Making Mocha test work for my db module

I am new to Mocha, and only a little experience with Node/Express. My DbProvider module works perfectly (mongodb) when I am access it through my Express app. And now I want to test it. I have read the Mocha site and some tutorials I could find. But I have big trouble of finding an real-world example out there that I could follow (any links much appreciated!).

Here is my unsuccessful attempt to write a testfile:

var DbProvider = require('../db').DbProvider;
var assert     = require('assert');
var dbProvider = new DbProvider('localhost', 27017, 'mydb');
var util       = require('util');

console.log(util.inspect(dbProvider));

describe('DbProvider', function(){
  describe('findAllNotes', function(){
    it('should return some notes', function(){
      dbProvider.findAllNotes({}, function (err, result){
        assert(result.length > 0);
      });
    })
  })
})

The output I get is this:

$ mocha
{}

  ✖ 1 of 1 test failed:

  1) DbProvider findAllNotes should return some notes:
     TypeError: Cannot call method 'collection' of undefined
      at DbProvider.doOperation (/Users/frode/Node/json/db.js:46:11)
      at DbProvider.findAllNotes (/Users/frode/Node/json/db.js:56:8)
      at Context.<anonymous> (/Users/frode/Node/json/test/test.js:15:18)
(cutting out the rest)

It seems that I am unsuccessful to create the dbProvider. This works perfectly in my app... How can I make this work? (And perhaps also: Is the way I have set it up in general ok?)

Edit: Here is the db.js file:

// Database related
'use strict';

var MongoClient       = require('mongodb').MongoClient;
var BSON              = require('mongodb').BSONPure;
var ObjectID          = require('mongodb').ObjectID;
var checkForHexRegExp = new RegExp("^[0-9a-fA-F]{24}$");
var Validator         = require('validator').Validator
var fieldMaxLength    = 1024;
//var util              = require('util');

var DbProvider = function(host, port, database) {
  var dbUrl = "mongodb://"+host+":"+port+"/"+database;
  var self = this;
  MongoClient.connect(dbUrl, function(err, db) {
    self.db = db;
  });
};

// Do some basic validation on the data we get from the client/user
var validateParams = function(params, callback) {
  // Let´ do a quick general sanity check on the length on all fields
  for(var key in params) {
    if(params[key].length > fieldMaxLength) callback(new Error('Field ' + key + ' is too long.'));
  }
  // and the let us check some specific fields better
  if (params._id) {
    if(checkForHexRegExp.test(params._id)) {
      // In case of '_id' we also need to convert it to BSON so that mongodb can use it.
      params._id = new BSON.ObjectID(params._id);
    } else {
      var err = {error: 'Wrong ID format'};
    }
  }
  if(err) callback(err);
}

// Generalized function to operations on the database
// Todo: Generalize even more when user authenication is implemented
DbProvider.prototype.doOperation = function(collection, operation, params, callback) {
  validateParams(params, callback);
  var operationCallback = function(err, result) {
    callback(err, result);
  };
  this.db.collection(collection, function(err, collection) {
    if(operation==='find') {
      collection.find().toArray(operationCallback);
    } else {
      collection[operation](params, operationCallback);
    }
  });
}

DbProvider.prototype.findAllNotes = function(params, callback) {
  this.doOperation('notes', 'find', params, callback);
};

DbProvider.prototype.findNoteById = function(params, callback) {
  this.doOperation('notes', 'findOne', params, callback);
};

DbProvider.prototype.saveNote = function(params, callback) {
  params.created_at = new Date();
  this.doOperation('notes', 'save', params, callback);
};

DbProvider.prototype.deleteNote = function(params, callback) {
  this.doOperation('notes', 'remove', params, callback);
};

DbProvider.prototype.findUser = function(params, callback) {
  this.doOperation('users', 'findOne', params, callback);
};

exports.DbProvider = DbProvider;

SOLUTION:

After Benjamin told me to handle the async nature of mongodb connecting to the database, and inspired by his suggestion on how to adapt the code, I split the constructor function DbProvider into two parts. The first part, the constructor DbProvider now just saves the db-parameters into a variable. The second part, a new function, DbProvider.connect does the actual async connection. See below.

var DbProvider = function(host, port, database) {
  this.dbUrl = "mongodb://"+host+":"+port+"/"+database;
};

DbProvider.prototype.connect = function(callback) {
  var self = this;
  MongoClient.connect(this.dbUrl, function(err, db) {
    self.db = db;
    callback();
  });
};

So I can now make a Mocha test like this (and async tests also need the "Done" included, like you see in the code below):

var assert     = require('assert');
var DbProvider = require('../db').DbProvider;
var dbProvider = new DbProvider('localhost', 27017, 'nki');

describe('DbProvider', function(){
  describe('findAllNotes', function(){
    it('should return some notes', function(done){
      dbProvider.connect(function(){
        dbProvider.findAllNotes({}, function (err, result){
          assert(result.length > 0);
          done();
        });
      });
    })
  })
})

Note that the acutal test ("should return some notes") is nothing to be proud of. What I wanted here was to get set up so I am able to test something. Now that I finally acutally can do that, I need to write good tests (something in the the line of having a test database, clear it, test insert a document, test search for a document, and so on...).

And in my Express app, I used to set up the database like this:

var DbProvider    = require('./db').DbProvider;
// Setup db instance
var dbProvider = new DbProvider(
  process.env.mongo_host       || 'localhost',
  process.env.mongo_port       || 27017,
  process.env.mongo_db         || 'nki'
);

Now I do the same, but in addition, I call the new connect-function:

// Connect to db. I use (for now) 1 connection for the lifetime of this app.
// And I do not use a callback when connecting here (we do in the testing)
dbProvider.connect(function(){});

Benjamin actually pointed out that it may be ok but not the best practice to have the database set up like this in an Express app. But until I figure out what the best practice really is, I will leave this code as it is. Here is a couple of links reagarding the subject I found (but I have still not concluded of how I will solve it myself):
What's the best practice for MongoDB connections on Node.js? and
[node-mongodb-native] MongoDB Best practices for beginner

If you like, you are very welcome to follow/fork/whatever this project on github. My goal is to get it as production ready I can. The link is https://github.com/frodefi/node-mongodb-json-server