Mern stacks deployment

// this is my index.js pubblished on vercel

if (process.env.NODE_ENV !== 'production') {
  require('dotenv').config({path: __dirname+'/.env'});
}
const express = require("express");
const multer = require("multer");
const mongoose = require("mongoose");
const dotenv = require("dotenv");
const authRoute = require("./routes/auth");
const userRoute = require("./routes/users");
const postRoute = require("./routes/posts");
const categoryRoute = require("./routes/categories");
const path = require("path");

const app = express();
dotenv.config();

mongoose
  .connect(process.env.MONGO_URL, {
    useNewUrlParser: true,
    useUnifiedTopology: false,
  })
  .then(() => {
    console.log("MongoDB connected!");
  })
  .catch((err) => {
    console.log(err);
  });

app.use(express.json());

app.use("/images", express.static(path.join(__dirname, "/images")));

const storage = multer.diskStorage({
  destination: (req, file, cb) => {
    cb(null, "images");
  },
  filename: (req, file, cb) => {
    cb(null, req.body.name);
  },
});

const upload = multer({ storage: storage });
app.post("/api/upload", upload.single("file"), (req, res) => {
  res.status(200).json("File has been uploaded");
});
//endpoints
app.use("/api/auth", authRoute);
app.use("/api/users", userRoute);
app.use("/api/posts", postRoute);
app.use("/api/categories", categoryRoute);

app.get('/', (req,res)=>{
  res.json({
    "ciao":"io sono il backend",
  });
});

// static files (build of your frontend)
if (process.env.NODE_ENV === 'production') {
  app.use(express.static(path.join(__dirname, '../frontend', 'build')));
  app.get('/*', (req, res) => {
    res.sendFile(path.join(__dirname, '../frontend', 'build', 'index.html'));
  })
}

app.listen(process.env.PORT || 5000, () => {
  console.log("Listening at port 5000");
});