ikramuzzaman455173 / Resolving-Vercel-Serverless-Function-Issues-with-Express

Resolving Vercel Serverless Function Issues with Express

Table of Contents

1. Introduction
2. Problem Description
3. Solution Overview
4. Step-by-Step Solution
   • 1. Modify api/index.js
   • 2. Update vercel.json
   • 3. Test Locally
   • 4. Deploy to Vercel
5. Full Code Example
6. Go to Top

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Introduction

This documentation provides a comprehensive guide to resolving issues with deploying an Express application as a serverless function on Vercel. The common issue faced is the TypeError: listener is not a function, which often arises due to incorrect setup for serverless functions.

Problem Description

When deploying an Express application to Vercel, you might encounter the error:

TypeError: listener is not a function

This error indicates that Vercel's serverless environment is not handling the Express app correctly due to improper setup.

Solution Overview

To resolve the issue, follow these steps:

1. Modify api/index.js: Adjust the serverless function to export a handler compatible with Vercel.
2. Update vercel.json: Ensure the configuration points to the correct file and setup.
3. Test Locally: Verify the setup using local testing.
4. Deploy to Vercel: Deploy your application and ensure it works correctly on Vercel.

Step-by-Step Solution

1. Modify api/index.js

Update your api/index.js file to export a handler function that Vercel can use. Here’s the revised code:

import express from 'express';
import dotenv from 'dotenv';
import cookieParser from 'cookie-parser';
import fileUpload from 'express-fileupload';
import cors from 'cors';
import mongoose from 'mongoose';
import morgan from 'morgan';
import router from '../routes/allRoutes.js';

// Load environment variables from .env file
dotenv.config();

// Initialize Express app
const app = express();

// Setup CORS
app.use(
  cors({
    origin: ['http://localhost:5173', 'http://localhost:3001'],
    credentials: true,
  })
);

// Middleware setup
app.use(cookieParser());
app.use(express.json());
app.use(express.urlencoded({ extended: true }));

// Setup basic Morgan logger
app.use(morgan('dev'));

// File upload configuration
app.use(
  fileUpload({
    useTempFiles: true,
    tempFileDir: '/tmp/',
  })
);

// MongoDB connection
const MongoUrl = process.env.MONGO_URI;

mongoose
  .connect(MongoUrl)
  .then(() => {
    console.log('Connected to MongoDB successfully!');
  })
  .catch((err) => {
    console.error(`Failed to connect to MongoDB: ${err.message}`);
  });

// Setup API routes (add your routes here)
app.use('/api/v1', router);

// Root path with a welcome message and list of all GET routes for fetching data
app.get('/', (req, res) => {
  res.json({
    message: 'Welcome to the IJ portfolio backend API!',
    status: 200,
    'GET Routes': {
      'Get User': '/api/v1/user/me',
      'Get User for Portfolio': '/api/v1/user/portfolio/me',
      'Get All Timelines': '/api/v1/timeline/getAllTimeline',
      'Get All Software Applications': '/api/v1/softwareApplication/getAllSoftwareApplication',
      'Get All Skills': '/api/v1/skill/getAllSkill',
      'Get Grouped Skills': '/api/v1/skill/getGroupedAllSkills',
      'Get All Projects': '/api/v1/project/getAllProject',
      'Get All Messages': '/api/v1/message/getAllMessages',
      'Get All Certificates': '/api/v1/certificate/getAllCertificates',
      'Get All Blogs': '/api/v1/blog/getAllBlogs',
    },
  });
});

// Error handling middleware
app.use((err, req, res, next) => {
  console.error('Error:', err.stack);
  res.status(500).send('Something broke!');
});

// Export the handler for Vercel
export default (req, res) => {
  return app(req, res);
};

2. Update vercel.json

Ensure your vercel.json file is configured to point to the correct function:

{
  "version": 2,
  "builds": [
    {
      "src": "api/index.js",
      "use": "@vercel/node"
    }
  ],
  "routes": [
    {
      "src": "/(.*)",
      "dest": "/api/index.js",
      "methods": ["GET", "POST", "PUT", "PATCH", "DELETE", "OPTIONS"]
    }
  ]
}

3. Test Locally

Test your setup locally using the Vercel CLI:

vercel dev

Ensure that your Express app runs as expected on http://localhost:3000.

4. Deploy to Vercel

Deploy your application to Vercel:

vercel --prod

Ensure that the deployment succeeds and the application works correctly on Vercel.

Full Code Example

// Full `api/index.js` code provided in the "Modify `api/index.js`" section

Go to Top

Go to Top

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