Code:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html xmlns="">
<head>
<title>New document</title>
<script type="text/javascript">
var i = 1;
function thingy()
{
var string = "fake stuff";
document.getElementById("stuff").value = string.substr(0,i);
i++;
}
</script>
</head>
<body>
<input type="text" id="stuff" onkeyup="thingy();"/>
</body>
</html>