Connecting an Express backend to MongoDB using Mongoose — with proper error handling and connection events.

MongoDB Connection

This replaces the placeholder db.js from the previous file with a real MongoDB connection using Mongoose — including error handling and connection event logging.

db.js — Full Connection Setup

// src/config/db.js
import mongoose from "mongoose";

const connectDB = async () => {
  try {
    // Step 1 — connect using the URI from environment variables
    const conn = await mongoose.connect(process.env.MONGO_URI);

    console.log(`MongoDB connected: ${conn.connection.host}`);

    return conn;
  } catch (error) {
    // Step 2 — if connection fails, throw so server.js can catch it
    // and exit the process instead of running with no database
    throw new Error(`MongoDB connection failed: ${error.message}`);
  }
};

// Step 3 — listen for connection events after the initial connect
// useful for catching disconnects that happen later, not just on startup
mongoose.connection.on("disconnected", () => {
  console.log("MongoDB disconnected");
});

mongoose.connection.on("reconnected", () => {
  console.log("MongoDB reconnected");
});

mongoose.connection.on("error", (error) => {
  console.error("MongoDB connection error:", error.message);
});

export default connectDB;

How It Connects With server.js

server.js from the previous file already handles both outcomes:

connectDB()
  .then(() => {
    app.listen(PORT, () => console.log(`Server running on port ${PORT}`));
  })
  .catch((error) => {
    console.error("Failed to connect to database:", error.message);
    process.exit(1);
  });

Getting a MONGO_URI — Local vs Atlas

Local MongoDB

# .env
MONGO_URI=mongodb://localhost:27017/myapp

Requires MongoDB installed and running locally — mongod running in the background.

MongoDB Atlas (cloud — recommended)

# .env
MONGO_URI=mongodb+srv://username:password@cluster0.mongodb.net/myapp?retryWrites=true&w=majority

Create a free cluster at mongodb.com/atlas
Create a database user with a username and password
Add your IP to the network access list (or 0.0.0.0/0 for development)
Copy the connection string and replace <password> with your actual password

Never commit a real MONGO_URI with credentials to git. It always lives in .env, which is in .gitignore.

Testing the Connection

npm run dev

# expected output on success
MongoDB connected: cluster0-shard-00-01.mongodb.net
Server running on port 5000

# expected output on failure (e.g. wrong password)
Failed to connect to database: MongoDB connection failed: bad auth : authentication failed

Summary

connectDB() uses mongoose.connect(process.env.MONGO_URI) wrapped in try/catch
Connection events — disconnected, reconnected, error — are logged for visibility after the initial connection
Local development can use a local MongoDB instance or MongoDB Atlas — Atlas is recommended even for development
Failed connections throw an error that server.js catches, logging it and exiting the process rather than running with no database
Real credentials always go in .env, never committed to git

MongoDB Connection

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