code: function onListen(type, err, server) {
      servers[type] = server || true;
      if (err) {
        errors[type] = err;
      }
  
      if (servers.http && servers.https) {
        Object.keys(servers)
          .forEach(function (key) {
            if (typeof servers[key] === 'boolean') {
              delete servers[key];
            }
          });
  
        if (errors.http || errors.https) {
          return listening(errs.create({
            message: (errors.https || errors.http).message,
            https: errors.https,
            http:  errors.http
          }), servers);
        }
  
        listening(undefined, servers);
      }
    }
docs: ### function onListen(type, err, server) Responds to the `listening` callback if necessary with the appropriate servers.