calendar.ts

interface CalendarDay {
    label: number;
    timestamp: number;
    isCurrentMonth: boolean;
}
type CalendarWeek = CalendarDay[];
type Calendar = CalendarWeek[];

/**
 * @example
 * getCalendar('2018-02-15')
 *
 * 29 30 31 01 02 03 04
 * 05 06 07 08 09 10 11
 * 12 13 14 15 16 17 18
 * 19 20 21 22 23 24 25
 * 26 27 28 01 02 03 04
 */
export function getCalendar(dateConstructValue: string | number | Date): Calendar {
    const currentDate = new Date(dateConstructValue);
    const currentMonth = currentDate.getMonth();
    const currentYear = currentDate.getFullYear();
    const calendarStartDate = new Date(
        currentYear,
        currentMonth,
    );
    const currentDay = calendarStartDate.getDay()
        ? calendarStartDate.getDay()
        : 7;
    calendarStartDate.setDate(calendarStartDate.getDate() - currentDay + 1);

    let weekCounter = -1;
    const calendar: Calendar = [];

    while (
        currentMonth >= calendarStartDate.getMonth()
        || currentYear !== calendarStartDate.getFullYear()
    ) {
        if (currentYear < calendarStartDate.getFullYear()) {
            return calendar;
        }

        if (calendarStartDate.getDay() === 1) {
            weekCounter += 1;
            calendar[weekCounter] = Array.from(
                {length: 7},
                (): CalendarDay => ({
                    label: 0,
                    timestamp: 0,
                    isCurrentMonth: false,
                })
            );
        }

        const currentWeek: CalendarDay[] = calendar[weekCounter];

        currentWeek.forEach((_, index): void => {
            const calendarDay: CalendarDay = {
                label: calendarStartDate.getDate(),
                timestamp: calendarStartDate.getTime(),
                isCurrentMonth: currentMonth === calendarStartDate.getMonth(),
            };
            currentWeek[index] = calendarDay;
            calendarStartDate.setDate(calendarStartDate.getDate() + 1);
        });
    }

    return calendar;
}