Turbopack "unreachable code" string causing warnings in Firefox

[OleSkaar]

Link to the code that reproduces this issue

https://github.com/OleSkaar/with-turbopack-app

I've used the with-turbopack example here, with two dependencies that were giving the same error in my project:

"@tanstack/react-query": "5.62.2", "@tanstack/react-query-devtools": "5.62.2",

To Reproduce

1. Start the application with pnpm run dev
2. Open the app at localhost:3000
3. Check the browser console in Firefox. You should see several warnings about unreachable code:

This is happening because of the "TURBOPACK unreachable"; statement that's inserted after the return statement in the code from react query dev tools:

Current vs. Expected behavior

I expect being able to load up this project without warnings in the console.

I believe Firefox is detecting the "TURBOPACK unreachable"; string as unreachable code and therefore reporting a warning (see this page).

The string appears before a function declaration, and this seemed to be the case for the other warnings reported here as well. This function is declared below the return the statement, but because of hoisting it is reachable (and used) higher up in the code. Could the "TURBOPACK unreachable"; string be inserted after the function declaration instead?

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 23.6.0: Mon Jul 29 21:14:30 PDT 2024; root:xnu-10063.141.2~1/RELEASE_ARM64_T6030
  Available memory (MB): 36864
  Available CPU cores: 12
Binaries:
  Node: 20.13.1
  npm: 10.5.2
  Yarn: 1.22.22
  pnpm: 9.14.4
Relevant Packages:
  next: 15.1.2 // Latest available version is detected (15.1.2).
  eslint-config-next: N/A
  react: 18.3.1
  react-dom: 18.3.1
  typescript: 5.7.2
Next.js Config:
  output: N/A

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

Developer Experience, Module Resolution, Turbopack

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

next dev (local)

Additional context

No response