trasferire dati in un textfield con un click

[badaze]

<script language="JavaScript" type="text/javascript">
<!--
function inc(textfield) {
textfield.value++;
}
function dec(textfield) {
textfield.value--;
}
//-->
</script>

<input type="text" id="txt">
<input type="button" value="Piu" onclick="inc(document.getElementById('txt'))"/>
<input type="button" value="Menu" onclick="dec(document.getElementById('txt'))"/>