Tiny Table-of-Contents JS Snippet

const toc = document.getElementById('table-of-contents');
for (const x of document.querySelectorAll('h2, h3')) {
  const id = x.innerText.replaceAll(/[^a-z0-9]/gi,'');
  x.id = id;
  const item = `<li><a href="#${id}">${x.innerText}</a></li>`;
  switch (x.tagName.toLowerCase()) {
    case "h2": toc.insertAdjacentHTML('beforeend', `${item}<ul></ul>`); break;
    case "h3": [...toc.querySelectorAll('ul')]?.pop()?.insertAdjacentHTML('beforeend', item); break;
  }
}

Example

The following markdown…

# Title
lorem ipsum

<ul id="table-of-contents"></ul>

## Chapter 1
lorem ipssum

### Section A
lorem ipssum

## Chapter 2
lorem ipssum

### Section B
lorem ipssum

### Section C
lorem ipssum

…produces this HTML…

<h1>Title</h1>
<p>lorem ipsum</p>
<ul id="table-of-contents"></ul>
<h2>Chapter 1</h2>
<p>lorem ipsum</p>
<h3>Section A</h3>
<p>lorem ipsum</p>
<h2>Chapter 2</h2>
<p>lorem ipsum</p>
<h3>Section B</h3>
<p>lorem ipsum</p>
<h3>Section C</h3>
<p>lorem ipsum</p>

…and JS transforms it on page load.

<h1>Title</h1>
<p>lorem ipsum</p>
<ul id="table-of-contents">
  <li><a href="#chapter1">Chapter 1</a></li>
  <ul>
    <li><a href="#sectiona">Section A</a></li>
  </ul>
  <li><a href="#chapter2">Chapter 2</a></li>
  <ul>
    <li><a href="#sectionb">Section B</a></li>
    <li><a href="#sectionc">Section C</a></li>
  </ul>
</ul>
<h2 id="chapter1">Chapter 1</h2>
<p>lorem ipsum</p>
<h3 id="sectiona">Section A</h3>
<p>lorem ipsum</p>
<h2 id="chapter2">Chapter 2</h2>
<p>lorem ipsum</p>
<h3 id="sectionb">Section B</h3>
<p>lorem ipsum</p>
<h3 id="sectionc">Section C</h3>
<p>lorem ipsum</p>