An express.js application that uses async.js to initialize a MongoDB database connection pool before the application starts listening for HTTP requests. The database connection pool is shared using an application-wide variable named 'mongodb'.

app.js

var async = require('async'),
  MongoClient = require('mongodb').MongoClient,
  express = require('express');

var MONGODB_URL = 'mongodb://<username>:<password>@<host>:<port>/<database>',
    PORT = -1;

async.waterfall([
  function(cb) {
    MongoClient.connect(MONGODB_URL, function(err, db) {
      if (err) {
        return cb(err);
      } else {
        console.log("Connected to MongoDB");
        cb(null, db);
      }
    });
  },
  function(db, cb) {

    try {
      app = express();
      app.set('mongodb', db);

      // Simple inline example of using the connection.
      // You should use async or similar here as well
      // (kriskowal/q is also popular) instead 
      // of nesting callbacks, and creating "callback hell".

      app.get('/users/:userId', function(req, res, next) {
        var db = req.app.get('mongodb'),
            userId = req.params['userId'];

        db.collection('users').findOne({'_id': userId}, function(err, userDoc) {
          if (err) {
            res.send(500);
          }
          if (!userDoc) {
            res.send(404);
          } else {
            res.send(userDoc);
          }
        });
      });

      app.listen(PORT);

      cb();
    } catch (err) {
      cb(err);
    }
  }
],
function(err, results) {
  if (err) {
    console.error(err.message);
  } else {
    console.log("App Started");
  }
});