How to use KaTeX with Eleventy

When building this blog using Eleventy, I had some equations that I wanted to have rendered as LaTeX. I wrote them directly into the markdown of the posts, surrounded by two dollar signs:

One equation is $$e = mc^2$$.

I found some ways to do this online that involved extending the markdown renderer, but I honestly really didn’t understand them. Finally, I just pulled together my own solution using Eleventy’s filters to modify content.

First, go to the layout for your blog posts, and pipe the page’s contents through a latex filter that we’ll create in a moment:

{# before: #}
{{ content | safe }}
{# after: #}
{{ content | latex | safe }}

Now, we have to create that latex filter. First, install the KaTeX package to render math equations:

npm install katex

and import it in your .eleventy.js file:

const katex = require("katex");

Now, we can write the latex filter in your .eleventy.js file:

eleventyConfig.addFilter("latex", (content) => {
  return content.replace(/\$\$(.+?)\$\$/g, (_, equation) => {
    const cleanEquation = equation.replace(/&lt;/g, "<").replace(/&gt;/g, ">");

    return katex.renderToString(cleanEquation, { throwOnError: false });
  });
});

What this does is it registers a new Eleventy filter called latex, which will affect the content of our page.

We take the content of the page and use a regex to replace every occurrence of $$something$$. We’re using \$ to escape the dollar sign, because $ has a special meaning in regex but we want the actual dollar sign character (not its special meaning).

When rendering markdown to HTML, Eleventy likes to change characters like > to >, etc. This stops those characters from rendering as actual HTML. However, here we want to turn these characters back into what they were before, since we might’ve used the > or < characters in our equations.

We use KaTeX’s renderToString method to render this equation so it looks like an actual equation, and replace the $$something$$ with that rendered KaTeX HTML.

Finally, add this CSS file to your layout’s <head>. It loads the necessary fonts and CSS to display the equations.

<link
  rel="stylesheet"
  href="https://unpkg.com/katex@latest/dist/katex.min.css"
/>

And that’s it! Now, any LaTeX written in your markdown in the format $$equation here$$ will be beautifully rendered on the page.