get-browser-locales.js

function getBrowserLocales(options = {}) {
  const defaultOptions = {
    languageCodeOnly: false,
  };

  const opt = {
    ...defaultOptions,
    ...options,
  };

  const browserLocales =
    navigator.languages === undefined
      ? [navigator.language]
      : navigator.languages;

  if (!browserLocales) {
    return undefined;
  }

  return browserLocales.map(locale => {
    const trimmedLocale = locale.trim();

    return opt.languageCodeOnly
      ? trimmedLocale.split(/-|_/)[0]
      : trimmedLocale;
  });
}