It is possible to use Date's setDate function (in combination with getDate) to add days onto a date i.e. -

var myDate = new Date(); // Tue 22/11/2016

myDate.setDate(myDate.getDate() + 3); // Fri 25/11/2016

So once you've calculated the number of weekend days within the workdays period you can add that and the required number of workdays to the start date to get the final date.

This function should work though obviously this will not take account of national holidays -

function addWorkDays(startDate, days) {

    if(isNaN(days)) {

        console.log(Value provided for days was not a number);

        return

    }

    if(!(startDate instanceof Date)) {

        console.log(Value provided for startDate was not a Date object);

        return

    }

    // Get the day of the week as a number (0 = Sunday, 1 = Monday, .... 6 = Saturday)

    var dow = startDate.getDay();

    var daysToAdd = parseInt(days);

    // If the current day is Sunday add one day

    if (dow == 0)

        daysToAdd++;

    // If the start date plus the additional days falls on or after the closest Saturday calculate weekends

    if (dow + daysToAdd >= 6) {

        //Subtract days in current working week from work days

        var remainingWorkDays = daysToAdd - (5 - dow);

        //Add current working week's weekend

        daysToAdd += 2;

        if (remainingWorkDays > 5) {

            //Add two days for each working week by calculating how many weeks are included

            daysToAdd += 2 * Math.floor(remainingWorkDays / 5);

            //Exclude final weekend if remainingWorkDays resolves to an exact number of weeks

            if (remainingWorkDays % 5 == 0)

                daysToAdd -= 2;

        }

    }

    startDate.setDate(startDate.getDate() + daysToAdd);

    return startDate;

}



//And use it like so (months are zero based)

var today = new Date(2016, 10, 22);

today = addWorkDays(today, 5); // Tue Nov 29 2016 00:00:00 GMT+0000 (GMT Standard Time)

It could also be added to the Date prototype -

Date.prototype.addWorkDays = function (days) {

    if(isNaN(days)) {

        console.log(Value provided for days was not a number);

        return

    }



    // Get the day of the week as a number (0 = Sunday, 1 = Monday, .... 6 = Saturday)

    var dow = this.getDay();

    var daysToAdd = parseInt(days);

    // If the current day is Sunday add one day

    if (dow == 0) {

        daysToAdd++;

    }

    // If the start date plus the additional days falls on or after the closest Saturday calculate weekends

    if (dow + daysToAdd >= 6) {

        //Subtract days in current working week from work days

        var remainingWorkDays = daysToAdd - (5 - dow);

        //Add current working week's weekend

        daysToAdd += 2;

        if (remainingWorkDays > 5) {

            //Add two days for each working week by calculating how many weeks are included

            daysToAdd += 2 * Math.floor(remainingWorkDays / 5);

            //Exclude final weekend if the remainingWorkDays resolves to an exact number of weeks

            if (remainingWorkDays % 5 == 0)

                daysToAdd -= 2;

        }

    }

    this.setDate(this.getDate() + daysToAdd);

};



//And use it like so (months are zero based)

var today = new Date(2016, 10, 22)

today.addWorkDays(5); // Tue Nov 29 2016 00:00:00 GMT+0000 (GMT Standard Time)