JavaScript实现小球在指定区域内反弹运动

<!DOCTYPE html>
<html>
 <head>
 <meta charset="utf-8">
 <title>小球运动</title>
 <style type="text/css">
 #box {
 width: 600px;
 height: 300px;
 border: 1px solid red;
 position: relative;
 }
 #ball {
 width: 50px;
 height: 50px;
 border-radius: 25px;
 background-color: yellow;
 position: absolute;
 top: 0;
 left: 0;
 }
 button {
 position: relative;
 }
 </style>
 </head>
 <body>
 <div id="box">
 <div id="ball">
 
 </div>
 </div>
 <button id="stop" onclick="fly()">开始</button>
 <button id="stop" onclick="stop()">停止</button>
 <script type="text/javascript">
 var ball = document.getElementById("ball");
 //console.log(ball.offsetLeft);
 var sport;
 function fly() {
 var speedx = 1;
 var speedy = 1;
 
 sport = setInterval(function () {
  ball.style.left = ball.offsetLeft + speedx + 'px';
  ball.style.top = ball.offsetTop + speedy + 'px';
  
  if (ball.offsetTop >= 300 -50 || ball.offsetTop <= 0) {
  speedy *= -1; 
  }
  if (ball.offsetLeft >= 600 -50 || ball.offsetLeft <= 0) {
  speedx *= -1; 
  }
 }, 10)
 }
 function stop() {
 clearInterval(sport); //停止小球的运动
 }
 </script>
 </body>
</html>