hai dimenticato
var f=document.a;

oppure

<SCRIPT>
function stampa(f) {
var str=""
for( var i=0; i<f.elements.length; i++){
str+=f.elements[i].value;
}
document.write(str);
}
</SCRIPT>
<form name=pippo>
<input type=text name=nome>
<input type=text name=cognome>
<input type=button value="VAI" onClick="stampa(this.form)">
</form>

ciao