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
- 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
If you would like to use this component in your project, you can install it from npm
npm i toc-observer-componentor use it from UNPKG
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);
}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.
| 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 |
Install dependencies:
npm i<toc-observer> uses the TypeScript compiler to produce JavaScript that runs in modern browsers.
To build the JavaScript version of your component:
npm run buildTo watch files and rebuild when the files are modified, run the following command in a separate shell:
npm run build:watchBoth the TypeScript compiler and lit-analyzer are configured to be very strict. You may want to change tsconfig.json to make them less strict.
<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 serveThere 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.
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 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 lintPrettier 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.
To lint the project run:
npm run formatThis component doesn't include any build-time optimizations like bundling or minification. We recommend publishing components as unoptimized JavaScript modules, and performing build-time optimizations at the application level. This gives build tools the best chance to deduplicate code, remove dead code, and so on.
For information on building application projects that include LitElement components, see Build for production on the Lit site.
- Get started on the lit.dev site for more information.
- Open Web Component scaffold generators (an alternative for this starter project)
- Lit & Friends Slack
- Smooth Scrolling Sticky ScrollSpy Navigation by Bramus Van Damme