ordinal-formatting.js

const ordinalSuffixes = {
  "en": {    
    "one": "st",
    "two": "nd",
    "few": "rd",
    "other": "th"
  }
};

export class OrdinalFormat {
  
  #rules;
  #suffixes;
  
  constructor(locale) {
    if(!(locale in ordinalSuffixes)) throw new Error(`Unhandled locale: ${locale}`);
    this.#suffixes = ordinalSuffixes[locale];
    this.#rules = new Intl.PluralRules(locale, { type: "ordinal" });    
  }
  
  withOrdinalSuffix(x) {
    if(typeof x != "number") throw new TypeError(`Expected Number but received ${typeof x}`);
    if(x < 1) throw new RangeError(`Expected a number > 0 but received ${x}`);
    const ordinal = this.#rules.select(x);
    if (!ordinal in this.#suffixes) throw new Error(`Unexpected ordinal ${ordinal}`);
    const suffix = this.#suffixes[ordinal];
    return `${x}${suffix}`;
  }
  
}