Express server does not work on Vercel, works on localhost

Question

I am building live chat app, and It is working, and I want to run it on vercel. Everything is working fine on localhost, but it is not working on vercel or netlify.

This is my backend part folder structure.

IMAGE1

IMAGE2

Like everyone says, I made vercel.json file in root of folder and here it is code.

{
 "version": 2,
 "builds": [
  {
  "src": "../backend/server.js",
  "use": "@vercel/node"
  }
  ],
 "routes": [
 {
  "src": "/(.*)",
   "dest": "/"
  }
 ]
}

While finishing deploying on vercel, I added also env.variables.

But when deploying is finished, I always have this error. https://backend-chat-app-rouge.vercel.app/

ERROR

I don't have any errors or warnings on Biulding logs on Vercel.

I think problem might be realted to this from vercel.json : "src": "../backend/server.js", because I need to import files like this:

EDIT

server.js file:

const PORT = process.env.PORT || 6001;
const dotenv = require(`dotenv`);
dotenv.config({ path: `./config.env` });
const app = require(`../backend/app`);
const mongoose = require(`mongoose`);

const DB = process.env.MONGO_URL.replace(`<PASSWORD>`, 
process.env.PASSWORD);

 mongoose
   .connect(DB, {})
   .then((con) => {
   console.log(`DB is connected`);
 })
 .catch((err) => {
 console.log(err);
 });

 const http = require("http");
 const server = http.createServer(app);
 const io = require("socket.io")(server);

 io.on("connection", (socket) => {
 console.log("A client connected");

 socket.on("message", (message) => {
 console.log("Received message:", message);
 io.emit("message", message);
   });
 });

 server.listen(PORT, () => {
 console.log(`Aplikacija radi na portu broj ${PORT}`);
 });

app.js file:

 const express = require(`express`);
 const app = express();
 const userRouter = require(`../backend/routes/userRoutes`);
 const chatRouter = require(`../backend/routes/ChatRoutes`);
 const messageRouter = require(`../backend/routes/MessageRoutes`);
 const cors = require(`cors`);
 const bodyParser = require(`body-parser`);

 //MIDLEWARE

 app.use(bodyParser.json());

 app.use((err, req, res, next) => {
 if (err instanceof SyntaxError && err.status === 400 && "body" in err) 
  {
  res.status(400).json({ status: "failed", message: "Invalid JSON 
  payload" });
  } else {
  next();
    }
  });

 app.use(
  cors({
     origin: "*",
     credentials: true,
  })
 );

 //ROUTES
 app.use(`/`, userRouter);
 app.use(`/chat`, chatRouter);
 app.use(`/messages`, messageRouter);

 module.exports = app;

Answer 1

in vercel.json file you have to add

"installCommand": "npm install"

and remove ../backend from src after making changes the vercel.json file looks like this

{
 "version": 2,
 "builds": [
  {
  "src": "./server.js",
  "use": "@vercel/node"
  }
  ],
 "routes": [
 {
  "src": "/(.*)",
   "dest": "/"
  }
 ],
"installCommand": "npm install"
}

finally make sure your server is listening on some port(like 5000) that you have added in your env variable in vercel