Extending feature (add, format) for Date object in Vanilla Javascript (:see_no_evil: Monkey Patch)

date.extend.js

(function () {
  'use strict';

  var MONTHS_NAME = ['January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October', 'November', 'December'];
  var DAYS_NAME = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday']; // Weekdays
  var DEFAULT_FORMAT = 'YYYY-MM-DDTHH:mm:ssZ';
  var FORMAT_REGEX = /\[.*?\]|Y{2,4}|M{1,4}|D{1,2}|d{1,4}|H{1,2}|h{1,2}|a|A|m{1,2}|s{1,2}|Z{1,2}|SSS/g;
  var STRIP_BRACKETS_REGEX = /\[|\]/g;

  Date.prototype.addDate = function (value) { // Day
    this.setDate(this.getDate() + value);
    return this;
  };

  Date.prototype.addFullYear = function (value) {
    this.setFullYear(this.getFullYear() + value);
    return this;
  };

  Date.prototype.addHours = function (value) {
    this.setHours(this.getHours() + value);
    return this;
  };

  Date.prototype.addMilliseconds = function (value) {
    this.setMilliseconds(this.getMilliseconds() + value);
    return this;
  };

  Date.prototype.addMinutes = function (value) {
    this.setMinutes(this.getMinutes() + value);
    return this;
  };

  Date.prototype.addMonth = function (value) {
    this.setMonth(this.getMonth() + value);
    return this;
  };

  Date.prototype.addSeconds = function (value) {
    this.setSeconds(this.getSeconds() + value);
    return this;
  };

  Date.prototype.addTime = function (value) {
    this.setTime(this.getTime() + value);
    return this;
  };

  Date.prototype.addYear = function (value) {
    this.setYear(this.getYear() + value);
    return this;
  };

  Date.prototype.getMonthName = function () {
    return MONTHS_NAME[this.getMonth()];
  };

  Date.prototype.getDayName = function () { // Weekday
    return DAYS_NAME[this.getDay()];
  };

  // https://github.com/iamkun/dayjs
  Date.prototype.toFormatString = function (format, options) {
    format = typeof format === 'string' ? format : DEFAULT_FORMAT;
    options = typeof options === 'object' ? options : {};
    options.utc = typeof options.utc === 'boolean' ? options.utc : false;

    var digitPadStart = function (value, length) {
      length = typeof length === 'number' ? length : 2;
      var str = String(value);
      if (!str || str.length >= length) return str;
      return Array((length + 1) - str.length).join('0') + str;
    };

    var year = options.utc ? this.getUTCFullYear() : this.getFullYear(),
      month = options.utc ? this.getUTCMonth() : this.getMonth(),
      date = options.utc ? this.getUTCDate() : this.getDate(), // Day
      day = options.utc ? this.getUTCDay() : this.getDay(), // Weekday
      hours = options.utc ? this.getUTCHours() : this.getHours(),
      minutes = options.utc ? this.getUTCMinutes() : this.getMinutes(),
      seconds = options.utc ? this.getUTCSeconds() : this.getSeconds(),
      milliseconds = options.utc ? this.getUTCMilliseconds() : this.getMilliseconds(),
      meridiem = hours < 12 ? 'AM' : 'PM',
      zone = (function () {
        if (options.utc) return '+00:00';
        var negMinuts = this.getTimezoneOffset();
        var minutes = Math.abs(negMinuts);
        return (negMinuts <= 0 ? '+' : '-') +
          digitPadStart(Math.floor(minutes / 60), 2) +
          ':' +
          digitPadStart(minutes % 60, 2);
      }.bind(this)());

    return format.replace(FORMAT_REGEX, function (match) {
      if (match.indexOf('[') > -1) return match.replace(STRIP_BRACKETS_REGEX, '');
      switch (match) {
        case 'YY':
          return String(year).slice(-2);
        case 'YYYY':
          return String(year);
        case 'M':
          return String(month + 1);
        case 'MM':
          return digitPadStart(month + 1, 2);
        case 'MMM':
          return MONTHS_NAME[month].substr(0, 3);
        case 'MMMM':
          return MONTHS_NAME[month];
        case 'D':
          return String(date);
        case 'DD':
          return digitPadStart(date, 2);
        case 'd':
          return String(day);
        case 'dd':
          return DAYS_NAME[day].substr(0, 2);
        case 'ddd':
          return DAYS_NAME[day].substr(0, 3);
        case 'dddd':
          return DAYS_NAME[day];
        case 'H':
          return String(hours);
        case 'HH':
          return digitPadStart(hours, 2);
        case 'h':
        case 'hh':
          if (hours === 0) return 12;
          return digitPadStart(hours < 13 ? hours : hours - 12, match === 'hh' ? 2 : 1);
        case 'a':
          return meridiem.toLowerCase();
        case 'A':
          return meridiem;
        case 'm':
          return String(minutes);
        case 'mm':
          return digitPadStart(minutes, 2);
        case 's':
          return String(seconds);
        case 'ss':
          return digitPadStart(seconds, 2);
        case 'SSS':
          return digitPadStart(milliseconds, 3);
        case 'Z':
          return zone;
        case 'ZZ':
          return zone.replace(':', '');
        default:
          return '';
      }
    });
  };
}());