Example: calculate the date after the current time has passed 5 working days (set as N working days).
Train of thought:
Get the weekend and legal holiday days between the current system time + n (after N working days) +

Because legal holidays are immutable, we first set them as an array

  //Holiday array
          var Holiday = ["2018/1/1","2018/1/15","2018/2/19","2018/5/28","2018/7/4","2018/9/3","2018/10/8","2018/11/12","2018/11/22","2018/12/25"];

          /**
           * Loop to determine whether an element exists in an array
           * @param {Object} arr array
           * @param {Object} value Element value
           */
           function isInArray(arr,value){
            for(var i = 0; i < arr.length; i++){
              if(value === arr[i]){
                  return true;
                  }
            }
              return false; 
         }

Then get the current system time

//Format the current system time format as (yyyy MM DD)
        function times(){

          var date = new Date();
          var nowMonth = date.getMonth() + 1;
          var hours = date.getHours();
          if(hours >= 15){
              var strDate = date.getDate();
          }else{
              var strDate = date.getDate()-1;
          }
          // Get the current number
          //var strDate = date.getDate();
          // Add separator '-'
          var seperator = "-";
          // Process the month, add a "0" before January to September
          if (nowMonth >= 1 && nowMonth <= 9) {
             nowMonth = "0" + nowMonth;
          }
          // Process the month, adding a "0" before the 1st-9th
          if (strDate >= 0 && strDate <= 9) {
             strDate = "0" + strDate;
          }
          // Finally, concatenate the string to get a date in the format of (yyyy MM DD)
          var nowDate = date.getFullYear() + seperator + nowMonth + seperator + strDate;
          return nowDate;
        }

Here is the function to calculate holidays

 /**
   * Calculate the date after a working day of the current period
   * @param {date} startDate current time
   * @param {string} limitDay Working day
   */
   function getWorkDate(startDate,limitDay){
        var time = Date.parse(startDate);
        var startTime = new Date(Date.parse(startDate));
         var startTime = startTime.getTime();
         var T = 24*60*60*1000; 
         var endTime = startTime+(limitDay*T);
         if(limitDay>0){
          var holidays = 0;
          for(var i=startTime+T;i<=endTime;i+=T){
           var date = new Date(i);
           //Holiday logic here
           if(date.getDay()==0 || date.getDay()==6){  
            holidays++;
           }
           //Determine whether the date is in the holiday array
           if(isInArray(Holiday,date.toLocaleDateString()) == true){
                        holidays++;
            }
          }
          return getWorkDate(new Date(endTime),holidays);
         }else{
          return startDate.toLocaleDateString();

         }
        }

instantiation

//Get current system time
var current_time = times();

//The format of the date after three working days is (YYYY-MM-dd) 
var min_days = getWorkDate(current_time,"3");