Simple language matcher

example.ts

import { matchLanguages, getNearestSupportedLanguage } from './match-languages';

const supportedLanguages = ['en-GB', 'fr'];
console.log('navigator.languages: ', navigator.languages);
const matches = matchLanguages(navigator.languages, supportedLanguages);
console.log('preferred language matches:', matches);
const nearest = getNearestSupportedLanguage(matches, supportedLanguages);
console.log('nearest matching supported language:', nearest);

match-languages.ts

/**
 * Get all potential sub-languages from a given ISO 639-1 language code
 *
 * @param langCode - ISO 639-1 format language code
 * @returns Array of potential sub-languages, sorted by most to least specific
 */
const getAllSubLanguages = (langCode: string): string[] =>
  langCode
    .toLowerCase()
    .split('-')
    .flatMap((_, i, tags) => tags.slice(0, i + 1).join('-'))
    .reverse();

/**
 * Match preferred language list against a supported languages list
 *
 * @param preferred - Sorted language list in order of most preferable to least preferable
 * @param supported - List of supported languages to match from
 * @returns Preferred languages (including sub-language matches) that match supported languages list
 */
export const matchLanguages = (
  preferred: string[],
  supported: string[],
): string[] => {
  const matches = new Set<string>();
  const allSupportedLanguages = supported.flatMap(getAllSubLanguages);
  return preferred.flatMap(getAllSubLanguages).filter((language) => {
    if (!allSupportedLanguages.includes(language)) {
      return false;
    }
    const unique = !matches.has(language);
    if (unique) {
      matches.add(language);
    }
    return unique;
  });
};

/**
 * Get nearest matching language from a list of supported languages
 *
 * @param preferred - Sorted language list in order of most preferable to least preferable
 * @param supported - List of supported languages to match from
 * @returns Nearest supported language, sorted by preferred language list
 */
export const getNearestSupportedLanguage = (
  preferred: string[],
  supported: string[],
): string | undefined =>
  supported.find((language) =>
    getAllSubLanguages(language).some((lang) =>
      preferred.some((preferredLang) =>
        preferredLang.toLowerCase() === lang
      )
    ),
  );