Moltiplicazione Automatica

<form action="#" name="my_form" method="POST">
    
    <input type="text" name="prezzo1"><br>
    <input type="text" name="quantita1"><br>
    <input type="text" name="totaleanalitico1" value="0.00" disabled>
    
</form>


<script>
    
    window.onkeypress = setInterval('changeTotale()',100);
    
    function changeTotale(){
        var num = parseFloat(document.my_form.prezzo1.value)*parseFloat(document.my_form.quantita1.value);
        
        if(num>=0)
        document.my_form.totaleanalitico1.value = num;
    }
    
</script>

document.my_form.totaleanalitico1.value = num.toFixed(2);