Wie kann ich mit Express ohne Mongoose eine Verbindung zu Mongodb herstellen?

Befolgen Sie das Beispiel aus meinem Kommentar und ändern Sie es so, dass die App Fehler behandelt, anstatt den Server nicht zu starten.

var express = require('express');
var mongodb = require('mongodb');
var app = express();

var MongoClient = require('mongodb').MongoClient;
var dbURL = "mongodb://localhost:27017/integration_test";
var db;

// Initialize connection once
MongoClient.connect(dbURL, function(err, database) {
  if(err) return console.error(err);

  db = database;

  // the Mongo driver recommends starting the server here 
  // because most apps *should* fail to start if they have no DB.
  // If yours is the exception, move the server startup elsewhere. 
});

// Reuse database object in request handlers
app.get("/", function(req, res, next) {
  var collection = "replicaset_mongo_client_collection";
  db.collection(collection).find({}, function(err, docs) {
    if(err) return next(err);
    docs.each(function(err, doc) {
      if(doc) {
        console.log(doc);
      }
      else {
        res.end();
      }
    });
  });
});

app.use(function(err, req, res){
  // handle error here.  For example, logging and 
  // returning a friendly error page
});

// Starting the app here will work, but some users 
// will get errors if the db connection process is slow.  
app.listen(3000);
console.log("Listening on port 3000");