The Rollup Miracle

• ☕ The Rollup Miracle: How I Untangled a Massive Legacy Bundle and Hit Warp Speed
• 😱 The Anatomy of the Nightmare: What Was the Problem?
• 🛠️ Rollup to the Rescue
• 1. Decoupling and Config Clean-up
• 2. Output Format and Caching
• 3. The Actual Tree-Shaking
• 🥳 The Result: Less is More (and Faster!)

☕ The Rollup Miracle: How I Untangled a Massive Legacy Bundle and Hit Warp Speed

Hey folks! You know the drill, right? You inherit a project, and lurking in the src/bundles folder is a JavaScript monolith that looks like it's been vacuuming up every script written in the last ten years. In my case, we affectionately called this thing the "God-Bundle." It was huge. It was slow. And building it gave me enough time to brew a double espresso, attend a meeting, and maybe squeeze in a quick yoga session.

But I'm not just here for the coffee breaks. I'm here to build fast frontends! Today, I'm going to tell you how I used Rollup to transform this monstrosity into an arsenal of lean, dedicated modules.

😱 The Anatomy of the Nightmare: What Was the Problem?

The "God-Bundle" suffered from a classic issue often found in growing projects: lack of separation.

Massive File Size: We were talking several megabytes here. Loading it on mobile devices or poor connections was a straight-up nightmare.

Zero Tree-Shaking: Since everything was imported in one go, dozens of unused legacy functions ended up in the final bundle. Dead Code galore!

Poor Caching Efficiency: Even if a tiny feature changed, the entire behemoth had to be downloaded again because the hash changed. That's like having to repaint your whole house just to change a lightbulb. Absurd!

My goal was clear: Split, Split, Split!

🛠️ Rollup to the Rescue

Why Rollup and not Webpack? Well, for application bundles (like a full SPA), Webpack with its feature richness might be unbeatable. But for creating JavaScript libraries or, as in my case, dedicated utility bundles with a focus on clean ESM output, Rollup is the clear champion. Rollup has unbeatable tree-shaking and often produces a cleaner, more compact output.

The first step was to bring order to the chaos. What started as one giant, undocumented rollup.config.js turned into a collection of specific configurations, based on the modules you could extract from the old code:

/**
 * @type {import('rollup').RollupOptions[]}
 */
export default [
    rollupShariffConfig,
    rollupSCSSConfig,
    rollupTemplateConfig,
    rollupConfigTracing,
    rollupPurchaseConfig,
];

This is the game-changer! We now define a separate, dedicated Rollup bundle for each domain—be it the Shariff integration, SCSS handling, Template logic, or the Purchase flow.

1. Decoupling and Config Clean-up

Instead of a huge array internally trying to manage various entry points, we created separate config files (e.g., rollup.config.purchase.mjs). This brings two massive benefits:

• Clarity: Each configuration file does exactly one thing. Debugging shifts from "Where is the error in these 500 lines?" to "Is the entry point in purchase.mjs correct?"
• Performance: Rollup can process these configurations in parallel. The total build process, which used to take 60 seconds, was suddenly under 10!

2. Output Format and Caching

For every new, small bundle, we strictly adhered to modern standards:

• ESM Output (format: 'es'): This allows for optimal native tree-shaking if the bundle is later included in another application via import.
• Hashed Filenames: We made sure the output filenames included a content hash (e.g., [name]-[hash].js).

Now, if only purchase.js changes, only that single small file is re-downloaded. The other large, stable parts (like shariff.js or tracing.js) remain in the browser cache. Boom! The initial load time for returning visitors dropped dramatically.

3. The Actual Tree-Shaking

The most important part: By implementing the new setup, where each bundle only imports what it actually needs, Rollup could unleash its magic.

We used the @rollup/plugin-commonjs (for old NPM modules) and the @rollup/plugin-node-resolve (so Rollup knows where to find modules). But the key was the consistent module-based writing of the legacy code. Each new bundle had a clear, small entry point that explicitly imported only the functions it required.

My Pro-Tip: If you're working with Rollup and encountering unwanted code, always start with the external array in your config. Define everything there that should not be in the bundle (e.g., react, jquery). This forces you to define dependencies cleanly.

🥳 The Result: Less is More (and Faster!)

The build process is now not only ten times faster but also more maintainable.

Cleaning up legacy code isn't the sexiest job, but it is essential. It's like cleaning out the basement—it stinks at first, but the feeling afterward is priceless. And thanks to the clean Rollup structure, I now even have time for a second cup of coffee... guilt-free!

Happy Bundling! And remember: every kilobyte counts.