Connecting to MongoDB

Before your app can read or write any data, it needs to connect to MongoDB. Every connection uses a connection string — a single line of text that tells your app where MongoDB is running and how to reach it.

What is a Connection String?

A connection string is like an address for your database. It tells your app:

• What protocol to use
• Where the server is running
• Which port to connect on
• Which database to use
• Any extra options like authentication

The basic structure looks like this:

mongodb://username:password@host:port/database

For a local MongoDB server with no username or password, it simplifies to:

mongodb://localhost:27017

Let's break that down:

Connecting with mongosh

When you open mongosh without any arguments, it automatically connects to mongodb://localhost:27017. But you can also pass a connection string directly:

mongosh "mongodb://localhost:27017"

To connect directly to the school database:

mongosh "mongodb://localhost:27017/school"

Once connected, switch to the school database:

use school

Check which database you are currently using:

db

Output:

school

List all collections in the school database:

show collections

Output:

students

Connecting from Node.js

To connect to MongoDB from a Node.js application, you need the official MongoDB Node.js driver.

Install it

npm install mongodb

Basic connection

Create a file called connect.js:

const { MongoClient } = require('mongodb');

// Connection string pointing to our local MongoDB server
const uri = 'mongodb://localhost:27017';

// Create a client
const client = new MongoClient(uri);

async function main() {
  // Connect to the server
  await client.connect();

  console.log('Connected to MongoDB');

  // Select the school database
  const db = client.db('school');

  // Select the students collection
  const students = db.collection('students');

  // Find all students
  const allStudents = await students.find().toArray();
  console.log(allStudents);
}

main()
  .catch(console.error)
  .finally(() => client.close());

Run it:

node connect.js

Output:

Connected to MongoDB
[
  {
    _id: ObjectId('64a1f2c3e4b0a1b2c3d4e5f6'),
    name: 'Ali Hassan',
    age: 16,
    grade: '10th'
  }
]

Your Node.js app is now talking to your MongoDB school database.

Connection Options

You can add options to the connection string to control how the connection behaves. Options go after a ? at the end:

mongodb://localhost:27017/school?option1=value1&option2=value2

Here are the most useful ones:

For a local development connection you rarely need these. They matter more in production.

Connection with Authentication

If your MongoDB server has a username and password set up, include them in the connection string:

mongodb://username:password@localhost:27017/school

In Node.js:

const uri = 'mongodb://admin:secret123@localhost:27017/school';
const client = new MongoClient(uri);

const uri = process.env.MONGODB_URI;

Common Connection Errors

Error: connect ECONNREFUSED 127.0.0.1:27017

MongoDB is not running. Start it:

• Windows: check Services and start the MongoDB service
• macOS: brew services start mongodb-community
• Linux: sudo systemctl start mongod

Error: Authentication failed

Wrong username or password in your connection string. Double check them.

Error: MongoServerSelectionError

MongoDB is running but your app cannot reach it. Check your firewall settings or whether the port 27017 is blocked.

Connecting to the School Database

From this point forward, every example in this guide connects to the school database. In mongosh:

use school

In Node.js, always use this as your starting point:

const client = new MongoClient('mongodb://localhost:27017');
await client.connect();
const db = client.db('school');

We will use this same pattern in every file going forward.