Verifying swc minifier using dbg-swc

Everyone wants a faster build. But the build time should never be the highest priority. There are many more important things - namely correctness and user experience. In this article, I'll describe a way to verify if the SWC minifier is better than the terser for your web application.

The SWC minifier is fast. Almost everyone reading this article will know. But there's a vast inherent problem with JavaScript minification tools. Changing the minifier is risky. Changing here includes updating. In other words, upgrading the terser may break your app in production This was a case for lots of people, including my colleague. So using the SWC minifier becomes something you want to avoid trying. And yes, I admit that there are bugs in the SWC minifier that no one knows.

If the SWC minifier makes your bundle larger, it's also a bug because it degrades the user experience. So it's also covered by the new tool.

Automatic reducing

But there's a way to check if the swc minifier will break your application without breaking the production. Also, You don't need to give me access to the source code. Additionally, you don't need to investigate the bug by yourself.

Today, I'm introducing dbg-swc, which can be used to verify SWC minifier for your application in a semi-automated way.

How it works

It uses creduce internally, a fantastic CLI tool to reduce an input file to have a minimal input file with the same property. It repeatedly tries the same operation after modifying the input slightly. The same operation here is the verification implemented by dbg-swc. Currently, there are two modes. Those are size and semantics. I'll describe them in the Usage section.


You need to install creduce to use the reducer. To install creduce, please see

For dbg-swc itself, you can download a binary for your platform from After downloading one for your platform, rename it to dbg-swc, make it executable (chmod +x dbg-swc), and store it in your PATH.


If you want to find an input that the SWC minifier breaks, you can do

  • dbg-swc es minifier reduce --mode semantics path/to/file_or_dir

Disclaimer: This is not 100% correct verification.

If you want to verify that the SWC minifier does not emit a larger output than terser, you can run

  • dbg-swc es minifier reduce --mode size path/to/file_or_dir


A single investment can make long-term gains. I'd recommend running it against your application.