js保留n位小数

function toDecimal(num) { 
    var f = parseFloat(num); 
    if (isNaN(f)) { 
　　    return; 
    } 
    f = Math.round(num*100)/100; 
    return f; 
} 
document.write(toDecimal(2.2291));
结果：2.23

function get(num)
{
    var str = num.substring(0,num.indexOf(".") + 3);
    return str;
}
document.write(toDecimal(2.2291));
结果：2.22

function getnum(num){
        var re = /([0-9]+\.[0-9]{2})[0-9]*/;
        aNew = num.replace(re,"$1");
        return aNew;
}
document.write(getnum("23.456322"));
结果：23.45

var num=22.127456;
document.write(num.toFixed(2)));
结果：22.13

function decimal(num,v)
{
        var vv = Math.pow(10,v);
        return Math.round(num*vv)/vv;
}           
document.write(decimal("2.129",2));
结果：2.13