Checking day for weekday or weekend ordinal (ie. first weekend day or third weekday).

weekDayEndOccurance.js

// imports from date-fns https://github.com/date-fns/date-fns
const isWeekend = require('date-fns/is_weekend');
const endOfMonth = require('date-fns/end_of_month');
const startOfDay = require('date-fns/start_of_day');
const startOfMonth = require('date-fns/start_of_month');
const addDays = require('date-fns/add_days');
const subDays = require('date-fns/sub_days');

// test day
const now = new Date(2018,1,28);

const obj = {
  weekdayWeekend: (pattern, ordinal) => {
    const ordinals = ['first', 'second', 'third', 'fourth'];
    let checkDay;

    const patterns = {
      weekend_day: (checkDay) => isWeekend(checkDay),
      weekday: (checkDay) => !isWeekend(checkDay),
    }

    // if looking for last, loop from end of month backwards
    if (ordinal === 'last') {
      // set check day to last day of month
      checkDay = endOfMonth(now);
      // set endDate for loop
      endDate = endOfMonth(now).getDate();
      for (let i = endDate; i >= endDate - 6; i -= 1) {
        // if day matches weekend/weekday pattern
        if (patterns[pattern](checkDay)) {
          // set time to start of day for checking and end for loop
          checkDay = startOfDay(checkDay);
          break;
        }
        // otherwise, step back a day in loop
        checkDay = subDays(checkDay, 1);
      }
    // otherwise, loop from start of month forward
    } else {
      // set check day to first day of month
      checkDay = startOfMonth(now);
      // set count to account for ordinals
      count = 0;
      // for loop from start of month forwards
      for (let i = 1; i <= 14; i += 1) {
        // if day matches weekend/weekday pattern
        if (patterns[pattern](checkDay)) {
          // if count matches with ordinal index in array
          if (ordinals.indexOf(ordinal) === count) {
            break;
          }
          // if ordinal doesnt match, but weekday/weekend does increase count by one
          count += 1;
        }
        // otherwise, step forward a day in loop
        checkDay = addDays(checkDay, 1);
      }
    }
    // return boolean for matching given date and actual
    return checkDay.getTime() === now.getTime();
  }
}

console.log('test', obj.weekdayWeekend('weekday','last'));