[ASP.NET][JavaScript]JavaScript取得radiobuttenlist 值傳給控制項

function displayrbl()
        {
            var selValue;
            var rbl = document.getElementById("ctl00_ContentPlaceHolder1_rblPORTCODE");
            var rbllist = rbl.getElementsByTagName("INPUT");

            for (var i = 0; i < rbllist.length; i++) {
                if (rbllist[i].checked) {
                    selValue = rbl.childNodes[(i*2)+1].innerText;
                }
            }

            document.getElementById('ctl00_ContentPlaceHolder1_ftbPORTNM').value = selValue;
        }
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<asp:RadioButtonList ID="rblPORTCODE" runat="server" RepeatDirection="Horizontal" RepeatLayout="Flow"  ColumnName="PORTCODE" onchange="displayrbl();">
                            </asp:RadioButtonList>