🇧🇪 Checks if a date is a Belgian holiday

belgian-holidays.js

// 🇧🇪 Checks if a date is a Belgian holiday
/*
  The code below is not fully mine.
  Everything is based on some snippets I found online (another github gist and something on stackoverflow if I am not mistaken).
  I was in a hurry and forgot to keep track of my sources, my apologies.
*/
// I did not bother translating all holiday names to English, was a bit easier to work with

// you could replace this with another date library or use native JS if bundle size is an issue
import { addDays, getYear, isSameDay } from 'date-fns';

export const getNewYearsDay = (year) => new Date(year, 0, 1);
export const getLaborDay = (year) => new Date(year, 4, 1);
export const getNationaleFeestdag = (year) => new Date(year, 6, 21);
export const getOlvHemelvaart = (year) => new Date(year, 7, 15);
export const getAllerheiligen = (year) => new Date(year, 10, 1);
export const getWapenstilstand = (year) => new Date(year, 10, 11);
export const getChristmas = (year) => new Date(year, 11, 25);
export function getEaster(year) {
  if (year < 325) {
    throw new RangeError('Cannot calculate Easter dates before 325 AD.');
  }

  function mod(a, b) {
    return a % b;
  }

  function div(a, b) {
    const q = a / b;
    if (q < 0) {
      throw new Error('Unexpected negative q');
    }
    return Math.floor(q);
  }

  const y = year,
    skipMarchDays = 21,
    a = mod(y, 19),
    b = div(y, 100),
    c = mod(y, 100),
    d = div(b, 4),
    e = mod(b, 4),
    f = div(b + 8, 25),
    g = div(b - f + 1, 3),
    h = mod(19 * a + b - d - g + 15, 30),
    i = div(c, 4),
    k = mod(c, 4),
    l = mod(32 + 2 * e + 2 * i - h - k, 7),
    m = div(a + 11 * h + 22 * l, 451),
    t = h + l - 7 * m + skipMarchDays,
    n = div(t, 31) + 3,
    p = mod(t, 31);

  return new Date(year, n - 1, p + 1);
}
// 1 day after easter
export const getEasterMonday = (year) => addDays(getEaster(year), 1);
// 39 days after easter
export const getOlhHemelvaart = (year) => addDays(getEaster(year), 39);
export const getPinksteren = (year) => addDays(getOlhHemelvaart(year), 10);
export const getPinksterMaandag = (year) => addDays(getPinksteren(year), 1);

const getHolidays = (year) => ({
  newYearsDay: getNewYearsDay(year),
  paasmaandag: getEasterMonday(year),
  laborDay: getLaborDay(year),
  olhHemelvaart: getOlhHemelvaart(year),
  pinkstermaandag: getPinksterMaandag(year),
  nationaleFeestdag: getNationaleFeestdag(year),
  olvHemelvaart: getOlvHemelvaart(year),
  allerheiligen: getAllerheiligen(year),
  wapenstilstand: getWapenstilstand(year),
  christmas: getChristmas(year),
});

const isInHolidayList = (date) => {
  const holidays = getHolidays(getYear(date));
  let found = false;
  for (const holidayName of Object.keys(holidays)) {
    if (isSameDay(date, holidays[holidayName])) {
      found = true;
      break; // exit the loop as soon as we found 1 match
    }
  }
  return found;
};

// export const getHolidaysFormatted = (year = new Date().getFullYear()) => {
//   const holidays = getHolidays(year);
//   return Object.keys(holidays).map((holidayName) => {
//     return format(holidays[holidayName], 'dd/MM/yyyy');
//   });
// };

export const isHoliday = (date) => isInHolidayList(date);