Use mermaid with code fences

Use mermaid with code fences


The Hugo theme Learn supports mermaid diagrams out of the box by using a custom shortcode. However, it’s not possible to use code fences syntax like

// Graph definition

When using Gitlab, this poses a problem. Gitlab requires the backticks for defining code blocks, which means that mermaid diagrams will not be rendered in Gitlab when using the shortcode provided by the theme.


When using code fences syntax for mermaid code blocks, you’ll notice that Hugo turns this into <code class="language-mermaid">...</code>. Turns out it’s possible to tell mermaid which classes to look for when searching for graph definitions.

Simply add this at the end of your body:

{{ $mermaid := resources.Get "/path/to/mermaid.min.js" }}
<script src="{{ $mermaid.RelPermalink }}"></script>
    window.onload = function() {
        mermaid.init(undefined, ".language-mermaid");

This configures mermaid to search for nodes with class “language-mermaid”. These will be rendered as diagrams. Gitlab will render the diagram correctly, too.

Granted, this solution is far from perfect (the <code> tag is wrapped in two other tags defining settings which are specific to the highlighting theme in use), but it works well with both Hugo and Gitlab.


Thanks for your tips! My approach reflects the one shown here:

Do you think it’s possible to implement your suggestions and still be in accordance with what the mermaid documentation says? I’m no JS expert, so it’s hard for me to tell.