_js-findClosestLocale - Gets the closest locale from the available locales

_js-findClosestLocale.js

// rev3 - https://gist.github.com/Noitidart/110c2f859db62398ae76069f4a6c5642
/**
 * Selects the closest matching locale from a list of locales.
 *
 * @param  aLocales
 *         An array of available locales
 * @param  aMatchLocales
 *         An array of prefered locales, ordered by priority. Most wanted first.
 *         Locales have to be in lowercase.
 * @return the best match for the currently selected locale
 *
 * Stolen from http://mxr.mozilla.org/mozilla-central/source/toolkit/mozapps/extensions/internal/XPIProvider.jsm
 */
function findClosestLocale(aLocales, aMatchLocales) {
  aMatchLocales = aMatchLocales;

  // Holds the best matching localized resource
  let bestmatch = null;
  // The number of locale parts it matched with
  let bestmatchcount = 0;
  // The number of locale parts in the match
  let bestpartcount = 0;

  for (let locale of aMatchLocales) {
    let lparts = locale.split("-");
    for (let localized of aLocales) {
      let found = localized.toLowerCase();
      // Exact match is returned immediately
      if (locale == found)
        return localized;

      let fparts = found.split("-");
      /* If we have found a possible match and this one isn't any longer
         then we dont need to check further. */
      if (bestmatch && fparts.length < bestmatchcount)
        continue;

      // Count the number of parts that match
      let maxmatchcount = Math.min(fparts.length, lparts.length);
      let matchcount = 0;
      while (matchcount < maxmatchcount &&
             fparts[matchcount] == lparts[matchcount])
        matchcount++;

      /* If we matched more than the last best match or matched the same and
         this locale is less specific than the last best match. */
      if (matchcount > bestmatchcount ||
         (matchcount == bestmatchcount && fparts.length < bestpartcount)) {
        bestmatch = localized;
        bestmatchcount = matchcount;
        bestpartcount = fparts.length;
      }
    }
    // If we found a valid match for this locale return it
    if (bestmatch)
      return bestmatch;
  }
  return null;
}

README

Rev1

• Copy paste from - https://github.com/mozilla/addon-sdk/blob/f5fab7b242121dccfa4e55ac80489899bb9f2a41/lib/sdk/l10n/locale.js#L70

Rev2

• Added rev and link comment

Rev3

• Removed fall back to getPreferredLocales() as that isn't available

Here's a more concise implementation of this utility using array methods:

const getAllSubLanguages = (langCode: string): string[] =>
  langCode
    .toLowerCase()
    .split('-')
    .flatMap((_, i, tags) => tags.slice(0, i + 1).join('-'))
    .reverse();

const getNearestSupportedLanguage = (
  preferred: string[],
  supported: string[],
): string | undefined =>
  supported.find((language) =>
    getAllSubLanguages(language).some((lang) =>
      preferred.some((preferredLang) =>
        preferredLang.toLowerCase() === lang
      )
    ),
  );

Thanks for sharing @eligrey! I've also been using another way recently I'll share soon.