//num 是传过来的值，del是要保留几位

function valueFmt(num, del) {

    if (num != '') {

        if (del != 0) {

            num = parseFloat(num).toFixed(del);

        }

        var source = String(num).split(".");

        source[0] = source[0].replace(new RegExp('(\\d)(?=(\\d{3})+$)', 'ig'), "$1,");

        return source.join(".") == "NaN" ? "0.00" : source.join(".");

    }

};

var num = 2.00;

var result=valueFmt(num,2);

console.log(result);//结果是：2.00