controllers/users.js

var usersModel = require('../models/users.js');

users.create = function(req, res){
  usersModel.create(req.data.username, req.data.password, function(error, user){
    if(error){
      res.statusCode = 500;
      res.end('server-error');
      return;
    }

    res.statusCode = 201;
    res.end(user._id);
  });
};

models/users.js

var db = require('../mongodb.js');

module.exports.create = function(username, password, callback){
  db.collection('main', 'users', function(error, collection){
    if(error){
      callback(error);
      return;
    }
    
    collection.insert({
      username: username,
      password: password,
      createdOn: new Date()
    }, callback);
  });
};

mongodb.js

var mongo = require('mongodb')
  , mongodb = module.exports
  , connections = {}
  , config = {
    main: {
      name: 'application-main',
      server: new mongo.Server('127.0.0.1', 27017, { auto_reconnect: true })
    }
  };

mongodb.database = function(key, callback){
  if(connections[key]){
    callback(null, connections[key]);
    return;
  }
  
  var db = mongo.Db(config[key].name, config[key].server);
  
  db.open(function(error, connection){
    if(error){
      callback(error);
      return;
    }

    connection.on('close', function(){
      delete(connections[key]);
    });

    connections[key] = connection;
    callback(null, connection);
  }
};

mongodb.collection = function(dbKey, collName, callback){
  mongodb.database(dbKey, function(error, connection){
    if(error){
      callback(error);
      return;
    }

    connection.collection(collName, function(error, collection){
      if(error){
        callback(error);
        return;
      }

      callback(null, collection);
    });
  });
};

As you can see, the errors are passed through every callback, but infact, it might be better to throw an error at higher levels for example, look at the mongodb.js database function, it passes through an error in the callback, however if there was an error at that level it might be better to deal with it at that point and just fall back on procces.on('uncaughtException')

It is alot of extra code each time, dealing with errors in callbacks. And I am wonder if there is a better way around it.