Find by specific code results in null

I'm very new to Nodejs and i'm trying to find a record in my db(mongo) by submitting a code and crosschecking the record to see if it exists.

I'm not really clear on how to approach this to get to an endresult stating "There is a code, please continue" and "There is no code".

This is what i have so far as a result of my fiddling:

userProvider.js:

UserProvider.prototype.findByCode = function(code, callback) {
    this.getCollection(function(error, user_collection) {
        user_collection.findOne({code: code}, function(error, result) {
            callback(error, result)
        });
    });
}

The get collection belonging to userProvider.js:

UserProvider.prototype.getCollection= function(callback) {
  this.db.collection('users', function(error, user_collection) {
    if( error ) callback(error);
    else callback(null, user_collection);
  });
};

app.js:

app.post('/game/code', function(req, res) {
    if (req.param('code')) {
        userProvider.findByCode(req.param('code'), function(error, result) {
            console.log(error);
            console.log(result);
        });
    }
});

And the jade form submitting the code:

extends layout

block content
    h1= "Gimme a code"
    div
        form( method="post")
            div
                div
                    span.label code :
                    input(type="text", name="code", id="code")
                div
                    input(type="submit", value="Verbind")

Console logging the param('code') results in the code. So i'm pretty sure that one's solid. So why's there a null returned?

From the question, you didn't say where the 'null' was returned. I am new to nodejs as well, however I noticed in your code:

app.post('/game/code', function(req, res) {
    if (req.param('code')) {
        userProvider.findByCode(req.param('code'), function(error, result) {
            console.log(error);
            console.log(result);
        });
    }
});

and you didn't call the 'res' object. You should try something like this:

app.post('/game/code', function(req, res) {
    if (req.param('code')) {
        userProvider.findByCode(req.param('code'), function(error, result) {
            if(error) 
              res.send(404, 'There is no code');
            else {
              console.log(result);
              res.send('There is a code:'+result);
            }
        });
    }
});

Check out http://expressjs.com/api.html#res.status for various things you do with res object