JavaScript lastIndexOf() Method

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="index2.aspx.cs" Inherits="MyApp.index2" %>

<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
    <script type="text/javascript">
        function getTopLevelDomain() {
            var url = document.getElementById("txtURL").value;
            if (url == "") {
                alert("Please enter a valid URL");
                document.getElementById("txtURL").focus();
            }
            else {
                var domain = url.substring(url.lastIndexOf("."));
                document.getElementById("txtDomain").value = domain;
            }
        }
    </script>
</head>
<body>
    <form id="form1" runat="server">
        <div>
            <table style="border: 1px solid blue; font-family: 'Comic Sans MS'">
                <tr>
                    <td>
                        <h3>Top level Domain Checker</h3>
                    </td>
                </tr>
                <tr>
                    <td>Enter the URL :
                    </td>
                    <td>
                        <input id="txtURL" type="text" />
                    </td>
                    <td>
                        <input id="btnFetch" type="button" value="Fetch" onclick="getTopLevelDomain();"
                            style="font-family: 'Comic Sans MS'" /></td>
                </tr>
                <tr>
                    <td>Top Level Domain : </td>
                    <td>
                        <input id="txtDomain" type="text" disabled="disabled" />
                    </td>
                </tr>
            </table>
        </div>
    </form>
</body>
</html>