date - add/subtract business days in Javascript

Question

I need a Date.prototype.addBusDays function that'll take an integer as the number of working days to add to the date.

However, there are two considerations: 1. Weekends, 2. Holidays (which I imagine would be a preset array to compare against. If beginning date and end date contain 3 holidays, then you push out the end date by 3)

I have come across some scripts online, one dilemma I can think of is, lets say you address all the weekends first, then you do the holidays, what if you +1 day (due to holiday), and your end date is pushed into a weekends again...<

Any ideas? Thanks!

EDIT:

This is a part of a scheduling tool I am developing, which mean the dates will be tied to tasks which are linked together. Adding 1 day to a task, will trigger a recalculation of everything tied to it, potentially all dates in the database.

Answer 1

Datageek's solution helped me but I needed to augment it. This still doesn't do holidays but does do working days with the option of including Sat and/or Sun, and does support adding negative days:-

function AddWorkingDays(datStartDate, lngNumberOfWorkingDays, blnIncSat, blnIncSun) {
    var intWorkingDays = 5;
    var intNonWorkingDays = 2;
    var intStartDay = datStartDate.getDay(); // 0=Sunday ... 6=Saturday
    var intOffset;
    var intModifier = 0;

    if (blnIncSat) { intWorkingDays++; intNonWorkingDays--; }
    if (blnIncSun) { intWorkingDays++; intNonWorkingDays--; }
    var newDate = new Date(datStartDate)
    if (lngNumberOfWorkingDays >= 0) {
        // Moving Forward
        if (!blnIncSat && blnIncSun) {
            intOffset = intStartDay;
        } else {
            intOffset = intStartDay - 1;
        }
        // Special start Saturday rule for 5 day week
        if (intStartDay == 6 && !blnIncSat && !blnIncSun) {
            intOffset -= 6;
            intModifier = 1;
        }
    } else {
        // Moving Backward
        if (blnIncSat && !blnIncSun) {
            intOffset = intStartDay - 6;
        } else {
            intOffset = intStartDay - 5;
        }
        // Special start Sunday rule for 5 day week
        if (intStartDay == 0 && !blnIncSat && !blnIncSun) {
            intOffset++;
            intModifier = 1;
        }
    }
    // ~~ is used to achieve integer division for both positive and negative numbers
    newDate.setTime(datStartDate.getTime() + (new Number((~~((lngNumberOfWorkingDays + intOffset) / intWorkingDays) * intNonWorkingDays) + lngNumberOfWorkingDays + intModifier)*86400000));
    return newDate;
}