Connect to mongodb on heroku using native drivers in express node

On my local machine i connect to mongoDb "tasksdb" database this way:

var server = new Server('localhost', 27017, {auto_reconnect: true, });
db = new Db('tasksdb', server);

db.open(function(err, db) {
if(!err) {
   console.log("Connected to 'tasksdb' database");
   db.collection('tasks', {safe:true}, function(err, collection) {
if (err) {
   console.log("The 'tasks' collection doesn't exist. Creating it with sample data...");
   populateDB();
  }
 });
 }
});

how to force it to work on heroku ? I tried:

var server = new Server(process.env.MONGOLAB_URI, process.env.PORT, {auto_reconnect: true, });
db = new Db('heroku_mydatabase', server);

but it doesn't work. On each example in web people use "connect" but i want to do it this way. Is it possible ?