foo.js

function getFeedsList() {
  return new RSVP.Promise(function(resolve, reject) {
    function getFeedsKeys() {
      client.smembers("feeds", function(error, obj) {
        if (error) throw(error);
        return obj;
      });
    }

    function getFeedsMetadata(feedsKeys) {
      client.mget(feedsKeys, function(error, obj) {
        if (error) throw(error);
        return obj.map(function(currentValue, index, array) {return JSON.parse(currentValue);});
      });
    }

    try {
      var keys = getFeedsKeys();
      var metadata = getFeedsMetadata(keys);
      resolve(metadata);
    } catch (err) {
      reject(err);
    }
  });
}

getFeedsList().then(fetchFeeds, handleFeedsListError);