var num =2.446242342;
num = num.toFixed(2); // 输出结果为 2.45

Math.floor(15.7784514000 * 100) / 100
// 输出结果为 15.77

Number(15.7784514000.toString().match(/^\d+(?:\.\d{0,2})?/)) ;
// 输出结果为 15.77,不能用于整数如 10 必须写为10.0000

<script type="text/javascript"> //保留两位小数  //功能：将浮点数四舍五入，取小数点后2位 function toDecimal(x) { var f = parseFloat(x); if (isNaN(f)) { return; } f = Math.round(x*100)/100; return f; } //制保留2位小数，如：2，会在2后面补上00.即2.00 function toDecimal2(x) { var f = parseFloat(x); if (isNaN(f)) { return false; } var f = Math.round(x*100)/100; var s = f.toString(); var rs = s.indexOf('.'); if (rs < 0) { rs = s.length; s += '.'; } while (s.length <= rs + 2) { s += '0'; } return s; } function fomatFloat(src,pos){   return Math.round(src*Math.pow(10, pos))/Math.pow(10, pos);
    } //四舍五入 alert("保留2位小数：" + toDecimal(3.14159267)); alert("强制保留2位小数：" + toDecimal2(3.14159267)); alert("保留2位小数：" + toDecimal(3.14559267)); alert("强制保留2位小数：" + toDecimal2(3.15159267)); alert("保留2位小数：" + fomatFloat(3.14559267, 2)); alert("保留1位小数：" + fomatFloat(3.15159267, 1)); //五舍六入 alert("保留2位小数：" + 1000.003.toFixed(2)); alert("保留1位小数：" + 1000.08.toFixed(1)); alert("保留1位小数：" + 1000.04.toFixed(1)); alert("保留1位小数：" + 1000.05.toFixed(1)); //科学计数 alert(3.1415.toExponential(2)); alert(3.1455.toExponential(2)); alert(3.1445.toExponential(2)); alert(3.1465.toExponential(2)); alert(3.1665.toExponential(1)); //精确到n位，不含n位 alert("精确到小数点第2位" + 3.1415.toPrecision(2)); alert("精确到小数点第3位" + 3.1465.toPrecision(3)); alert("精确到小数点第2位" + 3.1415.toPrecision(2)); alert("精确到小数点第2位" + 3.1455.toPrecision(2)); alert("精确到小数点第5位" + 3.141592679287.toPrecision(5)); </script>

function get()
{var s = 22.127456 + "";var str = s.substring(0,s.indexOf(".") + 3);alert(str);
}

<script type="text/javascript">
onload = function(){var a = "23.456322";var aNew;var re = /([0-9]+.[0-9]{2})[0-9]*/;aNew = a.replace(re,"$1");alert(aNew);
}
</script>

<script>
var num=22.127456;
alert( Math.round(num*100)/100);
</script>

function changeTwoDecimal_f(x) {var f_x = parseFloat(x);if (isNaN(f_x)) {alert('function:changeTwoDecimal->parameter error');return false;}var f_x = Math.round(x * 100) / 100;var s_x = f_x.toString();var pos_decimal = s_x.indexOf('.');if (pos_decimal < 0) {pos_decimal = s_x.length;s_x += '.';}while (s_x.length <= pos_decimal + 2) {s_x += '0';}return s_x;
}