<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
 "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
    <head>
        <title>$F</title>
        <script type="text/javascript" language="javascript"
         src="prototype.js" ></script>
        <script type="text/javascript" language="javascript">
        function test()
        {
            alert($F("userName"));
        }
        </script>
    </head>
    <body>
        <form>
            <input type="text" id="userName" value="test username">
            <input type="button" value="click" onclick="test()">
        </form>
</html>