This is inaccurate, because you have a 10% chance of getting a 4 tile added instead of a 2. That part makes the game tricky, because otherwise you could optimize every solution like an algorithm without ever losing.

    var value = Math.random() < 0.9 ? 2 : 4;

https://github.com/gabrielecirulli/2048/blob/master/js/game_...