js 格式化数字，输出格式化后的字符串。

Number.prototype.format = function (format) {
    if (!format) {
        return this.toString();
    }
    var comma = ',',
                dec = '.',
                i18n = false,
                neg = v < 0;

    var v = Math.abs(this);
    if (format.substr(format.length - 2) == '/i') {
        format = format.substr(0, format.length - 2);
        i18n = true;
        comma = '.';
        dec = ',';
    }

    var hasComma = format.indexOf(comma) != -1,
                psplit = (i18n ? format.replace(/[^\d\,]/g, '') : format.replace(/[^\d\.]/g, '')).split(dec);

    if (1 < psplit.length) {
        v = v.toFixed(psplit[1].length);
    } else if (2 < psplit.length) {
        throw ('NumberFormatException: invalid format, formats should have no more than 1 period: ' + format);
    } else {
        v = v.toFixed(0);
    }

    var fnum = v.toString();

    psplit = fnum.split('.');

    if (hasComma) {
        var cnum = psplit[0],
                    parr = [],
                    j = cnum.length,
                    m = Math.floor(j / 3),
                    n = cnum.length % 3 || 3,
                    i;

        for (i = 0; i < j; i += n) {
            if (i != 0) {
                n = 3;
            }

            parr[parr.length] = cnum.substr(i, n);
            m -= 1;
        }
        fnum = parr.join(comma);
        if (psplit[1]) {
            fnum += dec + psplit[1];
        }
    } else {
        if (psplit[1]) {
            fnum = psplit[0] + dec + psplit[1];
        }
    }

    return (neg ? '-' : '') + format.replace(/[\d,?\.?]+/, fnum);
}