Javascript - Get calendar view of month as an array of arrays representing a month, split into weeks 7 dates long.

getCalendarView.js

 /**
   * This function will return an array of arrays representing a month as a 6*7 data table. This includes leading/trailing days.
   *
   * This only uses native JS objects, so no external libs are needed
   *
   * It only iterates a single time, so it is highly performant
   * @param date The month/year of this Date object determines the calendar that will be generated.
   * @returns Date[][]
   */
  const getCalendarView = (date: Date) => {
    const startOfWeek = 0; // this can be adjusted to change the first day of the week. Sunday = 0, Monday = 1, etc
    const startOfMonth = new Date(date.getFullYear(), date.getMonth(), 1);

    let leadingDaysDifferential = startOfWeek - startOfMonth.getDay();
    if (leadingDaysDifferential > 0) {
      //invert the leading days so slack is evenly divided between top/bottom
      leadingDaysDifferential = -(7 - leadingDaysDifferential);
    }

    let dateIterator = new Date(
      startOfMonth.getFullYear(),
      startOfMonth.getMonth(),
      startOfMonth.getDate() + leadingDaysDifferential
    );

    const dates: Date[] = [dateIterator];
    // fixed sized for 6x7 calendar body
    for (let index = 0; index < 41; index++) {
      dateIterator = new Date(
        dateIterator.getFullYear(),
        dateIterator.getMonth(),
        dateIterator.getDate() + 1
      );
      dates.push(dateIterator);
    }

    const monthView = [...Array(Math.ceil(dates.length / 7))].map(() =>
      dates.splice(0, 7)
    );

    return monthView;
  };