Get calendar days

index.js

import dateFns from 'date-fns';

function getMonthDays(date) {
  const startOfMonth = dateFns.startOfMonth(date);
  const endOfMonth = dateFns.endOfMonth(date);

  let prevMonthDaysNeeded = startOfMonth.getDay();
  const prevMonthDays = prevMonthDaysNeeded ? dateFns.eachDay(
    dateFns.subDays(startOfMonth, prevMonthDaysNeeded),
    dateFns.subDays(startOfMonth, 1)
  ) : [];
  
  const currentMonthDays = dateFns.eachDay(startOfMonth, endOfMonth);

  const lastWeekDays = (prevMonthDays.length + currentMonthDays.length) % 7;
  const nextMonthDaysNeeded = lastWeekDays === 0 ? 0 : 7 - lastWeekDays;
  const nextMonthDays = nextMonthDaysNeeded ? dateFns.eachDay(
    dateFns.addDays(endOfMonth, 1),
    dateFns.addDays(endOfMonth, nextMonthDaysNeeded)
  ) : [];

  return {
    previous: prevMonthDays,
    current: currentMonthDays,
    next: nextMonthDays,
  };
}

console.log('Now', getMonthDays(new Date()));
console.log('September', getMonthDays(new Date(2017,8,1)));
console.log('October', getMonthDays(new Date(2017,9,1)));