code: function(now) {
        now || (now = this.attributes);
        var old = this._previousAttributes;
        var changed = false;
        for (var attr in now) {
          if (!_.isEqual(old[attr], now[attr])) {
            changed = changed || {};
            changed[attr] = now[attr];
          }
        }
        return changed;
      }
docs: Return an object containing all the attributes that have changed, or false if there are no changed attributes. Useful for determining what parts of a view need to be updated and/or what attributes need to be persisted to the server.