Simple MongoDB connection pool factory.

mongoFactory.js

/**
 * Creates and manages the Mongo connection pool
 *
 * @type {exports}
 */
var constants = require('../js/constants.js');
var Q = require('q');
var MongoClient = require('mongodb').MongoClient;
var dbPromise = null;

module.exports = function() {

  return {

    /**
     * Gets a connection to Mongo from the pool. If the pool has not been instantiated it,
     *    instantiates it and returns a connection. Else it just returns a connection from the pool
     *
     * @returns {*}   - A promise object that will resolve to a mongo db object
     */
    getConnection: function getConnection() {

      // If the connection pool has not been created or has been closed, create it, else return an existing connection
      if (dbPromise === null) {

        var def = Q.defer();

        // Initialize connection once
        MongoClient.connect(constants.mongoUrl, function (err, database) {
          if (err) {
            def.reject(err);
          }

          def.resolve(database);
        });

        return dbPromise = def.promise;
      } else {
        return dbPromise;
      }
    }
  }
}();

usage.js

var mongoFactory = require('./path/to/mongoFactory');

mongoFactory.getConnection().then(function(db) {
  .. do stuff with db inside here
});