currently im building a MERN stack project. Now i want to host my MERN in Vercel. The problem im encounter were, when i deploying it stuck at “Connected to MongoDB” and “Server is running on port 4000”. i suspect that it doesnt know what to do after that. But i dont know what the code should be. here is my server.js code

const express = require('express');
const mongoose = require('mongoose');
const studentRoute = require('./routes/Rstudent');
const staffRoute = require('./routes/Rstaff');
const userRoute = require('./routes/Ruser');
const cors = require('cors');

const app = express();
app.use(cors(
  {
    origin: ['my front end http'],
    methods: ['GET', 'POST', 'PUT', 'DELETE'],
    allowedHeaders: ['Content-Type', 'Authorization'],
    credentials: true
}))

// app.use(cors(
//   {
//     origin: 'http://localhost:5173',
//     methods: ['GET', 'POST', 'PUT', 'DELETE'],
//     allowedHeaders: ['Content-Type', 'Authorization'],
//     credentials: true
// }))
app.use(express.json());


app.use('/api/student', studentRoute);
app.use('/api/staff', staffRoute);
app.use('/api/user', userRoute);

//connect to mongoDB
mongoose.connect(process.env.MONGO_URI).then(() => {
  console.log("connected to MongoDB");
  //listen to request
  const PORT = process.env.PORT || 4000; // Default port is 3000 if PORT variable is not provided
  app.listen(PORT, () => {
    console.log("Server is running on port", PORT);
  });

}).catch(err => {
  console.log("error:", err.message);
});


 ```. 
This is the output in vercel terminal 
Cloning completed: 999.94ms
Restored build cache
Running "vercel build"
Vercel CLI 34.1.3
Running "install" command: `npm install`...
up to date, audited 122 packages in 621ms
25 packages are looking for funding
  run `npm fund` for details
found 0 vulnerabilities
> [email protected] build
> node server.js
connected to MongoDB
Server is running on port 4000. 

Then it just keep building