react-day-picker/src/helpers/getBroadcastWeeksInMonth.ts

Customizable Date Picker for React

import { DateLib } from "../classes/index.js";

const FIVE_WEEKS = 5;
const FOUR_WEEKS = 4;

/**
 * Return the number of weeks to display in the broadcast calendar.
 *
 * @since 9.4.0
 */
export function getBroadcastWeeksInMonth(month: Date, dateLib: DateLib): 4 | 5 {
  // Get the first day of the month
  const firstDayOfMonth = dateLib.startOfMonth(month);

  // Get the day of the week for the first day of the month (1-7, where 1 is Monday)
  const firstDayOfWeek =
    firstDayOfMonth.getDay() > 0 ? firstDayOfMonth.getDay() : 7;

  const broadcastStartDate = dateLib.addDays(month, -firstDayOfWeek + 1);

  const lastDateOfLastWeek = dateLib.addDays(
    broadcastStartDate,
    FIVE_WEEKS * 7 - 1
  );
  const numberOfWeeks =
    dateLib.getMonth(month) === dateLib.getMonth(lastDateOfLastWeek)
      ? FIVE_WEEKS
      : FOUR_WEEKS;

  return numberOfWeeks;
}