js四舍五入保留指定小数位

//四舍五入保留二位小数function round(num,dec){     var sNum = num + '';    var idx = sNum.indexOf(".");        if(idx<0)   return num;     var n = sNum.length - idx -1;     if(dec < n){         var e = Math.pow(10,dec);        return Math.round(num * e) / e;     }else{         return num;     } }