Yes i didn't copy it correctly.I was missing a logical "or" operator between the expressions of "if".Now it works :
<script language="javascript">
function convert (form)
{
if ((form.first.value == "") ||
(form.second.value ==""))
return;
form.result.value=form.first.value * form.second.value;
}
</script> and
<form method="post" >
Give first number:<input type="text" name="first" value=0 onchange=convert(this.form)>
<p>Give second number:<input type="text" name="second" value=0 onchange=convert(this.form)></p>
<p><input type="button" name="calculate" value="calculate" onclick=convert(this.form)></p>
<p>Result:<input type="text" name="result" value="not calculated" ></p><br>
</form>
Sorry for the stupid post