groupBy Backbone collection

gistfile1.js

var Backbone = require('backbone');

var CollectionModel = Backbone.Model.extend({
  initialize: function () {
    this.collection = new Backbone.Collection();
  },

  add: function () {
    this.collection.add.apply(this.collection, arguments);
  },

  remove: function () {
    this.collection.remove.apply(this.collection, arguments);
  },

  length: function () {
    return this.collection.length;
  }
});

return function groupBy (originalCollection, grouping, listener) {
  var groups = {};
  var models = {};
  var collection = new Backbone.Collection();

  var addGroup = function (key) {
    groups[key] = new CollectionModel();
    collection.add(groups[key]);
  };

  var removeGroup = function (key, group) {
    collection.remove(group);
    delete groups[key];
  };

  var add = function (model) {
    var key = grouping(model);
    if (!groups[key]) {
      addGroup(key);
    }
    var group = groups[key];
    group.add(model);
    models[model.id] = key;
  };

  var change = function (model) {
    var oldKey = models[model.id];
    var newKey = grouping(model);
    if (oldKey !== newKey) {
      var group = groups[oldKey];
      group.remove(model);
      if (group.length() === 0) {
        removeGroup(oldKey);
      }

      if (!groups[newKey]) {
        addGroup(newKey);
      }
      groups[newKey].add(model);

      models[model.id] = newKey;
    }
  };

  var remove = function (model) {
    var key = models[model.id];
    var group = groups[key];
    group.remove(model);
    if (group.length() === 0) {
      removeGroup(key);
    }
    delete models[model.id];
  };

  originalCollection.each(add);

  listener.listenTo(originalCollection, 'add', add);
  listener.listenTo(originalCollection, 'change', change);
  listener.listenTo(originalCollection, 'remove', remove);

  return collection;
};