`TypeError: Expected CommonJS module to have a function wrapper` when building with `bun --bun`

[shiny]

I'm running into an error when trying to build a Next.js project using Bun. The build fails with:

TypeError: Expected CommonJS module to have a function wrapper. If you weren't messing around with Bun's internals, this is a bug in Bun
    at anonymous (null)

This only happens when I run the build with:

bun --bun build

Building with other configurations works as expected.

Environment:

• Bun version: 1.2.19
• Next.js version: 15.3.5

Steps to reproduce:

1. Create a Next.js project
2. use atomEffect
3. Build using bun --bun build
4. See the error

Similar to oven-sh/bun#21137

Expected behavior:
The project should build successfully without throwing this error.

[shiny]

minimal code:

import { atomEffect } from "jotai-effect";
atomEffect(() => {})

you can put it into the default nextjs home page,then run bun --bun run build
error message shows below

bun --bun run build
$ next build
   ▲ Next.js 15.4.2

   Creating an optimized production build ...
 ✓ Compiled successfully in 0ms
 ✓ Linting and checking validity of types    
   Collecting page data  .TypeError: Expected CommonJS module to have a function wrapper. If you weren't messing around with Bun's internals, this is a bug in Bun
    at anonymous (null)
    at processTicksAndRejections (null)
    at processTicksAndRejections (null)

> Build error occurred
Error: Failed to collect page data for /
    at processTicksAndRejections (null)
    at processTicksAndRejections (null) {
  type: 'Error'
}
error: script "build" exited with code 1

[shiny]

import { observe } from "jotai-effect";
observe(() => {})

change to observe still the same

[dmaskasky]

Thanks for reporting.

I'm out of town for the weekend, I can take a deeper look next week.

Preliminary search yielded some info.

Thanks for sharing the issue. This appears to be an incompatibility between Bun’s CommonJS module handling and the way jotai-effect is bundled or imported.

This is a known class of issue in Bun:
• See oven-sh/bun#21137 — Bun can fail to load CommonJS modules that don’t follow its expectations for structure, especially when internally wrapped.

Some things to try

1. Avoid using --bun

Instead of:

bun --bun run build

Use:

bun run build

or even:

next build

Because --bun forces the use of Bun’s internal Node.js replacement, which is not fully compatible with Next.js or CommonJS interop yet.

2. Try using jotai-effect as ESM instead

If you’re using Bun with ESM-first expectations, you could try to force an alias to the ESM-compatible build.

import { atomEffect } from 'jotai-effect/dist/index.mjs'

3. Try clearing Bun cache

Sometimes Bun caches faulty module metadata:

bun pm cache clean

Then rebuild.

[shiny]

Thanks for getting back to me so quickly.

1. The reason for using the --bun flag is that our production Next.js app runs on Bun, and our Docker build environment doesn’t have Node installed also . That’s why I didn’t catch this issue locally and only saw it fail in CI.
2. import from 'jotai-effect/dist/index.mjs' (or index.js) would trigger Cannot find module error.