How to get radiobuttonlist selected value in aspx?

Hi

I need to get the selected value of a radio button list in aspx by javascript.

But it is undefined. I tried ctl01_op too as I saw the id changed to it in html.

function cald() {
var radioList = document.getElementsByName("op");
            for( var i=0; i < radioList.length; ++i)
            {
               if( radioList[i].checked)
              {
                   alert("Checked item: " + radioList[i].value)
              }
            }
}
 
 
<asp:RadioButtonList Runat="server" ID="op" name="op" RepeatDirection="Horizontal" TextAlign="Left" onchange='cald()'>
<asp:ListItem Value="*" Selected="true">*</asp:ListItem>
<asp:ListItem Value="/">/</asp:ListItem>
</asp:RadioButtonList>

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