Date Utilities in vanilla JavaScript

dateutils.js

/**
 * 
 * @param date - may be either a Date object or a String representing a Date 
 */
function lastDayOfMonth(date) {
  var d = new Date(date);
  d.setFullYear(d.getFullYear(), d.getMonth() + 1, 0);
  return d.getDate();
}

// Example
console.log(lastDayOfMonth('2020-02-14')); // returns 29


function addDays(date, days) {
  var result = new Date(date);
  result.setDate(result.getDate() + days);
  return result;
}

function addMonths(date, months) {
  var result = new Date(date);
  var d = date.getDate();
  result.setMonth(result.getMonth() + months);
  if (result.getDate() != d) {
    result.setDate(0); // the date will be set to the last day of the previous month
  }
  return result;
}