Check Radio button from JavaScipt

Find which Radio button is Checked from JavaScript : 

JS :

 function check(obj) {

           var a = null;

           var f = document.forms[0];

           var e = f.elements[obj];

if (e[0].checked == true) {               
alert(“1 radio button is checked”)

}

           else {

alert(“2 radio button is checked”)

           }

       }

 HTML :

  <asp:RadioButtonList ID="RDB" runat="server" >

           <asp:ListItem Value="1">Ajay</asp:ListItem>

           <asp:ListItem Value="2">Patel</asp:ListItem>

        </asp:RadioButtonList>

        <asp:Button ID="btn" runat="server" Text="Button" />

        <input type="submit" value="submit" />