Full Implementation

Table of contents, component:

attr :headers, :list

def table_of_contents(assigns) do
  ~H"""
  <div
    id="table-of-contents"
    class="sticky top-[calc(var(--header-height))] py-1 pr-5"
    data-file={__ENV__.file}
    data-line={__ENV__.line}
    phx-hook={Application.fetch_env!(:derpy_tools, :show_inspector?) && "SourceInspector"}
  >
    <h5
      id="toc"
      class="text-slate-900 font-semibold mb-2 text-sm leading-6 dark:text-slate-100"
      phx-hook="TableOfContents"
    >
      On this page
    </h5>
    <a
      href="#"
      class="block py-1 font-medium hover:text-slate-900 dark:text-slate-400 dark:hover:text-slate-300"
    >
      <i class="hero-chevron-up w-5.5 h-5.5 text-slate-500 dark:text-navy-100" /> Top
    </a>
     <.nested_header
      headers={@headers}
      class="max-h-[calc(100svh-(var(--header-height)))] overflow-auto"
    />
  </div>
  """
end

attr :id, :string, default: nil
attr :headers, :list
attr :class, :string, default: nil
attr :parent, :list, default: []

def nested_header(assigns) do
  ~H"""
  <ul id={@id} class={["space-y-1 font-inter font-medium list-none not-prose", @class]}>
    <li
      :for={%{header: header, id: id, title: title, children: children} <- @headers}
      class="not-prose"
    >
      <a
        id={"#{id}-link"}
        key={id}
        href={"##{id}"}
        tabindex="0"
        data-parent={@parent |> Enum.join(">")}
        class={[
          "block py-1 hover:text-slate-900 dark:text-slate-400 dark:hover:text-slate-300",
          case header do
            "h2" -> "font-semibold"
            "h3" -> "font-medium"
            "h4" -> "font-normal"
            "h5" -> "font-normal text-xs"
          end
        ]}
        phx-click={JS.toggle(to: "##{id}-container")}
      >
        <i :if={header in ~w{h3 h4 h5}} class="hero-chevron-right-mini" />
        <span><%= title %></span>
      </a>

      <.nested_header
        :if={children}
        id={"#{id}-container"}
        headers={children |> Enum.reverse()}
        class="pl-4 hidden"
        parent={[id, @parent |> Enum.join(">")]}
      />
    </li>
  </ul>
  """
end

Using the TOC component in Left Nav

Pass any function component to the parse_blog function, and it’ll work.

attr :post, :map
attr :class, :string, default: nil

def left_nav(assigns) do
  parsed_blog =
    __MODULE__
    |> apply(assigns.post.body, [assigns])
    |> parse_blog()

  assigns =
    assigns
    |> assign(
      headers: extract_headers(parsed_blog),
      reading_time: reading_time(parsed_blog)
    )

  ~H"""
  <aside class={["flex flex-col", @class]}>
    <span><%= @reading_time %></span>
    <.table_of_contents headers={@headers} />
  </aside>
  """
end

Parsing-related code in private functions:

defp parse_blog(function_component) do
  function_component.static
  |> Floki.parse_fragment!()
  |> Floki.find("article")
end

defp extract_headers([parsed_blog]) do
  parsed_blog
  |> Floki.children()
  |> Enum.filter(&match?({name, _, _} when name in ~w(h2 h3 h4 h5), &1))
  |> Enum.map(fn
    {header, meta, [{"a", _, [title | _]} | _rest]} ->
      {_, id} = Enum.find(meta, &match?({"id", _}, &1))
      {header, id, title |> String.trim()}

     {header, meta, [title | _rest]} ->
      {_, id} = Enum.find(meta, &match?({"id", _}, &1))
      {header, id, title |> String.trim()}
  end)
  |> Enum.reduce([], fn
    {"h2", id, title}, acc ->
      [%{header: "h2", id: id, title: title, children: []} | acc]

    {"h3", id, title}, acc ->
      children = path(0 / :children)

      Pathex.set!(acc, children, [
        %{header: "h3", id: id, title: title, children: []} | Pathex.get(acc, children)
      ])

    {"h4", id, title}, acc ->
      children = path(0 / :children / 0 / :children)

      Pathex.set!(acc, children, [
        %{header: "h4", id: id, title: title, children: []} | Pathex.get(acc, children)
      ])

    {"h5", id, title}, acc ->
      children = path(0 / :children / 0 / :children / 0 / :children)

      Pathex.set!(acc, children, [
        %{header: "h5", id: id, title: title, children: nil} | Pathex.get(acc, children)
      ])
  end)
  |> Enum.reverse()
end

defp reading_time(parsed_blog) do
  parsed_blog
  |> Floki.text()
  |> String.replace(~r/@|#|\$|%|&|\^|:|_|!|,/u, " ")
  |> String.split()
  |> Enum.count()
  |> div(@wpm)
  |> Timex.Duration.from_minutes()
  |> Timex.Format.Duration.Formatter.format(:humanized)
end

JS Hook

const TableOfContents = {
  mounted() {
    if (location.hash) {
      const hash = location.hash.replace("#", "");
      const header = document.getElementById(hash);

      header &&
        header.scrollIntoView({
          behavior: "instant",
          block: "start",
          inline: "end",
        });

      highlightNav(
        hash,
        [
          "hover:text-slate-900",
          "dark:text-slate-400",
          "dark:hover:text-slate-300",
        ],
        ["text-sky-500", "dark:text-sky-400"]
      );
    }
    window.addEventListener("hashchange", handleHashChange);
  },
  destroyed() {
    window.removeEventListener("hashchange", handleHashChange);
  },
};

function handleHashChange(event) {
  if (event.oldURL.includes("#")) {
    const hash = event.oldURL.split("#").pop();

    highlightNav(
      hash,
      ["text-sky-500", "dark:text-sky-400"],
      [
        "hover:text-slate-900",
        "dark:text-slate-400",
        "dark:hover:text-slate-300",
      ]
    );
  }

  if (location.hash) {
    const hash = location.hash.replace("#", "");

    highlightNav(
      hash,
      [
        "hover:text-slate-900",
        "dark:text-slate-400",
        "dark:hover:text-slate-300",
      ],
      ["text-sky-500", "dark:text-sky-400"]
    );
  }
}

function highlightNav(hash, remove, add) {
  const nav = document.getElementById(`${hash}-link`);

  if (nav) {
    nav.classList.remove(...remove);
    nav.classList.add(...add);

    const { parent } = nav.dataset;

    if (parent) {
      parent.split(">").forEach((parentId) => {
        const parent = document.getElementById(`${parentId}-link`);

        if (parent) {
          parent.classList.remove(...remove);
          parent.classList.add(...add);
        }

        const parentContainer = document.getElementById(
          `${parentId}-container`
        );

        if (parentContainer) {
          parentContainer.classList.remove("hidden");
          if (location.hash == `#${hash}`) parentContainer.style.display = null;
        }
      });
    }
  }
}

export default TableOfContents;

Example headers, with self linking.

<h2 id="installation" class="group flex whitespace-nowrap not-prose">
    <a href="#installation" class="relative flex items-center">
      Installation
      <span class="absolute -ml-8 opacity-0 group-hover:opacity-100 transition-opacity duration-500 group-focus:opacity-100 flex h-6 w-6 items-center justify-center rounded-md text-slate-400 shadow-sm ring-1 ring-slate-900/5 hover:text-slate-700 hover:shadow hover:ring-slate-900/10 dark:bg-slate-700 dark:text-slate-300 dark:shadow-none dark:ring-0">
        <svg width="12" height="12" fill="none" aria-hidden="true">
          <path
            d="M3.75 1v10M8.25 1v10M1 3.75h10M1 8.25h10"
            stroke="currentColor"
            stroke-width="1.5"
            stroke-linecap="round"
          >
          </path>
        </svg>
      </span>
    </a>
  </h2>
  <h3 id="linux">Linux</h3>
  <h4 id="ubuntu" class="group whitespace-nowrap not-prose">
    <a href="#ubuntu" class="relative flex items-center">
      Ubuntu
      <span class="absolute -ml-8 opacity-0 group-hover:opacity-100 transition-opacity duration-500 group-focus:opacity-100 flex h-6 w-6 items-center justify-center rounded-md text-slate-400 shadow-sm ring-1 ring-slate-900/5 hover:text-slate-700 hover:shadow hover:ring-slate-900/10 dark:bg-slate-700 dark:text-slate-300 dark:shadow-none dark:ring-0">
        <svg width="12" height="12" fill="none" aria-hidden="true">
          <path
            d="M3.75 1v10M8.25 1v10M1 3.75h10M1 8.25h10"
            stroke="currentColor"
            stroke-width="1.5"
            stroke-linecap="round"
          >
          </path>
        </svg>
      </span>
    </a>
  </h4>

Sorry about the wall of text, but I don’t know how to string some nice description for the code.
Perhaps I’ll write a blog post about it.

Features:

1. The nav stays selected, even on page refresh.
2. Only the highlight part is JS, rest is happening on the Elixir side.
3. Even the parent nav-links, light/open up, when it’s child is selected.
   …

Rest you can figure out the features and kinks.

Thanks everyone.

P.S. I left the reading time estimator in the code, in case someone finds better way to do that thing.

I would need to put a little more thought into the nesting part, but for now the stuff before it could be cleaned up a bit.

|> Enum.filter(&is_tuple/1)
|> Enum.filter(fn each -> Tuple.to_list(each) |> Enum.count() == 3 end)
|> Enum.filter(fn {name, _, _} -> name in ~w(h2 h3 h4) end)

Whenever you see type checking and size checks in loops that can often be consolidated with pattern matching. The following should be equivalent:

|> Enum.filter(&match?({name, _, _} when name in ~w(h2 h3 h4), &1))

But I think for would do better here and you can even include the following map too! You’d have to break the pipe, though I think that’s a good thing in this situation. This is how I would probably write everything before the big reduce:

[menu] =
  function_component.static
  |> Floki.parse_fragment!()
  |> Floki.find("article")

menu_items = Floki.children(menu)

for {header, meta, [title | _]} <- menu_items,
    header in ~w(h2 h3 h4) do
  {
    header,
    meta |> Enum.find(&(elem(&1, 0) == "id")) |> elem(1), # I can't mentally parse what this is doing atm
    String.trim(title) # Pretty sure `trim` is what you want here
  }
end

Sorry I can’t provide help on the nesting part as I haven’t done anything like that in a bit. I’m curious, though, and hopefully someone else can. I would actually probably write something like that just because it’s simple and explicit (since there will never be more than 3 levels), I would just extract some variables so it’s not so jarring to read.

Good job on the component—it looks really nice!

Oh it represents me too, lol! And then I’ve worked with people who would go so far as to add extra overhead just to make pipelines work. Personally, I actually prefer functional pipelines if they are clear, but comprehensions can often read much better, so I don’t shy away from them. I’ve said this recently already, but I really feel that Elixir provides such a nice succinct set of builtin constructs that there is no reason not to get to know all of them and use each where appropriate.