In Next.js 15 with Turbopack mode, the IIFE is not executing as expected, resulting in a strange bug.

[listen936]

Link to the code that reproduces this issue

https://github.com/listen936/nextjs_15_turbopack_bug_demo

To Reproduce

1. npm install
2. npm run dev

Current vs. Expected behavior

(function _(namespace, path) {
  // The expected output is "", "wrapper", "wrapper.data", "wrapper.data.key"
  // The actual output is "", "wrapper", "data", "key"
  console.debug("####", path)

  for (var i = 0, keys = Object.keys(namespace); i < keys.length; i++) {
    var key = keys[i];
    // --------------------------------------------------------------------------
    console.debug("####", path ? true : false) // always true? why?
    // --------------------------------------------------------------------------
    parent[key] = _(namespace[key], path ? `${path}.${key}` : key, parent[key] || {});
    // --- If the line below is replaced, it will meet the expectations.
    // parent[key] = _(namespace[key], Boolean(path) ? `${path}.${key}` : key, parent[key] || {});
  }
  return parent;
})({
  wrapper: {
    data: {
      key: {}
    },
  }
}, '');

Provide environment information

Operating System:
  Platform: darwin
  Arch: x64
  Version: Darwin Kernel Version 24.1.0: Thu Oct 10 21:02:27 PDT 2024; root:xnu-11215.41.3~2/RELEASE_X86_64
  Available memory (MB): 16384
  Available CPU cores: 8
Binaries:
  Node: 18.20.1
  npm: 8.19.4
  Yarn: 1.22.10
  pnpm: 8.14.1
Relevant Packages:
  next: 15.1.2 // Latest available version is detected (15.1.2).
  eslint-config-next: 15.1.1
  react: 19.0.0
  react-dom: 19.0.0
  typescript: 5.7.2
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

[listen936]

This code is from my actual development experience; I just removed the unnecessary parts.

[listen936]

update demo code

var root = {};
// namespace builder
(function _(namespace, path, parent) {
  // The expected output is "", "wrapper", "wrapper.data", "wrapper.data.key"
  // The actual output is "", "wrapper", "data", "key"
  console.debug("####", path)

  for (var i = 0, keys = Object.keys(namespace); i < keys.length; i++) {
    var key = keys[i];
    // --------------------------------------------------------------------------
    console.debug("####", path, path ? true : false) // always true? why?
    // --------------------------------------------------------------------------
    parent[key] = _(namespace[key], path ? `${path}.${key}` : key, parent[key] || {});
    // --- If the line below is replaced, it will meet the expectations.
    // parent[key] = _(namespace[key], Boolean(path) ? `${path}.${key}` : key, parent[key] || {});
  }
  return parent;
})({
  wrapper: {
    data: {
      key: {}
    },
  }
}, "", root);