get month's bussiness days (javascript)

gistfile1.txt

function getWeeksInMonth(year: number, month: number) {
        const weeks = [],
            firstDate = new Date(year, month, 1),
            lastDate = new Date(year, month + 1, 0),
            numDays = lastDate.getDate();
        
        let offset = 0;

        if(firstDate.getDay() === 0) offset = 1;
        else if(firstDate.getDay() === 6) offset = 2;

        let start = 1 + offset;
        let end: number;

        if(offset === 0) end = 5 - firstDate.getDay() + 1;
        else end = 5 - start + 1 + (offset * 2);

        while(start <= numDays) {
            weeks.push({start: start, end: end});
            start = end + 3;
            end = end + 7;
            end = start === 1 && end === 8 ? 1 : end;
            if(end > numDays) end = numDays;
        }

        return weeks;
}