Session

If I have a session set to 20 minutes
and is possible that user will play javascript game more than 20 minutes
, does that means, that at time user will want to save game poits
, that he/she will get 'session expired' ?

Something like that. You could increase the session timeout, 20 mins is short when you expect people to be there for a while. Or you could have the game keep the session alive every 5 minutes or so.