guyellis/nth-day

import moment from 'moment';

function dateFromDate(date) {
  if (!date) {
    return moment();
  }

  if (typeof date === 'string') {
    const dateObj = new Date(date);
    if (dateObj.toString() === 'Invalid Date') {
      // console.log('Invalid Date:', date);
      throw date;
    }
    return moment(dateObj);
  }

  if (typeof date === 'object') {
    if (moment.isMoment(date)) {
      return date;
    }
    return moment(date);
  }

  throw new Error('Do not know how to handle this type:', typeof date);
}

function dayFromString(dayOfWeek) {
  if (typeof dayOfWeek === 'string') {
    const dow = dayOfWeek.toLowerCase().slice(0, 3);
    const weekDays = ['sun', 'mon', 'tue', 'wed', 'thu', 'fri', 'sat'];
    const dayNumber = weekDays.indexOf(dow);
    if (dayNumber === -1) {
      throw new Error('Do not recognize this day: ', dayOfWeek);
    }
    return dayNumber;
  }
  if (typeof dayOfWeek !== 'number') {
    throw new Error('Expecting dayOfWeek to be a number or string but it was: ', typeof dayOfWeek);
  }
  if (dayOfWeek < 0 || dayOfWeek > 6) {
    throw new Error('Expecting dayOfWeek to be in the range of 0 to 6 but it was ', dayOfWeek);
  }
  return dayOfWeek;
}

// nth - value from 1 through 5 for the (say) 4th Friday
//   Negative numbers count backwards from the end of the month
//   (For example, the last Friday, or the second to last Sunday)
// dayOfWeek - 0 through 6 (Sun through Sat) or can be string (e.g. 'monday')
// relevantDate - a date as a string or Date object or moment object
// Returns a moment date
// eslint-disable-next-line import/prefer-default-export
export function nthDay(nth, dayOfWeek, relevantDateParam, afterDayParam) {
  const relevantDate = dateFromDate(relevantDateParam);
  const month = relevantDate.month();
  const year = relevantDate.year();
  const afterDay = afterDayParam || 0;

  if (nth < 0) {
    // nth-last day of the month
    const fourthDay = nthDay(4, dayOfWeek, relevantDate);
    let occurrences;

    if (fourthDay.add(1, 'week').month() > month) {
      // It's a month with four of this day of the week
      occurrences = 4;
    } else {
      // It's a month with five of this day of the week
      occurrences = 5;
    }
    return nthDay(occurrences + 1 + nth, dayOfWeek, relevantDate, afterDay);
  }

  const dow = dayFromString(dayOfWeek);

  let counter = nth - 1;
  const day = (counter * 7) + 1 + afterDay;

  const date = moment([year, month, day]);
  date.subtract(1, 'day');
  while (counter !== nth) {
    date.add(1, 'day');
    if (date.day() === dow) {
      counter += 1;
    }
  }
  return date;
}