Mine

{ 
  snap: function(x,y){ 
    if(x < 0) x=0; 
    if(y < 0) y=0; 
    if((x + your_draggable.offsetWidth) > document.body.clientWidth) 
      x = document.body.clientWidth; 
    if((y + your_draggable.offsetHeight) > document.body.clientHeight)
      y = docment.body.clientHeight;
    return [x,y] 
  }
}