ReferenceError for Heroku MongoLab Connect - node.js

Question

I am very close to an express app working in a sandbox environment on Heroku with a MongoDB addon..
I am still seeing a ReferenceError in the log, which seems to be the reason for the app crashing.
The first error was regarding this configuration:
//MongoDB
var mongodb = require('mongodb');
var db;
var MONGODB_URI = process.env.MONGOLAB_URI;
var PORT = process.env.PORT;
mongodb.MongoClient.connect(MONGODB_URI, function(err, database) {
if(err) throw err;
db = database;
app.listen(PORT);
console.log('Listening on port ' + PORT);
});
var testColl = db.collection('test');
That led to the log reading a TypeError, probably because db was defined locally..
app[web.1]: var testColl = db.collection('test');
app[web.1]: TypeError: Cannot call method 'collection' of undefined
And the recent error was a ReferenceError:
app[web.1]: var db = database;
app[web.1]: ReferenceError: database is not defined
Probably because database is defined globally, but not referenced within the context of the connection..
//MongoDB
var MongoClient = require('mongodb').MongoClient,
var Server = require('mongodb').Server;
var mongoUri = process.env.MONGOLAB_URI;
var port = process.env.PORT;
var db = database;
var mongoclient = new MongoClient(new Server(mongoUri,
{ 'native_parser' : true }));
var testColl = db.collection('test');
mongoclient.open(function (err, mongoclient) {
if (err) throw err
app.listen(port);
console.log("Express server started on port " + port);
});
How can I reformat the code to launch this app on Heroku?

Answer 1

Your first example looks close to working, but there's one thing wrong. mongodb.connect is an asynchronous call, which means it returns right away. That's why it has the callback parameter. Because the code runs so fast, it doesn't have time to actually connect to the database before you try to use the db object, which is what causes your crash in the first set of code.
If you move your database call inside the callback, it should work.
Edit: This is in reply to your comment, but I can't include much code in another comment, so you get an edit.
It really depends on your use case, but I realize that's not very helpful. So here's one way you could do it:
module.exports = function() {
return db;
};
By doing so, you would be able to require it from another file, and use it as such:
var http = require('http');
var db = require('./db.js');
http.createServer(function(req, res) {
db().collection('test', function(err, collection) {
console.log(err, collection);
res.writeHead(200, {
'Content-Type': 'text/plain'
});
res.end('We got it! Unless there is an error...\n');
});
}).listen(8080);
There's definitely ways to improve upon that, but it should get you started.