Medium: daysInMonth

daysInMonth-long.js

/**
 * Given a date, how many days are in the month?
 *  Peek ahead to "next month" and look back to "last day of target month" via date=0
 * @param {Date} targetMonthDate
 * @return int
 */
function daysInMonth(targetDate) {
  // Create a new Date for the "next month"
  //  Also discard date's date (set to 1) to avoid any overflow
  //  Example: Attempting to set date to February 31 when there are only 28 days
  const nextMonth = new Date(targetDate.getFullYear(), targetDate.getMonth() + 1, 1);
  
  // Setting nextMonth's date to 1 would give us the first date of nextMonth.
  // Setting nextMonth's date to 0 gives us the last day of previous month.
  const lastDayOfTargetMonth = new Date(nextMonth.getFullYear(), nextMonth.getMonth(), 0);
  
  // Example, if last day of month is October 31st, daysInMonth = 31;
  const daysInMonth = lastDayOfTargetMonth.getDate();
  
  return daysInMonth;
}

daysInMonth-short.js

/* The same thing, shortened up a bit */
const daysInMonth = (date) =>
  (new Date(date.getFullYear(), date.getMonth() + 1, 0)).getDate();