nodejs_express_mongodb.js

// database.js
sys = require('sys');

var mongo = require('./lib/node-mongodb-native/lib/mongodb'),
  Connection = mongo.Connection,
  Server = mongo.Server,
  BSON = mongo.BSONNative;
  
var host = process.env['MONGO_NODE_DRIVER_HOST'] != null ? process.env['MONGO_NODE_DRIVER_HOST'] : 'localhost';
var port = process.env['MONGO_NODE_DRIVER_PORT'] != null ? process.env['MONGO_NODE_DRIVER_PORT'] : Connection.DEFAULT_PORT;

sys.puts("Connecting to database at " + host + ":" + port);

var db = new mongo.Db('node-test', new Server(host, port, {}), {native_parser:true});
// didn't have the next line originally
db.open(function(){});

db.addListener("error", function(error) {
  console.log("Error connecting to mongo -- perhaps it isn't running?");
});

module.exports = db;


// middleware/testMiddle.js
db = require('../database');
module.exports = function(){
  return function(req,res,next){
    // don't open connections on every request
    // db.open(function(err,db){
      res.myvar = typeof(db);
      next();
    // });
  }
}


// app.js
sys = require('sys');
var foo = require('./middleware/testMiddle');
var express = require('express');
var app = module.exports = express.createServer();

app.configure(function(){
  app.set('views', __dirname + '/views');
  app.set('view engine', 'jade');
  app.use(express.bodyDecoder());
  app.use(express.methodOverride());
  app.use(express.cookieDecoder());
  app.use(express.compiler({ src: __dirname + '/public', enable: ['sass'] }));
  app.use(app.router);
  app.use(express.staticProvider(__dirname + '/public'));
});

app.configure('development', function(){
  app.use(express.errorHandler({ dumpExceptions: true, showStack: true })); 
});

app.configure('production', function(){
  app.use(express.errorHandler()); 
});

// Routes

app.get('/', foo(), function(req, res){          
  res.render('index', {
    locals: {
      title: 'Test Page',
      myvar: res.myvar
    }
  }); 
});

// Only listen on $ node app.js

if (!module.parent) {
  app.listen(3000);
  console.log("Express server listening on port %d", app.address().port)
}

this.host = process.env['MONGO_NODE_DRIVER_HOST'] !== undefined ? process.env['MONGO_NODE_DRIVER_HOST'] : 'localhost';
this.port = process.env['MONGO_NODE_DRIVER_PORT'] !== undefined ? process.env['MONGO_NODE_DRIVER_PORT'] : mongo.Connection.DEFAULT_PORT;

Seems to be better IMO. And this way it's passes JSLint validation ;)