code: function connectOp(mongoClient, err, callback) {
    // Did we have a validation error
    if (err) return callback(err);
    // Fallback to callback based connect
    connect(mongoClient, mongoClient.s.url, mongoClient.s.options, err => {
      if (err) return callback(err);
      callback(null, mongoClient);
    });
  }
docs: Connect to MongoDB using a url as documented at
  
  docs.mongodb.org/manual/reference/connection-string/
  
  Note that for replicasets the replicaSet query parameter is required in the 2.0 driver
  
  @method
  @param {MongoClient} mongoClient The MongoClient instance with which to connect.
  @param {MongoClient~connectCallback} [callback] The command result callback