A simple class for formatting dates.

date-format.js

class DateFormat {
  constructor(format) {
    this.format = format instanceof String ? format : String(format);
  }

  get _formatMap() {
    return ['yyyy', 'yy', 'mm', 'm', 'dd', 'd', 'hh', 'h', 'ii', 'i', 'ss', 's'];
  }

  format(date) {
    this.date = date instanceof Date ? date : new Date(date);
    let output = '';
    while (format.length) {
      const key = this._formatMap.find(key => {
        return format.startsWith(key);
      });
      if (key) {
        output += this[`_${key}`](this.date);
        format = format.substr(key.length);
      } else {
        output += format[0];
        format = format.substr(1);
      }

    }
    return output;
  }

  _yyyy(date) {
    const year = date.getFullYear();
    return String(year).padStart(4, 0);
  }

  _yy(date) {
    const year = date.getYear();
    return String(year).padStart(2, 0);
  }

  _mm(date) {
    const month = date.getMonth() + 1;
    return String(month).padStart(2, 0)
  }

  _m(date) {
    const month = date.getMonth() + 1;
    return String(month);
  }

  _dd(date) {
    const day = date.getDate();
    return String(day).padStart(2, 0);
  }

  _d(date) {
    const day = date.getDate();
    return String(day);
  }

  _hh(date) {
    const hours = date.getHours();
    return String(hours).padStart(2, 0);
  }

  _h(date) {
    const hours = date.getHours();
    return String(hours);
  }

  _ii(date) {
    const minutes = date.getMinutes();
    return String(minutes).padStart(2, 0);
  }

  _i(date) {
    const minutes = date.getMinutes();
    return String(minutes);
  }

  _ss(date) {
    const seconds = date.getSeconds();
    return String(seconds).padStart(2, 0);
  }

  _s(date) {
    const seconds = date.getSeconds();
    return String(seconds);
  }
}

Date.prototype.format = function(format) {
  return new DateFormat(format).format(date);
};