Accessing 11ty filters within data files to keep your code DRY

I spent a good half-hour searching for a solution to my problem the other night, only to realise it was a Javascript solution, not an 11ty problem.

My website contains several filters which allow me to process data (e.g. slugify).

These allow me to process data in my 11ty templates, layouts and content.

For example, with the slugify filter and the Nunjucks templating language, I can do

{{ entry.title | slugify }}

If entry.title was "This is a Title!", then it would become "this-is-a-title".

The problem I was having is I wanted to access the slugify filter within an 11ty global data file.

I originally had the function repeated at the top of my data file, but figured there must be a better way.

Step one: External filters

When setting up your filter, make sure you put your function in an "external" Javascript file. I like to put mine in a filters folder, next to my layouts and partials.

Then, in your .eleventy.js file, you can include the filter:

module.exports = function (config) {
	// other stuff

	config.addFilter('slugify', require('./app/filters/slugify.js'));

	return {
		// ...

Inside the slugify.js file, make sure you module.exports the function. For example, my slugify file looks like:

module.exports = function(str) {
	str = str.replace(/^\s+|\s+$/g, ''); // trim
	str = str.toLowerCase();

	// remove accents, swap ñ for n, etc
	var from = "àáäâèéëêìíïîòóöôùúüûñç·/_,:;";
	var to   = "aaaaeeeeiiiioooouuuunc------";
	for (var i = 0, l = from.length ; i<l ; i++) {
		str = str.replace(new RegExp(from.charAt(i), 'g'), to.charAt(i));

	str = str.replace(/[^a-z0-9 -]/g, '') // remove invalid chars
		.replace(/\s+/g, '-') // collapse whitespace and replace by -
		.replace(/-+/g, '-'); // collapse dashes

	return str;

Step two: Include the file

With your filter separated out from your 11ty config file, you can now include the file wherever you wish. My use-case was a data file - and it could be used like the following (make sure you update the require path)

const slugify = require('./../filters/slugify');

module.exports = function() {
	// ...
	return {
		title: "This is the title",
		slug: slugify("This is the title")

You can see this in action in the source code for my side project, Ale House Rock.

View this post on Github

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Mike Street

Written by Mike Street

Mike is a CTO and Lead Developer from Brighton, UK. He spends his time writing, cycling and coding. You can find Mike on Mastodon.