请指教验证表单复选框问题？验证表单复选框是否选中并输出选中个数．

<htlm>
<head>
<script Language="JavaScript">
function form_check(){
   var an=new Array("answer1","answer2","answer3","answer4","answer5");
   var j=0;
   for(var i=0;i<5;i++){
 if (document.form1.elements[an[i]].checked) j++;
 }
 window.alert(j); return false;
}
   </script>
</head>
<form name="form1">
<input type="checkbox" name="answer1"  value="1">
<input type="checkbox" name="answer2" value="1">
<input type="checkbox" name="answer3" value="1">
<input type="checkbox" name="answer4" value="1">
<input type="checkbox" name="answer5" value="1">
<input type="button" value="test" onClick="form_check()">
</form>