`<toc-observer>` highlight links in a table of contents

About

A table of contents (TOC) is a list of chapters, section titles and similiar. On a website, a blog, these are usually links to the corresponding sections. This web component takes any HTML and tries to highlight links inside it based on the visibile sections. Even if any kind of HTML would work, you should probably use a list.

toc-observer-60.mp4

Features

Configurable through attributes
Framework / platform agnostic, use it wherever you like as long as it has a DOM and JS enabled
Works with "unusual" markup structure as seen below
Usage of IntersectionObserver for performant DOM observation

Installation

If you would like to use this component in your project, you can install it from npm

npm i toc-observer-component

or use it from UNPKG

Usage

Your markup should resemble something like the following, note the slot attribute and how links only contain an ID to the corresponding element you would want to highlight.

<toc-observer>
  <!-- The "toc" slot and its name is mandatory -->
  <ul slot="toc">
    <li>
      <!-- Links must begin with a hash ('#') -->
      <a href="#possums" class="toc-item">Possums</a>
      <ul>
        <li><a href="#diet" class="toc-item">Diet</a></li>
        <li><a href="#reproduction" class="toc-item">Reproduction</a></li>
      </ul>
    </li>
  </ul>
</toc-observer>

Styling is up to you but you can override the tocActiveClass attribute or just style the default which would be something like this:

toc-observer a.toc-active {
  color: var(--toc-color-active);
  border-left-color: var(--toc-color-active);
}

/* Not yet supported everywhere but cool: */
toc-observer a:has(+ ul li > .toc-active) {
  border-left: 2px solid var(--toc-color-active);
}

Observing parent elements

Most blogs or documentation pages are based on the following markup so that only visible headings are highlighted. However, it can also be interesting to highlight all visible sections instead of just headings - because that is what is currently visible on the screen. In a case like this, where it is not possible to apply IDs to these sections, the following configuration helps.

<toc-observer observeParent>
  <!-- 
    ^set parentSelector="my-selector" 
    if it differs from 'section' 
  -->
  
  <!-- Previous markup -->
<toc-observer>

<!-- Content -->
<section>
  <h2 id="possums">Possums</h2>
  <p>Opossum are immune to rabies (...)</p>
</section>

<section>
  <h3 id="diet">Diet</h3>
</section>

<section>
  <h3 id="reproduction">Reproduction</h3>
</section>

The idea of using sections or parent elements as a reference of observable items stems from this post by Bramus Van Damme - which I think is what you want from such a component.

Attributes

Name	Required	Default	Description
`tocActiveClass`	No	`toc-active`	CSS class which is added to / removed from a TOC link
`rootElement`	No	`null`	The intersection element for your content.
`rootMargin`	No	`0px`	Bounding box inside `rootElement`
`observeParent`	If `parentSelector` is set	`false`	Useful to watch intersecting wrapper elements
`parentSelector`	No	`section`	Specifiy the wrapper element that should be selected

Setup

Install dependencies:

npm i

Build

<toc-observer> uses the TypeScript compiler to produce JavaScript that runs in modern browsers.

To build the JavaScript version of your component:

npm run build

To watch files and rebuild when the files are modified, run the following command in a separate shell:

npm run build:watch

Both the TypeScript compiler and lit-analyzer are configured to be very strict. You may want to change tsconfig.json to make them less strict.

Dev Server

<toc-observer> uses modern-web.dev's @web/dev-server for previewing the project without additional build steps. Web Dev Server handles resolving Node-style "bare" import specifiers, which aren't supported in browsers. It also automatically transpiles JavaScript and adds polyfills to support older browsers. See modern-web.dev's Web Dev Server documentation for more information.

To run the dev server and open the project in a new browser tab:

npm run serve

There is a development HTML file located at /dev/index.html & /dev/index-headings.html that you can view at http://localhost:8000/dev/index.html or http://localhost:8000/dev/index-headings.html. Note that this command will serve your code using Lit's development mode (with more verbose errors). To serve your code against Lit's production mode, use npm run serve:prod.

Editing

If you use VS Code, it is highly recommend the lit-plugin extension, which enables some extremely useful features for lit-html templates:

Syntax highlighting
Type-checking
Code completion
Hover-over docs
Jump to definition
Linting
Quick Fixes

The project is setup to recommend lit-plugin to VS Code users if they don't already have it installed.

Linting

Linting of TypeScript files is provided by ESLint and TypeScript ESLint. In addition, lit-analyzer is used to type-check and lint lit-html templates with the same engine and rules as lit-plugin.

The rules are mostly the recommended rules from each project, but some have been turned off to make LitElement usage easier. The recommended rules are pretty strict, so you may want to relax them by editing .eslintrc.json and tsconfig.json.

To lint the project run:

npm run lint

Formatting

Prettier is used for code formatting. It has been pre-configured according to the Lit's style. You can change this in .prettierrc.json.

Prettier has not been configured to run when committing files, but this can be added with Husky and and pretty-quick. See the prettier.io site for instructions.