Anchor headers and code lines in Jekyll

The default Jekyll toolbox (Jekyll, kramdown and rouge) doesn’t provide with a configuration option to add anchors to headers and code blocks.

The best way I found of doing this is by creating a simple Jekyll plugin, more specifically, a Jekyll hook. These allow you to jump in to the Jekyll build and add a processing stage before of after Jekyll performs something.

All you have to do is add the code to _plugins/my-jekyll-plugin-code.rb, and Jekyll knows to pick it up and call your code on the appropriate time.

Anchor on headers

Since I wanted to add anchors to headers in all documents, this Jekyll hook works on :documents after they have been transformed into HTML, the :post_render phase:

Jekyll::Hooks.register :documents, :post_render do |doc|
  if doc.output_ext == ".html"
    doc.output =
      doc.output.gsub(
        /<h([1-6])(.*?)id="([\w-]+)"(.*?)>(.*?)<\/h[1-6]>/,
        '<a href="#\3"><h\1\2id="\3"\4>\5</h\1></a>'
      )
  end
end

I’ve derived my implementations from two “official”¹ hooks, jemoji and jekyll-mentions.

All I did was to wrap the header tag inside an <a>, and set the href of that <a> to the existing id of the header. Before the hook the HTML looks like:

...some unmodified text...
<h2 id="my-header">
  My header
</h2>
...more unmodified text...

And after the hook should turn that into:

...some unmodified text...
<a href="#my-header">
  <h2 id="my-header">
    My header
  </h2>
</a>
...more unmodified text...

The used regexp tries to match only h1-h6 tags, and keep the rest of the HTML attributes untouched, since this isn’t a general HTML parser, but the generated HTML is somewhat under your control. Use at your own risk because you shouldn’t parse HTML with regexps. Also I used this strategy in my environment, where no other plugins are installed. I haven’t considered how this approach may conflict with other Jekyll plugins.

In the new anchor tag you can add your custom CSS class to style it as you wish.

Anchor on code blocks

Adding anchors to code blocks needs a little bit of extra work, because line numbers themselves don’t have preexisting ids, so we need to generate them without duplications between multiple code blocks in the same page.

Similarly, this Jekyll hook also works on :documents in the :post_render phase:

PREFIX = '<pre class="lineno">'
POSTFIX = '</pre>'
Jekyll::Hooks.register :documents, :post_render do |doc|
  if doc.output_ext == ".html"
    code_block_counter = 1
    doc.output = doc.output.gsub(/<pre class="lineno">[\n0-9]+<\/pre>/) do |match|
      line_numbers = match
                      .gsub(/<pre class="lineno">([\n0-9]+)<\/pre>/, '\1')
                      .split("\n")

      anchored_line_numbers_array = line_numbers.map do |n|
        id = "B#{code_block_counter}-L#{n}"
        "<a id=\"#{id}\" href=\"##{id}\">#{n}</a>"
      end
      code_block_counter += 1

      PREFIX + anchored_line_numbers_array.join("\n") + POSTFIX
    end
  end
end

This solution assumes the default Jekyll toolbox with code line numbers turned on in _config.yml:

kramdown:
  syntax_highlighter_opts:
    span:
      line_numbers: false
    block:
      line_numbers: true

The anchors go from B1-L1 to BN-LN, using the code_block_counter to track which code block we’re in and don’t duplicate anchor ids. Before the hook the HTML looks like:

...some unmodified text...
<pre class="lineno">1
2
3
4
5
</pre>
...more unmodified text...

And after the hook should turn that into:

...some unmodified text...
<pre class="lineno"><a id="B1-L1" href="#B1-L1">1</a>
<a id="B1-L2" href="#B1-L2">2</a>
<a id="B1-L3" href="#B1-L3">3</a>
<a id="B1-L4" href="#B1-L4">4</a>
<a id="B1-L5" href="#B1-L5">5</a></pre>
...more unmodified text...

Happy writing :)