e se scrivi:

codice:
<script>
function aumenta() {
document.forms['nomeform'].elements['qta[]'].value++;
}

function diminuisci() {
if(document.forms['nomeform'].elements['qta[]'].value>0)
document.forms['nomeform'].elements['qta[]'].value--;
}
</script>
<form name="nomeform">
<input type="text" name="qta[]" value="1" readOnly>
[img]meno.gif[/img]
[img]piu.gif[/img]
</form>