code: function(otherUserId, callback){
            // if there is only one other user or the other user is the same user
            if (otherUserIdsWhoRated.length === 1 || userId === otherUserId){
              // then call the callback and exciting the similarity check
              callback();
            }
            // if the userid is not the same as the user
            if (userId !== otherUserId){
              // calculate the jaccard coefficient for similarity. it will return a value between -1 and 1 showing the two users
              // similarity
              jaccardCoefficient(userId, otherUserId, function(result) {
                // with the returned similarity score, add it to a sorted set named above
                client.zadd(similarityZSet, result, otherUserId, function(err){
                  // call the async callback function once finished to indicate that the process is finished
                  callback();
                });
              });
            }
          }
docs: running a function on each item in the list