freeCodeCamp/server/boot/z-lang-redirect.js

import supportedLanguages from '../../common/utils/supported-languages';
import passThroughs from '../utils/lang-passthrough-urls';
// import debug from 'debug';

// const log = debug('fcc:controller:lang-redirect');
const toLowerCase = String.prototype.toLowerCase;

export default function redirectLang(app) {
  app.all('*', function(req, res, next) {
    const { url, path } = req;
    const langCode = toLowerCase.call(url.split('/')[1]);

    if (passThroughs[langCode]) {
      return next();
    }

    if (supportedLanguages[langCode]) {
      req.flash('errors', {
        msg: `We couldn't find path ${ path }`
      });
      return res.render('404', { title: '404'});
    }

    // language aware redirect
    return res.redirect(url);
  });
}