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Various template engines can be extended with custom filters to modify content. Here’s an example:

Syntax Nunjucks or Liquid
<h1>{{ name | makeUppercase }}</h1>
Syntax Handlebars
<h1>{{ makeUppercase name }}</h1>
Syntax JavaScript
New in v0.7.0
module.exports = function({name}) {
return `<h1>${this.makeUppercase(name)}</h1>`;

These can be added using the Configuration API. Here are a few examples:

Filename .eleventy.js
module.exports = function(eleventyConfig) {
// Liquid Filter
eleventyConfig.addLiquidFilter("makeUppercase", function(value) {});

// Nunjucks Filter
eleventyConfig.addNunjucksFilter("makeUppercase", function(value) {});

// Handlebars Filter
eleventyConfig.addHandlebarsHelper("makeUppercase", function(value) {});

// JavaScript Template Function (New in 0.7.0)
eleventyConfig.addJavaScriptFunction("makeUppercase", function(value) {});

// or, use a Universal filter (an alias for all of the above)
eleventyConfig.addFilter("makeUppercase", function(value) {});

Read more about filters on the individual Template Language documentation pages:

Universal Filters

Universal filters can be added in a single place and are available to multiple template engines, simultaneously. This is currently supported in JavaScript (New in 0.7.0), Nunjucks, Liquid, and Handlebars.

Filename .eleventy.js
module.exports = function(eleventyConfig) {
// Universal filters adds to:
// * Liquid
// * Nunjucks
// * Handlebars
// * JavaScript (New in 0.7.0)

eleventyConfig.addFilter("myFilter", function(value) {
return value;

Eleventy Provided Universal Filters

We also provide a few universal filters, built-in:

Related Docs