Lesson 8—Configuration with .eleventy.js

We can configure 11ty with `.eleventy.js` file.

In this section, we will learn:

What is .eleventy.js file?

.eleventy.js file is the configuration file for eleventy. It is a JavaScript file becasue Eleventy is built on JavaScript.

To get started configuring eleventy, First we need to create a JavaScript file named .eleventy.js under the root project folder.

module.exports = function(eleventyConfig) {

We will put all the configurations inside this function.

.eleventy.js is a JavaScript file becasue Eleventy is built on JavaScript.

Configuring addPassthroughCopy

One common usage is to copy the css folder and the images folder to the eleventy output.

By default, Eleventy will parse all the known file format into the corresponding output folder. That’s why our html and md files are handled automatically. But for the images and CSS files, Eleventy will by-pass the files until we explicitly tell Eleventy to copy the files. That’s why we have the following configuration.

module.exports = function(eleventyConfig) {

  // Folders to copy into output.

  // If we use Netlify and has the _redirects file.


Copying files from sub-directory to root folders

We can configure the pass-through copy to copy files into a different path. For example, the following code snippet tells 11ty to copy all files inside _misc_root_files folder into the root _site output folder.

eleventyConfig.addPassthroughCopy({ "_misc_root_files": "/" });

Adding filters to transform output rendering

Let’s say we have a collection of task data. We want to display the date and the title of the task. The date is a full date-time string which might be too long for display. We want the YYYY-MM-DD format. We can create a custom filter to transform the string via JavaScript.

module.exports = function(eleventyConfig) {

  eleventyConfig.addFilter("toISOString", function(date) {
    return date.toISOString().split('T')[0];


Here is how we use the toISOString when listing tasks.

<h1>All Tasks</h1>



In this lesson, we learnt that 11ty only process know file types. For example, .html and .md files. But for images and CSS files, 11ty won’t handle it by default and thus we won’t see those files in _site output folder.

addPassthroughCopy configuration function is designed for us to specific which folders or files we need 11ty to copy into the output folder.