Collecting my attempts to improve at tech, art, and life

Processing Sass in Eleventy

Tags: eleventy scss css site

relevant pictures are overrated; this dog and cat are cute together

Eleventy’s 1.0 release includes the ability to add your own custom processing based on file extension. Unsurprisingly, I love this feature.

Lots of big ideas, but let’s start small with the officially documented process for processing Sass.

First things first, let’s update devDependencies in my package.json:

  "devDependencies": {
    "@11ty/eleventy": "^1.0.0"

I keep my Sass files under src/assets/style:

$ tree src/assets/style/
├── _base.scss
├── _layout.scss
├── main.scss
└── modules
    └── _shiki.scss

1 directory, 4 files


Generally, I have been following the site organization guidelines described in Structuring Eleventy Projects. If I have some non-content file I need transformed — Sass, images, whatever — I put it under src/assets.

Eleventy provides instructions on how to set up custom templates for dealing with Sass, and even skipping a template so it doesn’t build _base.scss and so on.

And that’s great. It works. src/assets/style/main.scss becomes dist/assets/style/main.css. Course, it’s not quite perfect. I still need main.css rebuild if I write to _base.scss.

But I also don’t want it randomly rebuilding the stylesheet 20 times because it found 20 support files.

I need to think my way through this one.

Add a SassHandler

Keeping with Jérôme Coupé’s structural suggestions, I keep my more complex JavaScript logic under src/_11ty/.

$ tree src/_11ty/
├── collections
├── filters
├── handlers
│   ├── MarkdownHandler.js
│   └── SassHandler.js
└── shortcodes

handlers is my own variation — that’s where I put code for particular file and content types.

So what needs to go into SassHandler.js? Let’s see.

Time to open up the old text editor.

//- file:src/_11ty/handlers/SassHandler.js
// Handler for my Sass stylesheets

// ==> Import libraries.
// ==> Define input and output paths for sass.
// ==> Set build time guard variables.

module.exports = {
  outputFileExtension: "css",
  compileOptions: {
    permalink: false,
  compile: async function(inputContent, inputPath) {
    // ==> Ensure we need to build the CSS.
    // ==> Remember and log this build.

    return async (data) => {
      // ==> Compile the Sass and write the CSS.

With what I’ve figured out so far, even though Eleventy can watch and act on the files for me, keeping the paths under tight control will be my job. A change in _base.scss means a change in main.scss which means a write to main.css. I have not yet learned how to do that in any Eleventy-specific way. So I’ll disable the permalink compile option and handle it myself with the appropriate libraries.

//- Import libraries
const fs = require("fs-plus");
const path = require("path");
const sass = require("sass");

I know exactly which files I want to read and write. Let’s define those as constant.

//- Define input and output paths for sass
const curDir = process.cwd();
const sassInputPath = path.join(cirDir, "src/assets/style/main.scss");
const cssOutputPath = path.join(curDir, "dist/assets/style/main.css");

Yes this does look a lot like what Eleventy would do on its own. Since I turned off permalinks for scss files, I need to be careful. I start this with being extremely specific and figuring out what I can relax later — assuming I decide that relaxing is what I want to do.


Common sense suggests these should be defined in a config file somewhere. I’m still getting the hang of Eleventy, though. Common sense won’t be an option for some time.

I figure the easiest way to avoid extra recompilation is by watching the clock. If it hasn’t been long enough since the last build, skip it. Five seconds is a completely arbitrary value for “long enough” but it seems to be working.

//- Set build time guard variables
let lastSassBuild = 0;     // valueOf last sass build
const minimumWait = 5_000; // wait this many milliseconds before rebuilding

And what does a build time check look like? We use Date.valueOf to get the number of milliseconds since 1970-01-01, which is a handy numeric value for simple comparison. If the difference between that and lastSassBuild is less than minimumWait milliseconds, we don’t need to build.

//- Ensure we need to build the CSS
const now = new Date().valueOf();

if (now - lastSassBuild <= minimumWait) {

What if we do need to build? Update lastSassBuild and log what’s going on, since I enjoy a little feedback.

//- Remember and log this build
lastSassBuild = now;
const parsed = path.parse(inputPath);
console.log(`[${now}] SassHandler: ${inputPath} changed`);
console.log(`Building ${sassFilePath}`);

I still haven’t learned enough JavaScript to know why, but when I use sass.compile instead of the supposedly deprecated renderSync I get an exception. Eventually I’ll be forced to revisit that, but today is not that day.

//- Compile the Sass and write the CSS
let result = sass.renderSync({
  file: sassFilePath,

const cssText = result.css.toString("utf8");
fs.writeFileSync(cssOutputPath, cssText);

Again, I’ve disabled permalink so it’s on me to ensure the output directory exists and to write the generated CSS.

So there it is! Probably not optimal but hey we’re all learning something every day.

Load that SassHandler

Then the relevant bits of my .eleventy.js:

const SassHandler = require("./src/_11ty/handlers/SassHandler.js");

module.exports = function (eleventyConfig) {
  // ...
  eleventyConfig.addExtension("scss", SassHandler);

This site still builds and the styles update as expected, so: yay!

What’s next?

I dunno. Knowing me? Probably Asciidoctor.

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Added to vault 2024-01-15. Updated on 2024-06-25