weekdayList.js

/**
 * Returns the days of the week from a given start day, indicated with a three-letter code.
 * Normally, the start day is Monday, but any day is usable depending on locale.
 *
 * @param {String} startDay One of 'mon', 'tue', 'wed', 'thu', 'fri', 'sat', 'sun'
 * @returns {Array} List of days in the week starting from the indicated first day
 */
export const weekdayList = (startDay = 'mon') => {
  const days = ['mon', 'tue', 'wed', 'thu', 'fri', 'sat', 'sun']
  const start = days.indexOf(startDay)
  let rest

  // Ensure we have a valid starting day.
  switch (start) {
    case -1:
      throw RangeError(`Invalid start day: ${startDay}. Must be one of ${days}.`)
    case 0:
      return days
    default:
      rest = days.splice(0, start)
      return [...days, ...rest]
  }
}