使用私有字段 （#） 作为元素的类型

interface PriceFormatOptions {
  unit: string;
}
export default class PriceHelper {
  /**
   * Adds unit and separates thousands
   */
  static format(
    price: Parameters<typeof PriceHelper.#separateThousands>[0],
    options: PriceFormatOptions = {} as PriceFormatOptions
  ) {
    let unit = options.unit || "تومان";

    const separatedPrice = this.#separateThousands(price);
    if (unit) unit = ` ${unit}`;

    return separatedPrice + unit;
  }

  /**
   * Converts numeral prices to persian words
   */
  static toWords() {}

  static #separateThousands(price: string | number) {
    return String(price || 0).replace(/\B(?=(\d{3})+(?!\d))/g, ",");
  }
}