react-trash-calendar/src/lib/calendar/calendar.ts

import {
  getMonthNames,
  getRealDay,
  getWeekDays,
  getWeekNumber,
} from './dateutil';

export type Week = (Date | null)[];

export class Calendar {
  public weeks?: Week[];
  public monthNames = getMonthNames(this.locale);
  public dayNames = getWeekDays(this.locale);

  constructor(public currentDate: Date, private locale = 'en-US') {}

  public setDate(date: Date) {
    this.currentDate = date;
    this.generateMonth();
  }

  public set year(year: number) {
    this.currentDate.setFullYear(year);
    this.generateMonth();
  }
  public get year() {
    return this.currentDate.getFullYear();
  }

  public set month(month: number) {
    this.currentDate.setMonth(month);
    this.generateMonth();
  }
  public get month() {
    return this.currentDate.getMonth();
  }

  public get monthName() {
    return this.monthNames[this.currentDate.getMonth()];
  }

  public set day(day: number) {
    this.currentDate.setDate(day);
    this.generateMonth();
  }
  public get day() {
    return this.currentDate.getDate();
  }

  public get date() {
    return this.currentDate;
  }

  public get firstOfMonth() {
    return new Date(
      Date.UTC(this.currentDate.getFullYear(), this.currentDate.getMonth(), 1)
    );
  }

  public get lastOfMonth() {
    return new Date(
      Date.UTC(
        this.currentDate.getFullYear(),
        this.currentDate.getMonth() + 1,
        0
      )
    );
  }

  public get weekNumbers() {
    return this.weeks!.map(([first]) => getWeekNumber(first!));
  }

  public get weekCount() {
    return Math.ceil(
      ((this.firstOfMonth.getDay() || 7) - 1 + this.lastOfMonth.getDate()) / 7
    );
  }

  public nextMonth() {
    this.currentDate?.setMonth(this.currentDate.getMonth() + 1);
    this.generateMonth();
  }

  public previousMonth() {
    this.currentDate?.setMonth(this.currentDate.getMonth() - 1);
    this.generateMonth();
  }

  public isOutOfMonth(date: Date) {
    return date.getMonth() !== this.currentDate.getMonth();
  }

  public dateEquals(date: Date, target: Date) {
    return (
      date.getDate() === target.getDate() &&
      date.getFullYear() === target.getFullYear() &&
      date.getMonth() === target.getMonth()
    );
  }

  public isToday(date: Date) {
    const today = new Date();
    return this.dateEquals(date, today);
  }

  public static populatePartialWeeks(weeks: Week[]) {
    // This reducer fills the first and/or last week of the month
    // with dates from previous/next month
    weeks.forEach((week, weekIndex) => {
      if (week.includes(null)) {
        const dir = weekIndex === 0 ? -1 : 1;
        const reducer = (
          previous: Date | null,
          current: Date | null,
          index: number
        ) => {
          if (!current && previous) {
            week[index] = new Date(
              Date.UTC(
                previous.getFullYear(),
                previous.getMonth(),
                previous.getDate() + dir
              )
            );
          }

          return week[index];
        };

        const start = dir === -1 ? week[6] : week[0];
        dir < 0
          ? week.reduceRight(reducer, start)
          : week.reduce(reducer, start);
      }
    });
    return weeks;
  }

  public generateMonth() {
    const year = this.currentDate.getFullYear();
    const month = this.currentDate.getMonth();

    // Create week arrays, fill with nulls
    const weeks = Array.from({ length: this.weekCount }, (k, i) =>
      new Array(7).fill(null)
    );

    // Starting offset of the dates in this month
    let offset = getRealDay(this.firstOfMonth);

    // Create dates for each day of the week
    for (let ci = 1; ci <= this.lastOfMonth.getDate(); ci++, offset++) {
      const iterDate = new Date(Date.UTC(year, month, ci));
      const weekOfMonth = Math.floor(offset / 7);
      weeks[weekOfMonth][getRealDay(iterDate)] = iterDate;
    }

    // Populate dates that are in previous or next month
    Calendar.populatePartialWeeks(weeks);
    this.weeks = weeks;

    return weeks;
  }
}