javascript 验证 radiobox

<html>
 <head>
  <script type = "text/javascript">
   function clickk(){
    x = document.getElementsByName("aa");
    count = 0
    for (i=0;i< x.length;i++ )
    {
     if (x[i].checked)
     {
      count++;
     }
    }
    if (count == 0)
    {
     alert("no radio");
    }
   }
  </script>
 </head>
 <body>
 <form name = 'myform'>
  <input type ="radio" name = "aa">aaaa</input>
  <input type ="radio" name = "aa">bbbb</input>
  <input type ="radio" name = "aa">cccc</input>
  <br/>
  <input type = "button" value ="javascript" onclick="clickk();"></input>
  <br/>
  <input type = "text"></input>
 </form>
 </body>
</html>