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>