merge.js

              var prev = 'prev' in found ? found.prev : _.clone(found);
              var threeWayMerge = function(a, b, c) {
                var d = a || {};
                _.forEach(b, function(value, key) {
                  if (_.isArray(value) || _.isObject(value)) {
                    d[key] = threeWayMerge(a[key], b[key], c[key]);
                  }
                  else {
                    if (a && c && a[key] == c[key]) {
                      d[key] = b[key];
                    }
                    else if (c && b[key] == c[key]) {
                      d[key] = a[key];
                    }
                    else {
                      // conflict: prefer online version (b)
                      d[key] = b[key];
                    }
                  }
                });
                return d;
              };
              var orig = _.clone(found);
              // found - визит в оффлайне, visit - визит из онлайне, prev - прошлая версия визита
              found = threeWayMerge(found, visit, prev);
              found.prev = orig;
              promises.push(db.update('visits', {'data': JSON.stringify(found)}, {
                'id': found.id
              }));