format-date-with-ordinal.ts

/**
 * Options when formatting a date
 */
interface DateFormatOptions {
  includeYear?: Boolean;
}

/**
 * For English, format a date with given options, adding an ordinal
 * e.g. "May 1st, 1992" (note the "1st"). For non-English locales,
 * format a date with given options (and no ordinal);
 */
function formatLocalizedDateWithOrdinal(locale: Locale,
                                         date: Date,
                                         options: DateFormatOptions = { includeYear: false }) {
  if (locale.toLowerCase().startsWith("en")) {
    return formatEnglishDateWithOrdinal(date, options);
  }

  return formatNonEnglishDate(locale, date, options);
}

/**
 * Format an English date with it ordinal e.g. "May 1st, 1992"
 */
function formatEnglishDateWithOrdinal(date: Date,
                                       { includeYear }: DateFormatOptions): string {
  const month: string = date.toLocaleDateString("en", { month: "long" });

  const day: string = getOrdinal(date.getDate());

  let formatted: string = `${month} ${day}`;

  if (includeYear) {
    formatted += `, ${date.getFullYear()}`;
  }
  
  return formatted; 
}

/**
 * Format a non-English date
 */
function formatNonEnglishDate(locale: Locale,
                               date: Date,
                               { includeYear }: DateFormatOptions): string {
  const options: Intl.DateTimeFormatOptions = { day: "numeric", month: "long" };
  
  if (includeYear) {
    options.year = "numeric";
  }
  
  return date.toLocaleDateString(locale, options);
}

/**
 * Retrieve an English ordinal for a number, e.g. "2nd" for 2
 */
function getOrdinal(n: number): string {
  // From https://community.shopify.com/c/Shopify-Design/Ordinal-Number-in-javascript-1st-2nd-3rd-4th/m-p/72156
   var s=["th","st","nd","rd"],
       v=n%100;
   return n+(s[(v-20)%10]||s[v]||s[0]);
}