Javascript date helper functions

1. Get the current week first date.

const getCurrentWeekFirstDt = (currentDt = new Date()) => {
  const weekFirstDt = new Date(currentDt);
  const offset = currentDt.getDate() - currentDt.getDay();
  weekFirstDt.setDate(offset);
  return weekFirstDt;
}

2. Get the current week last date.

const getCurrentWeekLastDt = (currentDt = new Date()) => {
  const weekLastDt = getCurrentWeekFirstDt(currentDt);
  const offset = weekLastDt.getDate() + 6;
  weekLastDt.setDate(offset);
  return weekLastDt;
}

3. Subtract days.

const subDays = (daysCount, currentDate = new Date()) => {
  if (!daysCount) return currentDate;

  currentDate.setDate(currentDate.getDate() - Number(daysCount));
  return currentDate;
}

4. Add days.

const addDays = (daysCount, currentDate = new Date()) => {
  if (!daysCount) return currentDate;

  currentDate.setDate(currentDate.getDate() + Number(daysCount));
  return currentDate;
}

5. Get yesterday's date.

const yesterday = (currentDate) => subDays(1, currentDate)

5. Get tomorrow's date.

const tomorrow = (currentDate) => addDays(1, currentDate)

7. Get an array of dates between two dates.

const getDateArray = (startDate, endDate) => {
  // An additional check here would be to reverse the dates
  // if startDate is greater than endDate

  let currentDate = startDate;
  const dates = [];
  while (currentDate <= endDate) {
    dates.push(currentDate);
    currentDate = tomorrow(currentDate);
  }
  return dates;
}

8. Get all dates within the current week.

const getWeekDates = (currentDate = new Date()) => {
  return getDateArray(
    getCurrentWeekFirstDt(currentDate), 
    getCurrentWeekLastDt(currentDate)
  );
}

9. Get the current month first date.

const getMonthFirstDt = (currentDate = new Date()) => new Date(
  currentDate.getFullYear(), currentDate.getMonth(), 1
);

10. Get the current month last date.

const getMonthLastDt = (currentDate = new Date()) => new Date(
  currentDate.getFullYear(), currentDate.getMonth() + 1, 0
)