header.js

export default {
  name: 'header',

  lookup(req, res, options) {
    let found;

    if (typeof req !== 'undefined') {
      let headers = req.headers;
      if (!headers) return found;

      let locales = [];
      let acceptLanguage = options.lookupHeader ? headers[options.lookupHeader] : headers['accept-language'];

      if (acceptLanguage) {
        let lngs = [], i, m;
        const rgx = /(([a-z]{2})-?([A-Z]{2})?)\s*;?\s*(q=([0-9.]+))?/gi;
        
        do {
          m = rgx.exec(acceptLanguage);
          if (m) {
            const lng = m[1], weight = m[5] || '1', q = Number(weight);
            if (lng && !isNaN(q)) {
              lngs.push({lng, q});
            }
          }
        } while (m);

        lngs.sort(function(a,b) {
          return b.q - a.q;
        });

        for (i = 0; i < lngs.length; i++) {
          locales.push(lngs[i].lng);
        }

        if (locales.length) found = locales;
      }
    }

    return found;
  }
};