edx-platform/cms/static/js/collections/chapter.js

define(['backbone', 'js/models/chapter'], function(Backbone, ChapterModel) {
    var ChapterCollection = Backbone.Collection.extend({
        model: ChapterModel,
        comparator: 'order',
        nextOrder: function() {
            if (!this.length) { return 1; }
            return this.last().get('order') + 1;
        },
        isEmpty: function() {
            return this.length === 0 || this.every(function(m) { return m.isEmpty(); });
        }
    });
    return ChapterCollection;
});