What's the best way to do javascript validation on ASP.Net page with Masterpage?

<script language="javascript" type="text/javascript">
  var control = document.getElementById('<%= control.ClientID %>');
</script>

<script>
    var SomeControl = <%= Control1.ClientID %>;
    var AnotherControl = <%= Control2.ClientID %>;
    // etc, etc.
</script>

<%= Control1.ClientID%>

<script>
       var <%=c1.ClientID %> = new SomeJsClass(<%=c1.ClientID %>);
       var <%=c2.ClientID %> = document.getElementById('<%=c1.ClientID %>');
   </script>