A bit simpler

gistfile1.js

bc = {
    auth:{
        configuration_type: 'replicaset',
        replicaset: {
            db_name: 'auth',
            servers: [
                {host:'localhost',port:4000,options:{auto_reconnect:true}},
                {host:'localhost',port:4001,options:{auto_reconnect:true}}
                ,{host:'localhost',port:4002,options:{auto_reconnect:true}}
            ],

                options: {
                    rs_name: 'replset0',
                    read_secondary: true,
                    socketOptions: { timeout: 0, keepAlive: 1 }
                }

        },

        standalone: {
            db_name:'auth',
            host:'UAT-NJ-MONGO-01',
            port:36108,
            options:{
                auto_reconnect: true,
                poolSize: 3,
                socketOptions: { keepAlive: 1 }
            }
        }
    },

    session:{
        configuration_type: 'standalone',
        replicaset: {

            db_name: 'sessions',

            servers: [
            {host:'UAT-NJ-MONGO-01',port:36108,options:{auto_reconnect:true}},
            {host:'UAT-NJ-MONGO-02',port:36108,options:{auto_reconnect:true}}
            ,{host:'UAT-NJ-MONGO-03',port:36108,options:{auto_reconnect:true}}

            ],

                options: {
                    rs_name: 'LANDER',
                    read_secondary: true,
                    socketOptions: { timeout: 0, keepAlive: 1 }
                }

        },
        standalone: {
            db_name:'sessions',
            host:'UAT-NJ-MONGO-01',
            port:36108,
            options:{
                auto_reconnect: true,
                poolSize: 3,
                socketOptions: { keepAlive: 1 }
            }
        }
    },

    sso:{
        configuration_type: 'replicaset',
        replicaset: {

            db_name: 'sso',

            servers: [
            {host:'UAT-NJ-MONGO-01',port:36108,options:{auto_reconnect:true}},
            {host:'UAT-NJ-MONGO-02',port:36108,options:{auto_reconnect:true}}
            ,{host:'UAT-NJ-MONGO-03',port:36108,options:{auto_reconnect:true}}

            ],

                options: {
                    rs_name: 'LANDER',
                    read_secondary: true,
                    socketOptions: { timeout: 0, keepAlive: 1 }
                }

        },
        standalone: {
            db_name:'sso',
            host:'UAT-NJ-MONGO-01',
            port:36108,
            options:{
                auto_reconnect: true,
                poolSize: 3,
                socketOptions: { keepAlive: 1 }
            }
        }

    }

};

var connect = function connect(configurationName, configurationList, callback) {
	var config = configurationList[configurationName];
	var configType = config[config.configuration_type];
	var db = null;
	
	switch(configType) {
		case 'replicaset':
		  var servers = config[configType].servers.map(function(properties) { return new Server(properties.host, properties.port)});
			var replicaset = new ReplSet(servers, config[configType].options);
			db = new Db(config[configType].db_name, replicaset);
			break;
		case 'standalone':
		  db = new Db(config[configType].db_name, new Server(config[configType].host, config[configType].port, config[configType].options));
	}
	
	// Open db
	db.open(callback);
}

connect("auth", bc, function(err, db) {
	db.on("error", function(err) {		
	});

	db.on("close", function(err) {		
	});

	db.on("timeout", function(err) {		
	});
})