用javascript实现浮点数的截取小数位数，并四舍五入

以前做过一个，没考虑四舍五入，现在把代码改进了一下，如下：

/***********************************************************
Function formatnumber(value,num)
Written by zergling
javascript版本的FormatNumber函数，用法与VBScript相同，第一个参数是待格式化的数值，第二个是保留小数位数
注意：返回的是字符串类型
第一个函数需要调用第二个函数，所以第二个不能去掉
***********************************************************/
function adv_format(value,num)   //四舍五入
    {
    var a_str = formatnumber(value,num);
    var a_int = parseFloat(a_str);
    if (value.toString().length>a_str.length)
        {
        var b_str = value.toString().substring(a_str.length,a_str.length+1)
        var b_int = parseFloat(b_str);
        if (b_int<5)
            {
            return a_str
            }
        else
            {
            var bonus_str,bonus_int;
            if (num==0)
                {
                bonus_int = 1;
                }
            else
                {
                bonus_str = "0."
                for (var i=1; i<num; i++)
                    bonus_str+="0";
                bonus_str+="1";
                bonus_int = parseFloat(bonus_str);
                }
            a_str = formatnumber(a_int + bonus_int, num)
            }
        }
        return a_str
    }

function formatnumber(value,num)    //直接去尾
    {
    var a,b,c,i
    a = value.toString();
    b = a.indexOf('.');
    c = a.length;
    if (num==0)
        {
        if (b!=-1)
            a = a.substring(0,b);
        }
    else
        {
        if (b==-1)
            {
            a = a + ".";
            for (i=1;i<=num;i++)
                a = a + "0";
            }
        else
            {
            a = a.substring(0,b+num+1);
            for (i=c;i<=b+num;i++)
                a = a + "0";
            }
        }
    return a
    }