Rollup-based dev environment for JavaScript (part 2: Service Workers)

Camille Hodoul
2 min readMar 22, 2019


This is the second part of my Rollup dev environment guide.
In this article, we will briefly introduce Service Workers then precache our app using Workbox.

If you just want to see a working example, clone this repo.

Service Workers

Service Workers are JavaScript files that run “in the background” of your app. They have no direct access to the DOM and run in a different thread than your application code.
They do have access to other APIs, such as IndexedDB, and can act as a proxy between your application and the network.
Workbox uses these features to cache network requests using various strategies, so that our app can be used offline as well as possible, transparently for the user.


We want our app to be completely available offline, but we can’t expect the user to request every file himself as he’s using the application just so that we can cache them.
We will give Workbox a list of files we know need to be cached in advance, thus, precaching.

Precaching Rollup bundles

In the first part of this guide, we configured Rollup to output 2 formats : esm and system. Since we only want to cache relevant files, we need to generate a ServiceWorker for each format.

We’ll be using workbox-cli (there is a rollup plugin for workbox, but I couldn’t get it to work correctly with multiple outputs).
As we’re adding a new step to our build process, let’s installnpm-run-all as well to help us chain scripts together.

npm install --save-dev workbox-cli npm-run-all

We need a template src/sw.js file:

(The registerRoute() call with is obviously not necessary if you’re not using this CDN. It is included here because we use it in our examples).

We will create a configuration file for each output (esm and system):

Notice the modifyURLPrefix option : I’ve added it to fix problems with Netlify, but depending on your hosting solution, you probably won’t need it.

Add the following scripts to package.json:

HTTP server configuration

Browsers save ServiceWorker files in a different way than traditional HTTP requests, which means we need to disable HTTP cache: Cache-control: no-cache.
Additionally, we decided to put our sw.js files in subdirectories of the server root (public/js/esm/ and public/js/system/), so we need a header to avoid unenjoyable warnings: Service-Worker-Allowed: /.
Here’s a configuration for the server that we set up in the previous article, serve:

Registering our Service Workers

Finally, we need to actually register sw.js from the browser, depending on which output format is used. Here’s an example of how to do it in index.html from the example repository:

See how our app looks so far (branch for this post).

In part 3, we will cover testing tools (unit and end-to-end), continuous integration and deployment.

If you have suggestions, corrections or enhancements, please tell me in the comments and I’ll update this post.