countWeekDaysBetween

Counts the weekdays between two dates.

• Use Array.from() to construct an array with length equal to the number of days between startDate and endDate.
• Use Array.prototype.reduce() to iterate over the array, checking if each date is a weekday and incrementing count.
• Update startDate with the next day each loop using Date.prototype.getDate() and Date.prototype.setDate() to advance it by one day.
• NOTE: Does not take official holidays into account.

const countWeekDaysBetween = (startDate, endDate) =>
  Array
    .from({ length: (endDate - startDate) / (1000 * 3600 * 24) })
    .reduce(count => {
      if (startDate.getDay() % 6 !== 0) count++;
      startDate = new Date(startDate.setDate(startDate.getDate() + 1));
      return count;
    }, 0);

countWeekDaysBetween(new Date('Oct 05, 2020'), new Date('Oct 06, 2020')); // 1
countWeekDaysBetween(new Date('Oct 05, 2020'), new Date('Oct 14, 2020')); // 7