Our Cache Busting Setup on Eleventy

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Cache busting is necessary because browsers cache the static resources, namely, CSS and JS files, and if we update the website, visitors might not see their latest versions.

In theory, implementing web workers should solve the cache busting problem, and there are Eleventy plugins opens in a new tab for that, but, we decided not to enable the PWA setup until we test it properly.

Here's how we bust the cache in an old-school way, on Eleventy + Nunjucks + Parcel.

We started this website on a XITY opens in a new tab boilerplate. It gave a headstart providing a decent JS pipeline and PostCSS using Parcel.js opens in a new tab. We removed the pwa opens in a new tab plugin and decided to bust cache in old ways.

The Essence

Folder-Based Cache Busting in Eleventy, Nunjucks and Parcel
  1. Generate file-based Unix epoch timestamp;
  2. Pipe the assets in Eleventy to a subfolder named as that timestamp;
  3. Also, use that timestamp when templating, in asset paths.

If you need to reference the timestamp inside of any of the files from this subfolder (and you will need to, if you use web workers), then, an additional step:

  1. Run a find-and-replace script after the build completes.

Read on for a proper explanation and setup examples.


In Web Architecture, the Addressability principle opens in a new tab tells:

"A URL identifies one and only one resource." Leonard Richardson, Mike Amundsen, and Sam Ruby. "RESTful Web APIs"

Given a visitor's browser caches resources (CSS and JS files, for example) those resources are distinguished from one another by the URL.

For example, two clones of the same file but in different folders are still two different resources.

is not the same as

From here, it flows that all it takes to turn a resource into a new resource is to change its URL.

That's why, conceptually, cache busting is achieved by changing the URL, one way or another. It can be done three ways:

  1. Add unique characters to the file name. For example, /css/main.css/css/main-234802938402.css
  2. Put the resource file in a uniquely-named folder. For example, /css/main.css/css/234802938402/main.css
  3. Add unique URL parameters: /css/main.css/css/main.css?v=234802938402

That's at least how it works in the pre-PWA world.

The third option seems easiest, but it is unreliable: URL parameter can get discarded anywhere in the file journey. Sorry eleventy-plugin-cache-buster opens in a new tab.

That leaves us with file-changing and folder-changing options.

File Versus the Folder

Folder approach is easier because we just pipe the files into a sub folder during Eleventy build — a one-liner:

eleventyConfig.addPassthroughCopy({ bundle: `assets/${hash}` })

File approach would involve either:

  • producing JS and CSS files with timestamped/hashed names when they're created (not possible, because they come from Parcel), or
  • renaming them later during Eleventy asset ingestion (tricky, considering the addPassthroughCopy won't do and afterBuild opens in a new tab event hook is not released yet at the time of writing), or
  • find-and-replace script that runs after (both complicated and tricky, considering two layers of watch that would run in front of it, in the pipeline).

Hash vs Timestamp

Another consideration: what random value to put.

An easy way is JS timestamp opens in a new tab Date.now() (or in the terminal opens in a new tab date +%s > hash.txt) but it’s not file-content-based hash, each deployment will cause assets to be treated as new, even if they haven’t changed since the last release.

A "proper" way is to use file-content-based hashes (MD5 opens in a new tab for example). URL would change only if file contents changed. The drawback is that we'd have one folder-per-file.

In either case, file or folder length is not an issue, timestamps are ten characters-long, and MD5 hashes are 32 characters-long.

For the initial setup, we went with easier, timestamp-based cache-busting.

Simplified Explanation

We run a huge script from package.json. It does many things.

First, it outputs a current timestamp into a file hash.txt, into a root folder of a project. Probably there's an old file from a previous run, but it gets overwritten.

That hash.txt contains our reference timestamp value. Parcel and Eleventy will be run after this file has been written.

Parcel then runs, processing JS files (baking-in all dependencies) and PostCSS files (rendering all mixins etc. into normal CSS). Files from src/assets/ get rendered into bundle/.

Then Eleventy script at .eleventy.js (located in the root folder) fires. First, it reads the hash.txt. Second, it copies all files from the bundle/ folder into dist/assets/${hash} where ${hash} is the hash from the hash.txt file.

Our assets are now placed in a uniquely-named folder.

The HTML templates also need to "know" the hash, to write correct JS and CSS file paths. It's achieved by global data files opens in a new tab — a data file reads hash.txt and returns a plain object which templates consume like

<link rel="stylesheet" href="/assets/{{ cacheBusting.hash }}/css/app.css">

That's the essence of the setup.

Huge Script, Deconstructed

Let's go through our package.json npm script start, piece-by-piece.

But first, ingredients:

npm-run-all opens in a new tab runs the scripts from package.json sequentially or parallelly.

cross-env opens in a new tab is to make it work on Windows. That's fair, considering not everybody can afford a Mac.

The script start is triggered by npm run start and it builds a development version of this website and watches for changes:

"start": "cross-env ELEVENTY_ENV=dev run-s clean:* build:assets && cross-env ELEVENTY_ENV=dev run-p serve hash watch:*"

Destructured, it calls the following npm scripts:


"clean:cache": "rimraf ./.cache",
"clean:dist": "rimraf ./dist",
"clean:bundle": "rimraf ./bundle",

These commands above delete folders. DIY solution for #19 opens in a new tab.

"build:assets": "parcel build ./src/assets/css/app.pcss ./src/assets/js/app.js ./src/assets/js/interdeps.js ./src/assets/js/search-worker.js --out-dir ./bundle --no-source-maps",

This command above calls Parcel. JS file processing is configured by .babelrc and PostCSS .pcss file processing is configured by postcss.config.js — both configs are in the root.

Parcel doesn't need to be aware of our hash.


"serve": "date +%s > hash.txt && eleventy --serve --port=8080",

The command above does two things: date +%s > hash.txt writes the timestamp into a hash.txt. eleventy --serve --port=8080 being for development, builds and watches for changes.

"hash": "delay 15 && node utils/scripts/fixDistWorkerHash.js",

The command above might not be relevant for some, but our search runs off a web worker opens in a new tab to a) unblock the thread, and b) to make it easy to start and kill it after each keypress in the search field. Simply speaking, it's one JS file running another, pet JS file, in a separate CPU thread. There's messaging in-between.

The catch is, app.js, the main script, needs to know the URL of the file of a worker:

myWorker = new Worker(

When Eleventy is running in parallel, we give it 15 seconds using delay 15, and then we find-and-replace text in a file using Node's fs.readFileSync and fs.writeFileSync in a script located at utils/scripts/fixDistWorkerHash.js. For scripting like this, use what you know best — some people prefer Bash or Make or Perl, to name a few.

"watch:assets": "parcel watch ./src/assets/css/app.pcss ./src/assets/js/app.js --out-dir ./bundle --no-source-maps --no-hmr",

In the command above, Parcel runs in parallel and watches for file changes.

The watch scripts of all kinds — commands marked by CLI flags --serve or --watch — they never end and thus block all further scripts chained with &&. If you append something after, that will never trigger. For example,

"badexample": "eleventy --serve --port=8080 && echo \"oodles\""

The oodles will never be printed in the terminal!

That's valid for any "watching" script: BrowserSync on watch mode, serve package from npm, Eleventy on a --serve flag and all others. If there's watching, you can't put other npm scripts after it using &&. That's one of the reasons chain watcher scripts are ran using run-p — rule doesn't apply to npm-run-all opens in a new tab because it fires different processes.

Here we covered only the dev (development) build. The prod (production) build is similar except it doesn't watch and there's another, standalone script which renames the http://localhost:8080/ into https://codsen.com/.

If you wonder why we can't just use / in URL, that's because at the moment, if Parcel "sees" anything else than absolute URL, it will process it. In this case, it will do it wrong, so we use absolute URL's as a means to make Parcel ignore them. At the moment, it is not possible to force Parcel to opens in a new tab ignore opens in a new tab things opens in a new tab.


This setup is an MVP — "Minimal Viable Product". We'll continue improving it.

The source code of this website is Open Sourced. Feel free to fork it and play around with it. We all learn by doing; reading alone won't suffice.

Related packages:

📦 @11ty/eleventy opens in a new tab
Transform a directory of templates into HTML
📦 npm-run-all opens in a new tab
A CLI tool to run multiple npm-scripts in parallel or sequential
📦 cross-env opens in a new tab
Run scripts that set and use environment variables across platforms