server.js

//
// Currency Middleware
// -----------------------------------------------------------------------------
app.use((req, res, next) => {
  const cookie = {
    name: "currency",
    options: {
      path: "/",
      maxAge: 3650 * 24 * 3600 * 1000, // 10 years in miliseconds
    },
    url: "/currency/{currency}",
  };
  const currenciesPromise = new Promise(async (resolve, reject) => {
    try {
      const currencies = await Currency.findAll();
      resolve(currencies);
    } catch (e) {
      reject(e);
    }
  });
  currenciesPromise.then(
    (currencies) => {
      currencies.map(currency => currency.currency);
      const currencyUrl = `^${cookie.url}`;
      const changeCurrencyURL = new RegExp(currencyUrl.replace("/", "\\/").replace("{currency}", "(.*)"));
      const match = changeCurrencyURL.exec(req.url);

      if (match !== null) {
        if (currencies.indexOf(match[1]) !== -1) {
          res.cookie(cookie.name, match[1], cookie.options);
          return res.redirect("back");
        }
        res.status(404).send("The currency is not supported.");
      } else {
        const currency = req.cookies[cookie.name] || req.headers["accept-currency"];
        if (typeof currency !== "undefined") {
          res.cookie(cookie.name, currency, cookie.options);
        } else {
          res.clearCookie(cookie.name);
        }
      }
      next();
    },
    err => next(err),
  );
});