vanillajs-datepicker/test/main/_utils/date.js

A vanilla JavaScript remake of bootstrap-datepicker for Bulma and other CSS frameworks

var dateUtils = (function (exports) {
  'use strict';

  function stripTime(timeValue) {
    return new Date(timeValue).setHours(0, 0, 0, 0);
  }

  function today() {
    return new Date().setHours(0, 0, 0, 0);
  }

  // Get the time value of the start of given date or year, month and day
  function dateValue(...args) {
    switch (args.length) {
      case 0:
        return today();
      case 1:
        return stripTime(args[0]);
    }

    // use setFullYear() to keep 2-digit year from being mapped to 1900-1999
    const newDate = new Date(0);
    newDate.setFullYear(...args);
    return newDate.setHours(0, 0, 0, 0);
  }

  function addDays(date, amount) {
    const newDate = new Date(date);
    return newDate.setDate(newDate.getDate() + amount);
  }

  function addWeeks(date, amount) {
    return addDays(date, amount * 7);
  }

  function addMonths(date, amount) {
    // If the day of the date is not in the new month, the last day of the new
    // month will be returned. e.g. Jan 31 + 1 month → Feb 28 (not Mar 03)
    const newDate = new Date(date);
    const monthsToSet = newDate.getMonth() + amount;
    let expectedMonth = monthsToSet % 12;
    if (expectedMonth < 0) {
      expectedMonth += 12;
    }

    const time = newDate.setMonth(monthsToSet);
    return newDate.getMonth() !== expectedMonth ? newDate.setDate(0) : time;
  }

  function addYears(date, amount) {
    // If the date is Feb 29 and the new year is not a leap year, Feb 28 of the
    // new year will be returned.
    const newDate = new Date(date);
    const expectedMonth = newDate.getMonth();
    const time = newDate.setFullYear(newDate.getFullYear() + amount);
    return expectedMonth === 1 && newDate.getMonth() === 2 ? newDate.setDate(0) : time;
  }

  // Calculate the distance bettwen 2 days of the week
  function dayDiff(day, from) {
    return (day - from + 7) % 7;
  }

  // Get the date of the specified day of the week of given base date
  function dayOfTheWeekOf(baseDate, dayOfWeek, weekStart = 0) {
    const baseDay = new Date(baseDate).getDay();
    return addDays(baseDate, dayDiff(dayOfWeek, weekStart) - dayDiff(baseDay, weekStart));
  }

  function calcWeekNum(dayOfTheWeek, sameDayOfFirstWeek) {
    return Math.round((dayOfTheWeek - sameDayOfFirstWeek) / 604800000) + 1;
  }

  // Get the ISO week number of a date
  function getIsoWeek(date) {
    // - Start of ISO week is Monday
    // - Use Thursday for culculation because the first Thursday of ISO week is
    //   always in January
    const thuOfTheWeek = dayOfTheWeekOf(date, 4, 1);
    // - Week 1 in ISO week is the week including Jan 04
    // - Use the Thu of given date's week (instead of given date itself) to
    //   calculate week 1 of the year so that Jan 01 - 03 won't be miscalculated
    //   as week 0 when Jan 04 is Mon - Wed
    const firstThu = dayOfTheWeekOf(new Date(thuOfTheWeek).setMonth(0, 4), 4, 1);
    // return Math.round((thuOfTheWeek - firstThu) / 604800000) + 1;
    return calcWeekNum(thuOfTheWeek, firstThu);
  }

  // Calculate week number in traditional week number system
  // @see https://en.wikipedia.org/wiki/Week#Other_week_numbering_systems
  function calcTraditionalWeekNumber(date, weekStart) {
    // - Week 1 of traditional week is the week including the Jan 01
    // - Use Jan 01 of given date's year to calculate the start of week 1
    const startOfFirstWeek = dayOfTheWeekOf(new Date(date).setMonth(0, 1), weekStart, weekStart);
    const startOfTheWeek = dayOfTheWeekOf(date, weekStart, weekStart);
    const weekNum = calcWeekNum(startOfTheWeek, startOfFirstWeek);
    if (weekNum < 53) {
      return weekNum;
    }
    // If the 53rd week includes Jan 01, it's actually next year's week 1
    const weekOneOfNextYear = dayOfTheWeekOf(new Date(date).setDate(32), weekStart, weekStart);
    return startOfTheWeek === weekOneOfNextYear ? 1 : weekNum;
  }

  // Get the Western traditional week number of a date
  function getWesternTradWeek(date) {
    // Start of Western traditionl week is Sunday
    return calcTraditionalWeekNumber(date, 0);
  }

  // Get the Middle Eastern week number of a date
  function getMidEasternWeek(date) {
    // Start of Middle Eastern week is Saturday
    return calcTraditionalWeekNumber(date, 6);
  }

  // Get the start year of the period of years that includes given date
  // years: length of the year period
  function startOfYearPeriod(date, years) {
    /* @see https://en.wikipedia.org/wiki/Year_zero#ISO_8601 */
    const year = new Date(date).getFullYear();
    return Math.floor(year / years) * years;
  }

  // Convert date to the first/last date of the month/year of the date
  function regularizeDate(date, timeSpan, useLastDate) {
    if (timeSpan !== 1 && timeSpan !== 2) {
      return date;
    }
    const newDate = new Date(date);
    if (timeSpan === 1) {
      useLastDate
        ? newDate.setMonth(newDate.getMonth() + 1, 0)
        : newDate.setDate(1);
    } else {
      useLastDate
        ? newDate.setFullYear(newDate.getFullYear() + 1, 0, 0)
        : newDate.setMonth(0, 1);
    }
    return newDate.setHours(0, 0, 0, 0);
  }

  exports.addDays = addDays;
  exports.addMonths = addMonths;
  exports.addWeeks = addWeeks;
  exports.addYears = addYears;
  exports.dateValue = dateValue;
  exports.dayOfTheWeekOf = dayOfTheWeekOf;
  exports.getIsoWeek = getIsoWeek;
  exports.getMidEasternWeek = getMidEasternWeek;
  exports.getWesternTradWeek = getWesternTradWeek;
  exports.regularizeDate = regularizeDate;
  exports.startOfYearPeriod = startOfYearPeriod;
  exports.stripTime = stripTime;
  exports.today = today;

  return exports;

})({});