So you're publishing a long document online and don't have an easy mechanism to automatically add a table of contents on the server side? Well with JavaScript, you enslave the web browser to do it instead! This TOC generator...

needs no jQuery or other third-party library
creates links to each heading, adding an id attribute to each heading that doesn't already have one
can support pages that contain multiple content areas with multiple unrelated tables of contents

Usage

After installing the code below, call addTOC(contentElement, before, tocClass), where

contentElement is an element that contains the document or blog post for which you want a table of contents, and
before is an immediate child of contentElement; the table of contents will be inserted as a child of contentElement, just before before. before is optional and if unspecified, contentElement.firstChild is used.
tocClass is the CSS class of the table-of-contents blog (default value: "sidebox")

See example below.

The Code

Insert this code at the bottom of any page in which you want to add a TOC (before </body>).

<script>// Add table of contents! <![CDATA[
function $get(selector) { return document.querySelector(selector); };
function $all(selector) {
      return Array.prototype.slice.call(document.querySelectorAll(selector));
}

function buildTOC_ul(selector) {
      var levels=[document.createElement("ul"),null,null];
      levels[0].style
      var lvl=0, c=0;
      if (!selector) selector = "h2, h3, h4";
      $all(selector).forEach(function(el) {
            if (!el.id) el.id='section_'+ ++c;
            var newLvl=(el.tagName=="H2"?0:el.tagName=="H3"?1:2);
            for (;lvl<newLvl;lvl++)
                 levels[lvl].appendChild(levels[lvl+1]=document.createElement("ul"));
            lvl=newLvl;

            var li=document.createElement('li');
            li.innerHTML="<a href='#"+el.id+"'></a>";
            li.firstChild.innerHTML=el.innerHTML;
            levels[lvl].appendChild(li);
      });
      return levels[0];
}
function addTOC(contentElement, before, tocClass) {
      if (before===undefined) before=contentElement.firstChild;
      var prefix = "";
        var c = contentElement.className;
        if (c) prefix="." + 
        (c.indexOf(" ")==-1?c:c.substr(0,c.indexOf(" "))) + " ";
      var selector = prefix+"h2, "+prefix+"h3, "+prefix+"h4";
      var toc=document.createElement("div");
      toc.className=tocClass||"sidebox";
      toc.appendChild(document.createTextNode("Contents"));
      toc.appendChild(buildTOC_ul(selector));
      contentElement.insertBefore(toc, before);
}
// =========================
// TODO: CALL addTOC() HERE!
// =========================
//]]>
</script>

Example

The simplest thing you can do is to call addTOC(document.body), but that only works on very simple pages. Generally the page has a main content area; if you assign an id="content" to this content area, you can add the table of contents at the top like this:

addTOC($get("#content"));

(See definition of $get in the code above.) Here's how I'm calling addToc() on my blog:

var _post_ = $get("#post") || $get("#content");
addTOC(_post_, _post_.firstChild.nextSibling.nextSibling);

This allows the content area to have _post_.firstChild.nextSibling.nextSibling skip past the initial heading so that the table of contents is inserted underneath the heading.

Sidebar Style

The default class is sidebox, and if you insert the following CSS on your page, you will get a table of contents that floats beside the content, just as you see on this page.

<style>
.sidebox {
  border: 1px dotted rgb(127, 127, 127);
  padding: 4px 3px 4px 6px; /* top right bottom left */
  min-width: 100px ! important;
  float: right ! important;
  font-size: 90%;
  margin-top: 1px;
  margin-bottom: 1px;
  margin-left: 6px;
  visibility: visible;
  max-width: 50%;
  width: 35%;
}
.sidebox ul { padding:0em 0em 0em 1.3em; margin:0em; }//TRBL
</style>

Usage with GitHub Pages / Jekyll

If you're publishing with Jekyll, I suggest adding the above code to the bottom of /_layouts/default.html (and any other layouts you might use that might need a TOC, as long as they do not import default.html using a layout: default option), just before </body>, surrounded by a test like this:

{% if page.toc %}
<script>
...
...
var _post_ = $get("#post")||$get("#content");
addTOC(_post_, _post_.firstChild.nextSibling.nextSibling);
</script>
{% endif %}

This way, Jekyll only adds the TOC code to a page if the front-matter contains a toc: true option.

Original article published here.

Building a Table of Contents in JavaScript