<html>
<head>
<script>
Number.prototype.decimal = function (num)
{
pot = Math.pow(10,num);
return parseInt(this * pot) / pot;
}
function decimal(src) {
n=eval(document.getElementById(src).value);
alert(n.decimal(3)); //deja 3 decimales
}
</script>
</head>
<body>
<input type="text" id="numero" value="3.1415926">
<input type="button" onClick="decimal('numero')">
</body>
</html>
