es importante el evento que estés usando
Cita: <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title></title>
</head>
<script type="text/javascript">
function compruebaTecla (elm, e) {
var key = e.keyCode;
elm.value = key + ' --> ' + String.fromCharCode(key) + ' --> ';
}
</script>
</head>
<body>
<b>onkeydown</b>
<form name="f">
<textarea rows="5" cols="20" onkeydown="compruebaTecla(this, event)"></textarea>
</form>
<br /><br />
<b>onkeypress</b>
<form name="f">
<textarea rows="5" cols="20" onkeypress="compruebaTecla(this, event)"></textarea>
</form>
<br /><br />
<b>onkeyup</b>
<form name="f">
<textarea rows="5" cols="20" onkeyup="compruebaTecla(this, event)"></textarea>
</form>
</body>
</html>