WeekOfDay Count Calculator created by davidsk - https://repl.it/DFqJ/25

WeekOfDay Count Calculator.js

var DayOfWeek = {
	Sunday: 0,
	Monday: 1,
	Tuesday: 2,
	Wednesday: 3,
	Thursday: 4,
	Friday: 5,
	Saturday: 6
};
	
function CountWeekdayInPeriod(targetDayIndex, startDayIndex, periodDuration){

    // ensure values are integers; js will concatenate string values    
    startDayIndex = parseInt(startDayIndex, 10),
    periodDuration = parseInt(periodDuration, 10);
	
    // find the number of days in addition to whole weeks
    var partialWeekLagDayCount = periodDuration % 7;

    // whichever day is being targeted, there is always one instance every 7 days
    var dayCount = (periodDuration - partialWeekLagDayCount) / 7;    

    // determine the start day index having 'rotated' the days so that the target day is now index 7
    var firstDayIndex = ((startDayIndex + (6 - targetDayIndex)) % 7 ) + 1
    
    // calculate the end date's day index by adding the period to the adjusted start date index
    // 1 is deducted as the start date already considered
    var firstDayPlusDurationIndex = firstDayIndex + partialWeekLagDayCount - 1;
    
    // determine the weekday index of the last day of the remaining period
    var lastWeekdayIndex = (firstDayPlusDurationIndex % 7);
    
    // determine whether the remaining period has 'gone around the horn' 
    var secondIteration = (firstDayPlusDurationIndex - lastWeekdayIndex) / 7;
    
    dayCount += secondIteration;
	
    return dayCount;
}

// functions built on top of CountWeekdayInPeriod
function CountWeekendDaysInPeriod(startDayIndex, periodDuration)
{
    return CountWeekdayInPeriod(DayOfWeek.Saturday, startDayIndex, periodDuration) + CountWeekdayInPeriod(DayOfWeek.Sunday, startDayIndex, periodDuration);
}

function GetNextWeekday(startDate, periodDuration){
	
    var weekendDayCount, totalWeekendDays = 0, iterativePeriodDuration = periodDuration;
	
    if(!iterativePeriodDuration || iterativePeriodDuration === 0){
        iterativePeriodDuration = 1;
    }

    var direction = iterativePeriodDuration / Math.abs(iterativePeriodDuration);

    var internalStartDate = new Date(startDate);
	
    if(direction === -1){
        internalStartDate.setDate(internalStartDate.getDate() + iterativePeriodDuration);
    }

    var startDayIndex = internalStartDate.getDay();
	
    do{
        weekendDayCount = CountWeekendDaysInPeriod(startDayIndex, Math.abs(iterativePeriodDuration));
		
	iterativePeriodDuration = weekendDayCount;
		
	startDayIndex = (7 + startDayIndex + (direction * (iterativePeriodDuration % 7))) % 7;

	totalWeekendDays += weekendDayCount;

	} while(weekendDayCount > 0);
	
    var totalDelta = periodDuration + (direction * totalWeekendDays);

    return new Date(new Date(startDate).setDate(startDate.getDate() + totalDelta));
}

// usage
var weekdaysInPeriod = CountWeekdayInPeriod(DayOfWeek.Wednesday, DayOfWeek.Tuesday, 10);
console.log("weekdaysInPeriod: " + weekdaysInPeriod);

var weekendDaysInPeriod = CountWeekendDaysInPeriod(DayOfWeek.Tuesday, 11);
console.log("weekendDaysInPeriod: " + weekendDaysInPeriod);

var nextWorkingDay = GetNextWeekday(new Date(2016, 8, 24), 0);
console.log("nextWorkingDay: " + nextWorkingDay);