The $F() function returns the value of any field input control, like text boxes or drop-down lists. This is a convenience alias of Form.Element.getValue.
The function can take as argument either the element id or the element object itself.
Syntax:
$F(element)
Return Value:
Form's element Value
Example:
<html>
<head>
<title>Prototype examples</title>
<script type="text/javascript"
src="/javascript/prototype.js">
</script>
<script>
function ShowValue(){
var value = $F('userName');
alert( 'Entred Value :' + value);
}
</script>
</head>
<body>
<p>Enter any value in the box and then click "Show Value"</p>
<form>
<input type="text" id="userName" value="Madisetti, Praveen">
<br/>
<input type="button" value="Show Value" onclick="ShowValue();"/>
</form>
</body>
</html>