碰到上左边框判断后不越界，可碰到下右边框判断后越界？

JScript code

_x=Math.abs(dog_left-box_left);        _y=Math.abs(dog_top-box_top);            if(obj.value=="上" && !(_y==0))        {            dog.style.top=(dog_top-10)+"px";            dog_top=dog_top-10;        }else if(obj.value=="下" && !(_y==(box_height-dog_height)))//这里要越界        {            dog.style.top=(dog_top+10)+"px";            dog_top=dog_top+10;        }else if(obj.value=="左" && !(_x==0))        {            dog.style.left=(dog_left-10)+"px";            dog_left=dog_left-10;            }else if(obj.value=="右" && !(_x==(box.width-dog_width)))//这里要越界        {            dog.style.left=(dog_left+10)+"px";            dog_left=dog_left+10;        }