code: function subscribeNode (client, subscriptions, channel, callback, done_callback) {
      if (subscriptions[channel]===undefined) {
          subscriptions[channel] = [callback];
          client.subscribe(channel, {qos: 0}, function() {
              // If there is a done_callback defined, execute it
              if (done_callback!==undefined) done_callback();
          });
      } else {
          subscriptions[channel].push(callback);
      }
  }
docs: Helper function that subscribes to a channel in node