TypeError('app.use() requires a middleware function')
^
TypeError: app.use() requires a middleware function
at Function.use (/Users/Andres/Desktop/chat-app-2/server/node_modules/express/lib/application.js:210:11)
at Object.<anonymous> (/Users/Andres/Desktop/chat-app-2/server/index.js:13:5)
at Module._compile (internal/modules/cjs/loader.js:1063:30)
at Object.Module._extensions..js (internal/modules/cjs/loader.js:1092:10)
at Module.load (internal/modules/cjs/loader.js:928:32)
at Function.Module._load (internal/modules/cjs/loader.js:769:14)
at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:72:12)
at internal/main/run_main_module.js:17:47
[nodemon] app crashed - waiting for file changes before starting...
ROUTER.JS
const express = require('express');
const router = express.Router();
router.get(`/`, (req, res) => {
res.send('server is up and running');
});
module.export = router;
INDEX.JS
const express = require('express');
const socketio = require('socket.io');
const http = require('http');
const PORT = process.env.PORT || 5000;
const router = require('./router');
const app = express();
const server = http.createServer(app);
const io = socketio(server);
app.use(router);
server.listen(PORT, () => console.log(`Server has started on port ${PORT}`));
Can someone help me with these?

Answer 1

According to https://expressjs.com/en/guide/routing.html you need to provide a route where the imported router is support to run. Try:
app.use('/', router);
or you can use a different route, like /mysubroute.