JavaScript实现DIV元素左右往复滚动效果

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
</head>
<body>
<div id="xx" style=" position:relative;left:100px;width:100px;height:50px;border: solid 1px red;">本站</div>
<script type="text/javascript">
  function ls(){
    var p=true;//开关,true开,false关
    var xx=document.getElementById('xx');
    var xxleft = parseInt(xx.style.left);
    var speed = 20;
    var t=setInterval(rollRight,speed);
    //向左移动
    function rollLeft(){
      if(p){
        xxleft--;
        xx.style.left = xxleft+'px';
        if(xxleft <=0 ){
          clearInterval(t);
          t=setInterval(rollRight,speed);
        }
      }
    }
    //向右移动
    function rollRight(){
      if(p){
        xxleft++;
        xx.style.left = xxleft+'px';
        if(xxleft >= 300){
          clearInterval(t);
          t=setInterval(rollLeft,speed);
        }
      }
    }
  }
  ls();
</script>
</body>
</html>