code: function computeDelta(datasetId, clientRecords, serverRecords) {
    var creates = {};
    var updates = {};
    var deletes = {};
    _.each(serverRecords, function(serverRecord, serverRecordUid) {
      var serverRecHash = serverRecord.hash;
      //record is in both client and server
      if (clientRecords[serverRecordUid]) {
        //hash value doesn't match, needs update
        if (clientRecords[serverRecordUid] !== serverRecHash) {
          debug('[%s] Updating client record %s  client hash=%s', datasetId, serverRecordUid, clientRecords[serverRecordUid]);
          updates[serverRecordUid] = serverRecord;
        }
      } else {
        //record is not in the client, needs create
        debug('[%s] Creating client record %s', datasetId, serverRecordUid);
        creates[serverRecordUid] = serverRecord;
      }
    });
  
    _.each(clientRecords, function(clientRecordHash, clientRecordUid) {
      if (!serverRecords[clientRecordUid]) {
        //the record is in the client but not in the server, need delete
        debug('[%s] Deleting client record %s', datasetId, clientRecordUid);
        deletes[clientRecordUid] = {};
      }
    });
  
    return {
      create: creates,
      update: updates,
      delete: deletes
    };
  }
docs: Find out the differences between the clientRecords and the serverRecords
  @param {String} datasetId the id of the dataset
  @param {Object} clientRecords client records. The key should be the uid of each record, and the value should be the hash of each record
  @param {Object} serverRecords server records. The key should be the uid of each record, and the value should contain the `data` and the `hash` of each record
  @returns the records that should be created/updated/deleted from the client