https://hlyani.github.io/gitbook/gitbook-plugin-expandable-chapters/expandable-chapters.js

1require(['gitbook', 'jQuery'], function(gitbook, $) {
2  var TOGGLE_CLASSNAME = 'expanded',
3      CHAPTER = '.chapter',
4      ARTICLES = '.articles',
5      TRIGGER_TEMPLATE = '<i class="exc-trigger fa"></i>',
6      LS_NAMESPACE = 'expChapters';
7  var init = function () {
8    // adding the trigger element to each ARTICLES parent and binding the event
9    $(ARTICLES)
10      .parent(CHAPTER)
11      .children('a,span')
12      .append(TRIGGER_TEMPLATE)
13      .on('click', function(e) {
14        if (!$(e.target).is('a')) {
15          e.preventDefault();
16          e.stopPropagation();
17          toggle($(e.target).closest(CHAPTER));
18        }
19      });
20
21    expand(lsItem());
22    //expand current selected chapter with it's parents
23    var activeChapter = $(CHAPTER + '.active');
24    expand(activeChapter);
25    expand(activeChapter.parents(CHAPTER));
26
27
28  } 
29  var toggle = function ($chapter) {
30    if ($chapter.hasClass('expanded')) {
31      collapse($chapter);
32    } else {
33      expand($chapter);
34    }
35  }
36  var collapse = function ($chapter) {
37    if ($chapter.length && $chapter.hasClass(TOGGLE_CLASSNAME)) {
38      $chapter.removeClass(TOGGLE_CLASSNAME);
39      lsItem($chapter);
40    }
41  }
42  var expand = function ($chapter) {
43    if ($chapter.length && !$chapter.hasClass(TOGGLE_CLASSNAME)) {
44      $chapter.addClass(TOGGLE_CLASSNAME);
45      lsItem($chapter);
46    }
47  }
48  var lsItem = function () {
49    var map = JSON.parse(localStorage.getItem(LS_NAMESPACE)) || {}
50    if (arguments.length) {
51      var $chapters = arguments[0];
52      $chapters.each(function (index, element) {
53        var level = $(this).data('level');
54        var value = $(this).hasClass(TOGGLE_CLASSNAME);
55        map[level] = value;
56      })
57      localStorage.setItem(LS_NAMESPACE, JSON.stringify(map));
58    } else {
59      return $(CHAPTER).map(function(index, element){
60        if (map[$(this).data('level')]) {
61          return this;
62        }
63      })
64    }
65  }
66  gitbook.events.bind('page.change', function() {
67    init()
68  }); 
69});