Generate constants with months/weekdays names

locales.js

// More info on locales and codes for names on MDN 'toLocaleString'

weekdaysArray(locale = 'en-us', startMonday = false) {
  const initYear = 2006; // Year that started on the first day of week

  return Array.from({ length: 7 }, (v, k) => k)
    .map(weekday => new Date(initYear, 0, weekday + (startMonday ? 1 : 0) + 1)
    .toLocaleString(locale, { weekday: 'narrow' }));
}

monthsArray(locale = 'en-us') {
  return Array.from({ length: 12 }, (v, k) => k)
    .map(month => new Date(2000, month, 1)
    .toLocaleString(locale, { month: 'long' }));
}