Small utility library for formatting Date objects in JavaScript

formatDate.js

/**
 * Usage:
 *
 * const fmt = formatDate('%dd-%MMM-%yy', 'en-US');
 * console.log(fmt(new Date())); // 18-Apr-20
 */
function formatDate (fmt, locales, defaultOptions) {
  function formatPart (option, value, fallbackImpl, options = { [option]: value }) {
    // lazily initialize format implementation
    let formatImpl;

    return function format (date) {
      if (typeof formatImpl !== 'function' && typeof window?.Intl?.DateTimeFormat === 'function') {
        const dateTimeFormat = new window.Intl.DateTimeFormat(
          locales,
          { ...defaultOptions, ...options, [option]: value }
        );
        const resolved = Object.entries(options).every(
          function ([key, expectedValue]) {
            return this[key] === expectedValue;
          },
          dateTimeFormat.resolvedOptions()
        );

        if (resolved) {
          formatImpl = function dateTimeImpl (date) {
            return dateTimeFormat.formatToParts(date).find(
              ({ type }) => type === option
            ).value;
          };
        }
      }

      if (typeof formatImpl !== 'function' && typeof fallbackImpl === 'function') {
        formatImpl = fallbackImpl;
      }

      if (typeof formatImpl !== 'function') {
        formatImpl = function unsupportedImpl () {
          throw new Error(`Unsupported format option ${option}`);
        };
      }

      return formatImpl(date);
    };
  }

  const part = {
    d: formatPart('day', 'numeric', date => `${date.getDate()}`),
    dd: formatPart('day', '2-digit', date => part.d(date).padStart(2, '0')),
    ddd: formatPart('weekday', 'short'),
    dddd: formatPart('weekday', 'long'),
    f: date => part.fff(date).slice(0, 1),
    ff: date => part.fff(date).slice(0, 2),
    fff: date => `${date.getMilliseconds()}`.padStart(3, '0'),
    F: date => part.f(date).replace('0', ''),
    FF: date => part.ff(date).replace('00', ''),
    FFF: date => part.fff(date).replace('000', ''),
    g: formatPart('era', 'short'),
    gg: formatPart('era', 'long'),
    h: formatPart('hour', 'numeric', date => `${(date.getHours() % 12) || 12}`, { hour: 'numeric', hour12: true }),
    hh: formatPart('hour', '2-digit', date => part.h(date).padStart(2, '0'), { hour: '2-digit', hour12: true }),
    H: formatPart('hour', 'numeric', date => `${date.getHours()}`, { hour: 'numeric', hour12: false }),
    HH: formatPart('hour', '2-digit', date => part.H(date).padStart(2, '0'), { hour: '2-digit', hour12: false }),
    K: formatPart('timeZoneName', 'short'),
    KK: formatPart('timeZoneName', 'long'),
    m: formatPart('minute', 'numeric', date => `${date.getMinutes()}`),
    mm: formatPart('minute', '2-digit', date => part.m(date).padStart(2, '0')),
    M: formatPart('month', 'numeric', date => `${date.getMonth() + 1}`),
    MM: formatPart('month', '2-digit', date => part.M(date).padStart(2, '0')),
    MMM: formatPart('month', 'short'),
    MMMM: formatPart('month', 'long'),
    s: formatPart('second', 'numeric', date => `${date.getSeconds()}`),
    ss: formatPart('second', '2-digit', date => part.s(date).padStart(2, '0')),
    t: formatPart('dayPeriod', 'narrow', undefined, { hour: 'numeric', hour12: true }),
    tt: formatPart('dayPeriod', 'short', undefined, { hour: 'numeric', hour12: true }),
    ttt: formatPart('dayPeriod', 'long', undefined, { hour: 'numeric', hour12: true }),
    yy: formatPart('year', '2-digit', date => `${date.getFullYear() % 100}`.padStart(2, '0')),
    yyyy: formatPart('year', 'numeric', date => `${date.getFullYear()}`.padStart(4, '0'))
  };

  return (date = new Date()) => fmt.replace(
    /%(([A-Za-z])\2*)/g,
    (match, key) => {
      if (part.hasOwnProperty(key)) {
        return part[key](date);
      }

      throw new SyntaxError(`Invalid specifier ${match}`);
    }
  );
}