javascript not working

here is my javascript which was not working atrer submit button where is i'm wrong?
 function onSubmit()
     {
      var RB1 = document.getElementById("<%=rbGender.ClientID%>");  
      var radio = RB1.getElementsByTagName("input"); 
      var isChecked=false;
       var CB1 = document.getElementById("<%=ChkAlertMsg.ClientID%>");  
      var chk = CB1.getElementsByTagName("input"); 
      var ischk=false;
for (var i=0;i<radio.length;i++)
            {  
            if (radio[ i ].checked)
            {  
   ...