code: function connect(uri, options, singleNodeConnection, cb) {
    if(singleNodeConnection){
      return _singleNodeConnect(uri, options, cb)
    } else {
      // Don't call DB.connect() as it defers to the native Db.connect(), which
      // doesn't use MongoClient
      return syncOrAsync(
        MongoClient, 'connect', [uri, options], function(db) {
          return new DB(db)
        }, cb
      )
    }
  }
docs: connect
  
  @param {!String} uri
  @param {Object=} options
  @param {Boolean} singleNodeConnection
  @param {function} [cb=undefined] execute asynchronously if present (signature: cb(err, result))
  @return {Object} returns the database object
  
  Options are as specified for MongoClient.connect
  http://mongodb.github.io/node-mongodb-native/driver-articles/mongoclient.html#mongoclient-connect-options
  
  Unfortunately, MongoClient.connect will always return a connection to the
  master node if it can find one.  In order to connect to specific node, set
  singleNodeConnection to true and a different connection strategy will be
  used. singleNodeConnections cannot recognize URI options.