A function to convert any string to title case including non capitalizing words in english and spanish

toTitleCase.ts

function toTitleCase(str: string | undefined | null) {
  if (!str) return "";
  const nonCapitalizedWords = ["a", "an", "the", "and", "but", "or", "nor", "for", "yet", "so", "as", "at", "by", "for", "in", "of", "on", "per", "to", "with"];
  const nonCapitalizedWordsSpanish = ["un", "una", "unos", "unas", "el", "la", "los", "las", "y", "pero", "o", "ni", "por", "aún", "así", "como", "en", "por", "para", "de", "del", "al", "con"];

  return str.replace(/\w\S*/g, (txt: string) => {
    if ([
      ...nonCapitalizedWords,
      ...nonCapitalizedWordsSpanish
    ].includes(txt.toLowerCase())) {
      return txt.toLowerCase();
    } else {
      return txt.charAt(0).toUpperCase() + txt.substring(1).toLowerCase();
    }
  });
}