I wrote a 12-line Rollup plugin

November 30, 2019


I was building a web application using Svelte and Rollup this morning. I needed to use a web worker, which the worker script has to be in another file:

const worker = new Worker('/build/worker.js');

So naturally, I was thinking of having 2 entries for my rollup application: the main app and the worker.

It works fine, except the fact that the rollup-plugin-livereload injected a livereload script to every output file:


(function(l, r) { if (l.getElementById('livereloadscript')) return; r = l.createElement('script'); r.async = 1; r.src = '//' + (window.location.host || 'localhost').split(':')[0] + ':35729/livereload.js?snipver=1'; r.id = 'livereloadscript'; l.head.appendChild(r) })(window.document);

// worker code ...

The livereload script includes a reference to window, which is not available to the worker script:

window reference error
window is not defined

I looked into the docs of the rollup-plugin-livereload, it doesn’t seemed to have a option to exclude files from adding the livereload script.

At this point, I was thinking to myself, “I just need to copy the worker.js into the ‘build/’ folder, I don’t need anything else, how hard can that be?”

It turns out harder than I imagined. 🤮

Need something? Install a plugin!

In todays JavaScript landscape, there’s a “node_module” for everything.

So I googled “rollup plugin copy files”, without a suprise, there are multiple rollup plugins published to npm:

google search
Google result for "rollup plugin copy files"

So I decided to install the first plugin, because it has the highest weekly downloads:

rollup-plugin-copy weekly downloads
17K weekly downloads

When I installed the plugin, I realise I was installing much more than I needed:

rollup-plugin-copy dependencies

Remember, my use case is simple:

I just need to copy the worker.js into the ‘build/’ folder.

I don’t need any bells and whistles this plugin is providing me. 🙈

So I uninstalled the plugin, thinking:

How hard is it to write a plugin that just copy the worker.js into the ‘build/’ folder?

Writing a Rollup plugin

Senpai once told me, “writing rollup plugins is very straightforward,”, yet, no one told me how to get started writing it.

So, I dug into node_modules/, and start skimming through the rollup plugins I have installed: rollup-plugin-svelte, rollup-plugin-node-resolve, rollup-plugin-terser, …

And I noticed a common pattern:


module.exports = function(options) {
  // ...
  return {
    name: 'plugin-name',
    load() { /* ... */ },
    resolveId() { /* ... */ },
    generateBundle() { /* ... */ },
    // ...

So I guess, this is the general structure of a rollup plugin:

  • It’s an object, …
  • with a property called name for the name of the plugin,
  • and functions like “load”, “load”, … that would be called by rollup when the time is right 🤔

OK. I know what I need, I need to copy my worker.js when rollup is generating a bundle:


export default {
  plugins: [
    // ...
    {      name: 'copy-worker',      generateBundle() {        fs.copyFileSync(          path.resolve('./src/worker.js'),          path.resolve('./public/build/worker.js')        );      }    }  ],

Great! It works! 😎

But, when I change the worker.js file, the build/worker.js is not updated. 😞

That’s because the worker.js is not watched!

After much googling, I ended up reading through the official docs of Rollup.

I learned that the functions like “load”, “generateBundle”, … are called “hooks”, and the docs explained when these hooks will be called, the arguments and the expected return value.

In the docs, I found this.addWatchFile(id: string) under plugin context, which according to the docs,

[…] can be used to add additional files to be monitored by watch mode.

Sounds exactly what I am looking for! 😁

export default {
  plugins: [
    // ...
      name: 'copy-worker',
      load() {        this.addWatchFile(path.resolve('./src/worker.js'));      },      generateBundle() {

Great! It works! 🎉🎉

Closing Notes

After some researching, I wrote simple rollup plugin in 12 lines of code, that copies the worker.js into “build/” folder.

This is something custom and specific, and it works perfectly fine.

So, why would I install a package that has so many files and dependencies, just to do a simple and specific task?

Am I going to publish my plugin to npm?

No. If you have a similar use case, you are free to copy these 12 lines of code.

At the moment, I am having these 12 lines of code in my rollup.config.js and have no intention to extract it out into its own package.

What about DRY? What if you/someone else have the same use case, wouldn’t it great to have it as a package?

Sorry. No. Before DRY (Dont Repeat Yourself), there’s YAGNI (You aren’t gonna need it).

Abstract code only when you need to.

Thank you for your time reading through this article.
It means a lot to me.

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