[Javascript] Most Frequent Days

mostFrequentDays.js

const mostFrequentDays = year => {
  let dayName = d => d.toLocaleString('en',{ weekday:'long' });

  let firstDay = new Date(year, 0, 1);
  let day = dayName(firstDay);
  let lastDay = dayName(new Date(year, 11, 31));
  let daysResult = day == lastDay? [day] : [day, lastDay];

  return firstDay.getDay() ? daysResult : daysResult.reverse();
};