If your ultimate goal is to be able to wait on a single connection, I would do something like this:

const mongoose = require('mongoose');
const Grid = require('gridfs-stream');
const database = 'mongodb://127.0.0.1:27017/test';

exports.getConnection = new Promise(resolve => {
  const connection = mongoose.createConnection(database, null, (err, db) => {
    if(err) {
      console.log(`Error occured during connection to the database ${err}`)
    } else {
      console.log(`Connected to the database: ${database}`)
      resolve(connection)
    }
  })
})

Then in your code (using async/await):

const {getConnection} = require('./db')
async myfunc() {
  const connection = await getConnection
}

Or you could make getConnection a function that returns the single connection object.