如何在Connect-Mongo中未运行数据库时捕获错误？

Question

我有：

            var MongoStore = ConnectMongo(express);
            sessionStore = new MongoStore({
                db: databaseName,
                port: port,
                host: host,
            })
            debugger;
            sessionStore.db.on("error", function(err) {
                // code never reached and throws the error
                // I need to reach this code and don't throw that error.
                console.log("Check if your mongodb database is running at " + host + ':' + port + " ".yellow);
                return console.log(err.message.red);
            });

但它没有发现错误。你知道更好的方法吗？

抛出错误：

C:\web\easy-express\node_modules\connect-mongo\lib\connect-mongo.js:155
      throw new Error('Error connecting to database <' + err + '>');
            ^
Error: Error connecting to database <Error: failed to connect to [localhost:27017]>
at C:\web\easy-express\node_modules\connect-mongo\lib\connect-mongo.js:155:17
at C:\web\easy-express\node_modules\connect-mongo\node_modules\mongodb\lib\mongodb\db.js:229:16
at null.<anonymous> (C:\web\easy-express\node_modules\connect-mongo\node_modules\mongodb\lib\mongodb\connection\server.js:578:7)
at EventEmitter.emit (events.js:106:17)
at null.<anonymous> (C:\web\easy-express\node_modules\connect-mongo\node_modules\mongodb\lib\mongodb\connection\connection_pool.js:139:15)
at EventEmitter.emit (events.js:98:17)
at Socket.<anonymous> (C:\web\easy-express\node_modules\connect-mongo\node_modules\mongodb\lib\mongodb\connection\connection.js:475:10)
at Socket.EventEmitter.emit (events.js:95:17)
at net.js:441:14
at process._tickCallback (node.js:415:13)

感谢