mongo.js

const { MongoClient } = require('mongodb');
const { promisify } = require('es6-promisify');

let _connection;

const connect = () => {
  if (!process.env.MONGO_URL) {
    throw new Error(`Environment variable MONGO_URL must be set to use API.`);
  }
  return promisify(MongoClient.connect)(process.env.MONGO_URL, {
    useNewUrlParser: true
  });
};

/**
 * Returns a promise of a `db` object. Subsequent calls to this function returns
 * the **same** promise, so it can be called any number of times without setting
 * up a new connection every time.
 */
const connection = () => {
  if (!_connection) {
    _connection = connect();
  }
  return _connection;
};

/**
 * Returns a ready-to-use `collection` object from MongoDB.
 * Usage:
 *   (await getCollection('users')).find().toArray().then( ... )
 */
async function getCollection(collectionName) {
  const db = (await connection()).db(process.env.MONGO_DB_NAME);
  return db.collection(collectionName);
}

module.exports = {
  connection,
  getCollection
};