Use an Eleventy filter as an if statement

18 Apr 2021 in TIL

If you find yourself writing the same `if` statement multiple times, add it to `.eleventy.js` as a filter and keep a single source of truth

On this blog, til is a special category that I wanted to render differently. It has it's own template and does not follow the standard /category/:slug URL pattern. It lives at /til instead.

This meant that anywhere I output a category link (e.g. in the post sidebar on the right or on a topic page) I needed to write the same if statement repeatedly.

Instead of duplicating the logic multiple times, I extracted it to my .eleventy.js config file as a filter:

eleventyConfig.addShortcode("category_url", async function (category) {
const slug = slugify(category, { lower: true });
if (slug === "til") {
return "/til";
return "/category/" + slug;

This means that anywhere I need to output a category link, I can now call this filter:

<a href="{% category_url %}" class="meta-tag">

If you find yourself writing the same logic repeatedly, adding it as a filter is a great way to DRY your templates.