SOURCE

'use strict';
Date.prototype.format = function(format) {
  // yyyy-mm-dd HH:mm:ss
  if (typeof format !== 'string') return;
  let dateStr = format;
  let year = this.getFullYear();
  let month = this.getMonth();
  let day = this.getDate();
  let hour = this.getHours();
  let minute = this.getMinutes();
  let seconds = this.getSeconds();

  function add0(value) {
    return value < 10 ? '0' + value : '' + value;
  }
  month = add0(month);
  day = add0(day);
  hour = add0(hour);
  minute = add0(minute);
  seconds = add0(seconds);
  dateStr = dateStr.replace(/yyyy/, '' + year);
  dateStr = dateStr.replace(/MM/, '' + month);
  dateStr = dateStr.replace(/dd/, '' + day);
  dateStr = dateStr.replace(/HH/, '' + hour);
  dateStr = dateStr.replace(/mm/, '' + minute);
  dateStr = dateStr.replace(/ss/, '' + seconds);
  return dateStr;
};