Docs for 11ty/eleventy#2536

src/docs/filters.md

@@ -70,6 +70,9 @@ module.exports = function(eleventyConfig) {
 
   // or, use a Universal filter (an alias for all of the above)
   eleventyConfig.addFilter("makeUppercase", function(value) { … });
+
+  // New in 2.0.0-canary.15
+  eleventyConfig.addAsyncFilter("makeUppercase", async function(value) { … });
 };
 ```
 
@@ -97,6 +100,33 @@ module.exports = function(eleventyConfig) {
 };
 ```
 
+### Asynchronous Universal Filters {% addedin "2.0.0-canary.15" %}
+
+Eleventy has added a new universal filter API for asynchronous filters and extended the currently available `addFilter` method to be async-friendly. _Note that even though Handlebars is used for synchronous filters in `addFilter`, it is excluded from asynchronous filters because Handlebars is not async-friendly._
+
+If you are not yet on Eleventy 2.0, you can still add asynchronous filters to each async-friendly template language individually: [Liquid `addLiquidFilter`](/docs/languages/liquid/#filters), [Nunjucks `addNunjucksAsyncFilter`](/docs/languages/nunjucks/#asynchronous-nunjucks-filters), and [JavaScript `addJavaScriptFunction`](/docs/languages/javascript/#asynchronous-javascript-template-functions).
+
+{% codetitle ".eleventy.js" %}
+
+```js
+module.exports = function(eleventyConfig) {
+  // Async universal filters add to:
+  // * Liquid
+  // * Nunjucks
+  // * JavaScript
+
+  eleventyConfig.addFilter("myFilter", async function(value) {
+    // do some Async work
+    return value;
+  });
+
+  eleventyConfig.addAsyncFilter("myFilter", async function(value) {
+    // do some Async work
+    return value;
+  });
+};
+```
+
 ### Eleventy Provided Universal Filters
 
 We also provide a few universal filters, built-in:

src/docs/languages/liquid.md

@@ -92,11 +92,16 @@ Filters are used to transform or modify content. You can add Liquid specific fil
 
 Read more about [LiquidJS Filter syntax](https://liquidjs.com/tutorials/register-filters-tags.html)
 
+Note that Liquid supports asynchronous filters out of the box (without any additional code or API method changes).
+
 ```js
 module.exports = function(eleventyConfig) {
   // Liquid Filter
   eleventyConfig.addLiquidFilter("myLiquidFilter", function(myVariable) { … });
 
+  // Async-friendly too
+  eleventyConfig.addLiquidFilter("myAsyncLiquidFilter", async function(myVariable) { … });
+
   // Universal filters (Adds to Liquid, Nunjucks, and Handlebars)
   eleventyConfig.addFilter("myFilter", function(myVariable) { … });
 };