                        课程
                    
                        /前端开发
                        
                            /JavaScript
                        
                        /Tab选项卡切换效果

怎么不能实现呢

<!doctype html>
<html>
<head>
    <meta charset="UTF-8">
<title>Document</title>
    <style>
        #receive{display:none;}

    </style>
<script type="text/javascript">
window.onload=function(){
     var send=document.getElementById('send'),
            receive=document.getElementById('receive'),
         times=60,
         timer=null;
     send.onclick=function(){
       // 计时开始
          timer=setInterval('showTime',500);
     }
        function showTime(){
            if(times>=0){
              receive.style.display='block';
                receive.disabled ='false';
                receive.value=times+'秒后重试';
                times--;
    send.disabled='false';
            }
            else{
    clearInterval(timer);
      receive.style.display='none';
            }
        }
}
</script>
</head>
<body>
<input type="button" id="send" value="发送验证码" >
    <input type="button" id="receive" >
</body>
</html>