To connect MongoDB with a Node.js application, install mongoose, create a MongoDB database, and use mongoose to establish a connection in your code.

Connecting MongoDB with a Node.js application allows you to leverage the power of a NoSQL database to store and manage your data. In this guide, we will walk through the process of connecting a Node.js application to a MongoDB database using Mongoose, an elegant MongoDB object modeling tool designed to work in an asynchronous environment.

1. Prerequisites: Ensure you have Node.js and npm installed. You should also have MongoDB installed locally or use a cloud service like MongoDB Atlas.

2. Initialize Your Project: Create a new directory for your project and navigate to it in your terminal. Run npm init -y to generate a package.json file.

3. Install Required Packages: Install Mongoose by running:

   npm install mongoose
   

   This command adds Mongoose to your project dependencies.

4. Set Up Your MongoDB Database: If you’re using MongoDB locally, start the MongoDB server by running mongod. If you’re using MongoDB Atlas, create a new database cluster and obtain your connection string from the Atlas dashboard.

5. Create the Connection: In your project directory, create a new file named server.js. In this file, you’ll require Mongoose and establish a connection to your database:

   const mongoose = require('mongoose');
   
   mongoose.connect('mongodb://localhost:27017/mydatabase', { useNewUrlParser: true, useUnifiedTopology: true })
       .then(() => console.log('MongoDB connected successfully.'))
       .catch(err => console.error('MongoDB connection error:', err));
   

   Replace 'mongodb://localhost:27017/mydatabase' with your MongoDB connection string if you’re using Atlas.

6. Defining a Schema: After connecting to the database, define a schema for your data. For example:

   const userSchema = new mongoose.Schema({
       name: String,
       email: String,
       age: Number,
   });
   
   const User = mongoose.model('User', userSchema);
   

   This schema defines a User model with name, email, and age fields.

7. CRUD Operations: With your model defined, you can now perform CRUD (Create, Read, Update, Delete) operations. For example, to create a new user:

   const newUser = new User({ name: 'John Doe', email: '[email protected]', age: 30 });
   newUser.save()
       .then(() => console.log('User created successfully.'))
       .catch(err => console.error('Error creating user:', err));
   

   This code snippet creates a new user in the MongoDB database.

8. Querying Data: To retrieve data from the database, you can use methods like find():

   User.find().then(users => console.log(users));
   

   This retrieves all users from the User collection.

9. Error Handling: Ensure to implement proper error handling in your database operations. You can handle errors using .catch() on your promises and implement try-catch blocks in async functions.

10. Conclusion: By following these steps, you have successfully connected your Node.js application to a MongoDB database using Mongoose. You can now expand your application by implementing more complex data models and functionalities as needed.