ho riesumato ed adattato alcune vecchie funzioni a volte che non siano utili:
codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
	<head>
		<title>a</title>
		<meta name="vs_defaultClientScript" content="JavaScript">
		<meta name="vs_targetSchema" content="http://schemas.microsoft.com/intellisense/ie5">
		<meta name="GENERATOR" content="Microsoft Visual Studio .NET 7.1">
		<meta name="ProgId" content="VisualStudio.HTML">
		<meta name="Originator" content="Microsoft Visual Studio .NET 7.1">
		<script id="clientEventHandlersJS" language="javascript">
<!--

function Button1_onclick() 
{
	var number = (document.getElementById("numero").value);
	var dec = (document.getElementById("decimali").value); //if(dec == "") dec = null;
	var sepm = document.getElementById("sepmigliaia").value;
	var sepd = document.getElementById("sepdecimale").value;
	
	var n = fix(number, dec);
	document.getElementById("risultato_fix").innerHTML = n;
	n = formatCurrency(number,dec,sepm,sepd);
	document.getElementById("risultato_formatCurrency").innerHTML = n;
}

//------------------------------------------------
//Arrotonda number a dec cifre
//se dec non è specificato, arrotonda a 10 cifre
//------------------------------------------------
function fix(number, dec)
{ 
	var n =(dec >=0)? Math.pow(10,dec) : 1e10; 
	return Math.round(number * n) / n; 
} 

//-------------------------------------------------
//arrotonda il numero num a dec decimali
// num = numero da formattare
//dec  = numero di decimali, predefinito = 2
//sepm = separatore migliaia, predefinito = spazio
//sepd = separatore decimale, predefinito = virgola
//-------------------------------------------------
function formatCurrency(num, dec, sepm, sepd)
{
   	if(dec == undefined || dec == "") dec = 2;
   	if(sepm == undefined || sepm == "") sepm = " "; sepm = sepm.substr(0,1);
   	if(sepd == undefined || sepd == "") sepd = ","; sepd = sepd.substr(0,1);
   	
   	var d = Math.pow(10,dec);
   	
   	num = num.toString().replace(/\$|\,/g,'');
	if(isNaN(num)) num = "0";
	sign = (num == (num = Math.abs(num)));
		
	num = Math.floor(num * d + 0.50000000001);
	cents = num % d;
	num = Math.floor(num / d).toString();
	if(cents < 10) cents = "0" + cents;
	for (var i = 0; i < Math.floor((num.length - (1+i))/3); i++)
	    num = num.substring(0, num.length - (4 * i + 3)) + sepm + num.substring(num.length - (4 * i + 3));
    return (((sign)?'':'-') + '' + num + sepd + cents);
}


//-->
		</script>
	</head>
	<body MS_POSITIONING="FlowLayout">
		<table>
			<tr>
				<td>Numero:</td>
				<td><INPUT id="numero" type="text" value="10236425.9064" name="numero"></td>
			</tr>
			<tr>
				<td>Decimali:</td>
				<td><INPUT type="text" ID="decimali" NAME="decimali" value="2"></td>
			</tr>
			<tr>
				<td>Sep. migliaia:</td>
				<td><INPUT type="text" ID="sepmigliaia" NAME="sepmigliaia" value=" "></td>
			</tr>
			<tr>
				<td>Sep decimale:</td>
				<td><INPUT type="text" ID="sepdecimale" NAME="sepdecimale" value=","></td>
			</tr>
			<tr><td colspan="2"><INPUT type="button" value="Calcola" ID="Button1" NAME="Button1" language="javascript" onclick="return Button1_onclick()"></td></tr>
			<tr>
				<td>risultato fix:</td>
				<td><span id="risultato_fix"></span></td>
			</tr>
			<tr>
				<td>risultato formatCurrency:</td>
				<td><span id="risultato_formatCurrency"></span></td>
			</tr>
			
		</table>
	</body>
</html>