JavaScript moment-js style date formatting function

date-format.js

/**
 * Given a date, return a format. I've always wanted a cheap way to do this
 * without installing a full library.
 *
 * Credit: https://gist.github.com/justin-schroeder/41505a6be88bfa25467e4ed5161608c2
 *
 * @param {Date} date
 * @return {string}
 */
export function format (date, format) {
  // ----- Years -----
  const yr = date.getFullYear()
  let shortYr = yr % (Math.floor(yr / 100) * 100)
  // ----- Hours -----
  const hour = date.getHours()
  const shortHour = hour === 0 ? 12 : (hour > 12 ? hour - 12 : hour)
  shortYr = shortYr < 10 ? '0' + shortYr : '' + shortYr
  // ----- Minutes -----
  const minute = date.getMinutes()
  // ----- Seconds -----
  const second = date.getSeconds()
  // ----- Months -----
  const months = ['January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October', 'November', 'December']
  // ----- Days -----
  const dateOfMonth = date.getDate()
  // ----- Ordinal -----
  const ordinals = ['th', 'st', 'nd', 'rd'].concat(Array(10).fill('th'))

  const replacements = {
    YYYY: yr,
    YY: shortYr,
    HH: hour < 10 ? '0' + hour : '' + hour,
    H: hour,
    h: shortHour,
    kk: hour + 1,
    k: hour + 1 < 10 ? '0' + hour + 1 : hour + 1,
    a: hour >= 12 ? 'pm' : 'am',
    A: hour >= 12 ? 'PM' : 'AM',
    mm: minute < 10 ? '0' + minute : minute,
    m: minute,
    ss: second < 10 ? '0' + second : second,
    s: second,
    MMMM: months[date.getMonth()],
    MMM: months[date.getMonth()].substr(0, 3),
    MM: (date.getMonth() < 9 ? '0' : '') + (date.getMonth() + 1),
    M: '' + date.getMonth(),
    DD: dateOfMonth < 10 ? '0' + dateOfMonth : dateOfMonth,
    D: dateOfMonth,
    o: ordinals[dateOfMonth > 13 ? dateOfMonth % 10 : dateOfMonth]
  }
  return format.replace(/Y{2,4}|H{1,2}|h|k{1,2}|a|A|m{1,2}|s{1,2}|M{1,4}|D{1,2}|o/g, (match) => replacements[match])
}