bug: imported function becomes not defined with --turbopack

[vanaigr]

Description

import { err } from '@/lib/utils';
export async function POST() {
    console.log('first', err);
    for(let i = 0; i < 0; i++) return err();
    console.log('second', err);
}

This route handler successfully prints err in the first console.log(), but then something happens and it fails with err is not defined when trying to print it a second time.

This only happens with --turbopack.

Link to the code that reproduces this issue

https://github.com/vanaigr/next-not-defined-bug

Edited by @timneutkens: Simplified reproduction: https://codesandbox.io/p/github/vanaigr/next-not-defined-bug/csb-5qq4l8/draft/keen-field?file=%2Fapp%2Froute.ts%3A1%2C1-7%2C1. Just run npm run dev and open /.

To Reproduce

1. pnpm dev in a terminal
2. Go to http://localhost:3000/Dashboard
3. Observe that terminal has output with the following error:

> next@0.1.0 dev /home/artarar/tmp/next
> next dev --turbopack

   ▲ Next.js 15.1.8 (Turbopack)
   - Local:        http://localhost:3000
   - Network:      http://192.168.1.102:3000

 ✓ Starting...
 ✓ Ready in 1544ms
 ○ Compiling /Dashboard ...
 ✓ Compiled /Dashboard in 7.1s
 GET /Dashboard 200 in 7590ms
 ✓ Compiled /Dashboard/loadDashboardData in 410ms
first [Function: err]
 ⨯ ReferenceError: err is not defined
    at POST (app/Dashboard/loadDashboardData/route.ts:5:26)
  3 |     console.log('first', err);
  4 |     for(let i = 0; i < 0; i++) return err();
> 5 |     console.log('second', err);
    |                          ^
  6 | }
  7 |
 ⨯ ReferenceError: err is not defined
    at POST (app/Dashboard/loadDashboardData/route.ts:5:26)
  3 |     console.log('first', err);
  4 |     for(let i = 0; i < 0; i++) return err();
> 5 |     console.log('second', err);
    |                          ^
  6 | }
  7 |
 POST /Dashboard/loadDashboardData 500 in 789ms

4. Run pnpm dev-without-turbopack
5. Go to http://localhost:3000/Dashboard
6. Observe both first and second print statements are successful:

> next@0.1.0 dev-without-turbopack /home/artarar/tmp/next
> next dev

   ▲ Next.js 15.1.8
   - Local:        http://localhost:3000
   - Network:      http://192.168.1.102:3000

 ✓ Starting...
 ✓ Ready in 3.3s
 ○ Compiling /Dashboard ...
 ✓ Compiled /Dashboard in 2.3s (591 modules)
 GET /Dashboard 200 in 3375ms
 ○ Compiling /Dashboard/loadDashboardData ...
 ✓ Compiled /Dashboard/loadDashboardData in 901ms (599 modules)
first [Function: err]
second [Function: err]
 ⨯ [Error: No response is returned from route handler '/home/artarar/tmp/next/app/Dashboard/loadDashboardData/route.ts'. Ensure you return a `Response` or a `NextResponse` in all branches of your handler.]
 ⨯ [Error: No response is returned from route handler '/home/artarar/tmp/next/app/Dashboard/loadDashboardData/route.ts'. Ensure you return a `Response` or a `NextResponse` in all branches of your handler.]
 POST /Dashboard/loadDashboardData 500 in 1291ms

Current vs. Expected behavior

The route handler shouldn't crash with err is not defined.

Provide environment information

/bin/sh: 1: yarn: not found

Operating System:
  Platform: linux
  Arch: x64
  Version: #45-Ubuntu SMP PREEMPT_DYNAMIC Fri Aug 30 12:02:04 UTC 2024
  Available memory (MB): 5865
  Available CPU cores: 2
Binaries:
  Node: 23.11.0
  npm: 10.9.2
  Yarn: N/A
  pnpm: 9.7.1
Relevant Packages:
  next: 15.1.8 // Latest available version is detected (15.1.8).
  eslint-config-next: N/A
  react: 19.1.0
  react-dom: 19.1.0
  typescript: 5.8.3
Next.js Config:
  output: N/A

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

Turbopack

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

next dev (local)

Additional context

No response

_PACK-4738

[timneutkens]

Thanks for the report, verified the reproduction and simplified it a bit to make it easier to dig into:

https://codesandbox.io/p/github/vanaigr/next-not-defined-bug/csb-5qq4l8/draft/keen-field?file=%2Fapp%2Froute.ts%3A1%2C1-7%2C1

[mischnic]

This is probably also fixed by #77964