match-supported-locales.js

function matchSupportedLocale(acceptsLocales, supportedLocales, defaultLocale) {
  return (
    firstExactMatch(acceptsLocales, supportedLocales) ||
    firstLanguageMatch(acceptsLocales, supportedLocales) ||
    defaultLocale
  );
}

function firstExactMatch(acceptsLocales, supportedLocales) {
  return acceptsLocales.find((al) =>
    supportedLocales.includes(al)
  );
}

function firstLanguageMatch(acceptsLocales, supportedLocales) {
  for (let acceptedLang of languagesFor(acceptsLocales)) {
    const match = supportedLocales.find(
      (sl) => languageFor(sl) === acceptedLang
    );

    if (match) {
      return match;
    }
  }
}

function languagesFor(locales) {
  return locales.map((loc) => languageFor(loc));
}

function languageFor(locale) {
  return locale.split('-')[0];
}

module.exports = matchSupportedLocale;