docs: function unsubscribe(topicArn) {
  
          // if not subscribed then return now
          if (!subscriptions[topicArn]) {
              debug('Not subscribed to ' + topicArn);
              return Promise.resolve();
          }
  
          return new Promise(function(resolve, reject) {
              const params = { SubscriptionArn: topicArn };
              config.sns.unsubscribe(params, function (err) {
                  debug('Unsubscribe from ' + topicArn + (err ? ' failed: ' + err.message : 'succeeded'));
                  if (err) return reject(err);
                  delete subscriptions[topicArn];
                  resolve();
              });
          });
      }