src/helpers/getPreviousMonth.ts

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

/**
 * Returns the previous month the user can navigate to, based on the given
 * options.
 *
 * The previous month is not always the previous calendar month:
 *
 * - If it is before the `calendarStartMonth`, it returns `undefined`.
 * - If paged navigation is enabled, it skips back by the number of displayed
 *   months.
 *
 * @param firstDisplayedMonth The first month currently displayed in the
 *   calendar.
 * @param calendarStartMonth The earliest month the user can navigate to.
 * @param options Navigation options, including `numberOfMonths` and
 *   `pagedNavigation`.
 * @param dateLib The date library to use for date manipulation.
 * @returns The previous month, or `undefined` if navigation is not possible.
 */
export function getPreviousMonth(
  firstDisplayedMonth: Date,
  calendarStartMonth: Date | undefined,
  options: Pick<
    DayPickerProps,
    "numberOfMonths" | "pagedNavigation" | "disableNavigation"
  >,
  dateLib: DateLib,
): Date | undefined {
  if (options.disableNavigation) {
    return undefined;
  }
  const { pagedNavigation, numberOfMonths } = options;
  const { startOfMonth, addMonths, differenceInCalendarMonths } = dateLib;
  const offset = pagedNavigation ? (numberOfMonths ?? 1) : 1;
  const month = startOfMonth(firstDisplayedMonth);
  if (!calendarStartMonth) {
    return addMonths(month, -offset);
  }
  const monthsDiff = differenceInCalendarMonths(month, calendarStartMonth);

  if (monthsDiff <= 0) {
    return undefined;
  }

  return addMonths(month, -offset);
}