code: function $createClient(options) {
          /*
           * Ensure we have default options.
           */
          options = extend({}, {}, options);
  
          const { url, transport } = options;
  
          if (_clients[url]) {
              return _clients[url];
          }
  
          const client = mqtt.connect(url, transport);
  
          _clients[url] = client;
  
          return client;
      }
docs: Factory function to create MQTT client.
  The default factory creates a regular MQTT.js
  client.
  @param {Object} options MQTT configuration object