Create a date that iterates on either multiple dates of the month or every X number of days

SeriesDate.js

/*

// EXAMPLES: 
var payday = new SeriesDate('DAILY', [14], '1/15/2010')
var gas = new SeriesDate('MONTHLY', [1,15], '1/15/2010')
var electricity = new SeriesDate('MONTHLY', [20], '1/15/2010')

payday.next(); // move to next payday
payday.prev(); // move to previous payday


// TODO: fix bad hack to subclass Date (http://www.2ality.com/2013/03/subclassing-builtins-es6.html)
// only tested in chrome

*/

function SeriesDate( seriesType, series, start )
{
  var _this = new Date(start);
  _this.__proto__ = SeriesDate.prototype;
  for(var i in series){ if (series[i]>28)series[i]=28}
  
  var _seriesType = seriesType,
  _series = series.sort(),
  _start = new Date(start.toString()),
  
  setLastDate = function(d)
  {
    nextMonth(d);
    d.setDate(0); /* set date 0 will subtract one day */
  },
  
  nextMonth = function(d)
  {
    if (d.getMonth() < 11)
    {
      d.setMonth( (d.getMonth()+1) );
    }
    else
    {
      d.setMonth(0);
      d.setFullYear(d.getFullYear()+1);
    }
    return d;
  },
  
  nextDate = function(d, date)
  {
    if (d.getDate() < date)
    {
      (date < 28) ? d.setDate(date) : setLastDate(d);
    }
    else
    {
      if (date < 28)
      {
        d.setDate(1)
        nextMonth(d);
        d.setDate(date);
      }
      else
      {
        setLastDate(d);
      }
      
    }
    return d;
  },
  
  previousMonth = function(d)
  {
    if (d.getMonth() > 0)
    {
      d.setMonth(d.getMonth()-1);
    }
    else
    {
      d.setMonth(11);
      d.setFullYear(d.getFullYear()-1);
    }
    return d;
  },
  
  previousDate = function(d, date)
  {
    
    if (d.getDate() > date)
    {
      (date < 28) ? d.setDate(date) : d.setDate(0);
    }
    else
    {
      if (date < 28) 
      {
        previousMonth(d);
        d.setDate(date);
      }
      else
      {
        d.setDate(1);
        previousMonth(d);
        setLastDate(d);
      }
      
    }
    return d;
  },
  
  nextMonthlyDate = function(date, monthSeries, start)
  {
    var isBeforeStartDate = (start.getTime() > date.getTime()),
    d = [date.getFullYear(), date.getMonth(), date.getDate()];
    
    if (isBeforeStartDate) 
      return new Date(start.toString());
    
    for (var i in monthSeries)
    {
      if (monthSeries[i] > d[2])
        return nextDate(date, monthSeries[i])
    }
    
    return nextDate(date, monthSeries[0])
  },
  
  previousMonthlyDate = function(date, monthSeries, start)
  {
    var isBeforeStartDate = (start.getTime() > date.getTime()),
    d = [date.getFullYear(), date.getMonth(), Math.min(date.getDate(), 28)];
    
    if (isBeforeStartDate) 
      return new Date(start.toString());
    
    for (var i=monthSeries.length-1; i>=0; i--)
    {
      if (monthSeries[i] < d[2])
        return previousDate(date, monthSeries[i])
    }
    
    return previousDate(date, monthSeries[monthSeries.length-1])
  },
  
  nextDailyDate = function(date, day, start)
  {
    var isBeforeStartDate = (start.getTime() > date.getTime());
    
    if (isBeforeStartDate) 
      return new Date(start.toString());
    
      day *= 86400000;
      
      date.setTime(date.getTime() + day)
      
      return date;
  },
  
  previousDailyDate = function(date, day, start)
  {
    var isBeforeStartDate = (start.getTime() > date.getTime());
    
    if (isBeforeStartDate) 
      return new Date(start.toString());
    
      day *= 86400000;
      
      date.setTime(date.getTime() - day)
      
      return date;
  };
  
  if (seriesType ==='MONTHLY')
  {
    _this.next = function(){
      return nextMonthlyDate(this, _series, _start);
    };
    _this.prev = function(){
      return previousMonthlyDate(this, _series, _start);
    };
  }
  else 
  {
    _this.next = function(){
      return nextDailyDate(this, _series, _start);
    };
    _this.prev = function(){
      return previousDailyDate(this, _series, _start);
    };
  }
  
  /* move to first future date */
  var now = new Date().getTime();
  while (now > _this.getTime() )
  {
    _this.next()
  }
  
  return _this;
}
SeriesDate.prototype = Object.create(Date.prototype);

// TODO: upload unit tests