why when i Deploy in vercel MERN APP shows me Error 404?


I’am trying to deploy my backend mern app in vercel and it shows me this error :

ID: cdg1::bbhc7-1691147707623-efc5cd10ff98

Here is my server.js file:

const express = require("express");
const mongoose = require("mongoose");
const cors = require("cors");
const cookieParser = require("cookie-parser");
const SocketServer = require("./socketServer");

const app = express();

// Socket
const http = require('http').createServer(app);
const io = require("socket.io")(http);

io.on("connection", socket => {

app.use("/api", require('./routes/authRouter'));
app.use("/api", require('./routes/userRouter'));
app.use("/api", require('./routes/postRouter'));
app.use("/api", require('./routes/commentRouter'));

const URI = process.env.MONGODB_URL;

.connect(URI, {
useNewUrlParser: true,
useUnifiedTopology: true,
.then(() => {
console.log("Connected successfully to MongoDB");
.catch((err) => {
console.error("Error connecting to MongoDB", err);

const port = process.env.PORT || 5000;
http.listen(port, () => {
console.log("Server is running on port: ", port);

And here is my backend’s package.json:

"scripts": {
"dev": "nodemon server.js"


In order for Vercel to turn Express into a serverless function, you have to export the Express instance for Vercel’s build process.
add this at the end of the file server.js:

module.exports = app;

After exporting Express, we have to tell Vercel what files to build, how to build them, and how to route them using a vercel.json file. So create vercel.json. Then, specify your server.js file and the NPM module Vercel will use to turn it into a serverless function:

  "version": 2, // Vercel version
  "builds": [
      // Specify file to convert to a serverless function
      "src": "server.js",
      // Specify the NPM module that is used for the build
      "use": "@vercel/node"

specify which paths will route to the server.js file’s built serverless function using regex.

  "version": 2,
  "builds": [
      "src": "server.js",
      "use": "@vercel/node"
  "routes": [
      // Specify which paths will route to a destination using a regex
      "src": "/(.*)",
      // Specify the paths' destination
      "dest": "server.js"

Finally, deploy your app

source: https://shadowsmith.com/thoughts/how-to-deploy-an-express-api-to-vercel

Answered By – Husain Mohamedi

This Answer collected from stackoverflow, is licensed under cc by-sa 2.5 , cc by-sa 3.0 and cc by-sa 4.0

Leave a Reply

(*) Required, Your email will not be published