Link to the code that reproduces this issue

https://github.com/joshwcomeau/next-16-prettier-repro

To Reproduce

1. Run a production build (npm run build)
2. Start a server (npm run start)
3. You should view "Application error" displayed on the page
4. Opening the browser console shows a ReferenceError, la is not defined.

Quick overview

This app uses Prettier to format code in-browser. The real-world use case I’m reproducing here is having a live-editable code playground that uses Prettier to format user-generated code. This repro is simplified, and what we’re trying to do is format a small test snippet.

I realize that this really seems like an issue with Prettier, but I don't believe it is. More info below.

Current vs. Expected behavior

I’ve included a testCode string with poorly-formatted code, and the expected behaviour is that the correctly-formatted code should be displayed in the console. This works fine in development, but breaks in production.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 23.5.0: Wed May  1 20:12:58 PDT 2024; root:xnu-10063.121.3~5/RELEASE_ARM64_T6000
  Available memory (MB): 65536
  Available CPU cores: 10
Binaries:
  Node: 22.11.0
  npm: 10.9.0
  Yarn: 1.22.22
  pnpm: 9.1.4
Relevant Packages:
  next: 16.0.1 // Latest available version is detected (16.0.1).
  eslint-config-next: N/A
  react: 19.2.0
  react-dom: 19.2.0
  typescript: 5.9.3
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Module Resolution, Runtime, Webpack, Turbopack

Which stage(s) are affected? (Select all that apply)

next start (local)

Additional context

I believe this issue was introduced with 16.0.0-canary.1. The issue is still present on the latest canary version (16.0.2-canary.3)

The issue appears to be specifically with prettier/plugins/babel. The build succeeds, but the code doesn't actually get bundled correctly in production.

I don't believe this is an issue with Prettier. The issue appears to have been introduced in Next 16 (the same code works fine with Next 15.5.6), and the issue isn’t specific to the latest version of Prettier (I tested a two-year-old version of Prettier, 3.0.3, and it had the same issue).

The issue happens with both Turbopack and Webpack, which was a bit of a surprise; I originally thought this was a Turbopack issue. It also happens in both the Pages and App router.

I thought for a second that maybe it was related to 83925, but the issue was not fixed by switching from SWC to Babel (by adding a .babelrc). It also wasn't fixed by switching to the latest canary version of Next (which is believed to fix the SWC issue).