我如何添加警告框，说“游戏结束”当球触及顶部边界?

I am creating a 2 player tennis sort of game right now in JavaScript and I got the ball to make an alert box popup saying "gameover" when it hits the top, but I can't seem to make it work on the top border. If someone could tell me what I'm doing wrong thanks in advance. I am creating a 2 player tennis sort of game ri