Print a simple HTML calendar using javascript with no dependencies (except pure.css)

calendar.js

String.prototype.format = function() {
  var args = arguments;
  return this.replace(/{(\d+)}/g, function(match, number) {
    return typeof args[number] != 'undefined'
      ? args[number]
      : match
    ;
  });
};

Date.prototype.monthDays = function() {
  var d = new Date(this.getFullYear(), this.getMonth()+1, 0);
  return d.getDate();
}

Date.prototype.firstWeekdayOfMonth = function() {
  var d = new Date(this.getFullYear(), this.getMonth(), 1);
  return d.getDay();
}

Date.prototype.formatMonth = function () {
  const monthNames = [
    "January", "February", "March",
    "April", "May", "June", "July",
    "August", "September", "October",
    "November", "December"
  ];
  return monthNames[this.getMonth()];
}

function calendar (month) {
  const calendarDayHTML = '<div class="pure-u-1-8 calendar-day">{0}</div>';
  var html = `
    <div class="pure-g calendar-table">
    <div class="pure-u-1">
      <h3>${month.getFullYear()} ${month.formatMonth()}</h3>
    </div>`;
  // Insert blank space for weekdays falling on previous month
  for (i=month.firstWeekdayOfMonth(); i>0; --i) {
    html += calendarDayHTML.format('&nbsp;');
  }
  // Print days
  for (i=0; i<month.monthDays(); ++i) {
    ((month.firstWeekdayOfMonth() % 7) == 0) ? html += calendarDayHTML.format('&nbsp;'): '';
    html += calendarDayHTML.format(i+1);
  }
  return html;
}

var root = document.getElementById('calendar');

root.innerHTML = calendar(new Date);

index.html

<div id="calendar"></div>