We will define our MongoDB connection in a ./db.js file. This will help keep our index.js file neat and allow us to keep our configurations modular.

To connect to our database, if you haven't already, create a db.js file in the root directory as given:

touch db.js

Then, insert the following into the file:

// ./db.js

const mongoose = require('mongoose');

module.exports = () => {
  mongoose.connect(
    'mongodb://localhost:27017/koa-blog',
    { useNewUrlParser: true }
  );

  const db = mongoose.connection;
  db.on('error', error => {
    throw new Error(`error connecting to db: ${error}`);
  });
  db.once('open', () => console.log('database connected'));
};

In the preceding code block, first, we require the mongoose ODM library, then we export a simple function that creates a connection to our database using the mongoose.connect(dbUrl, [options]) method. We are creating a connection to the koa-blog database.

It should also be noted that we set up listeners in our database connection function. The db.on('error') listener is for when an error occurs when a connection is trying to be established. The db.on('open') listener is called when a successful connection occurs.

Now that we have exported our database connection function, we can make use of it in the index.js file as seen here:

// ./index.js

// ...
const initDb = require('./db');

// initialize database
initDb();

// ...

The complete index.js file will now look like this:

const Koa = require('koa');
const initDb = require('./db');

// initialize database
initDb();

// create app instance
const app = new Koa();

// start server
const port = process.env.PORT || 3000;
app.listen(port, () => console.log(`Server running on http://localhost:${port}`));

The next time our application starts, we should see messages similar to this one:

Server running on http://localhost:3000
database connected

This indicates that the connection to the database was successful.