保留N位小数（不四舍五入）的

 
util.pointDecimal=function(value,num) {
    if(value===null||value===undefined||value==NaN){
        return "0.00";
    }
    value = value+"";
    value = parseFloat(value.replace(/(\.\d{num})\d+$/,"$1")).toFixed(num).toString().split(".");
    value[0] = value[0].replace(new RegExp('(\\d)(?=(\\d{3})+$)','ig'),"$1,");
    return  value.join(".");;
}