Optimizing barrel file import from local workspace packages does not work with optimizePackageImports and turbopack

[PMudra]

Link to the code that reproduces this issue

https://github.com/PMudra/reproduction-app-optimize-package-imports-npm-workspaces

To Reproduce

1. Install packages in root of workspace: npm install.
2. Start the application:
   1. cd packages/frontend
   2. npm run dev -- --turbopack
3. Inspect the page using your browser dev tools. The "Sources" and "Coverage" tabs are helpful.

Current vs. Expected behavior

Example

In my example project, I am using npm workspaces to organize a Next.js app with a library "my-lib". The folders are like this:

├── packages
│   ├── frontend
│   │   ├── app
│   │   │   ├── favicon.ico
│   │   │   ├── layout.tsx
│   │   │   └── page.tsx
│   │   ├── next.config.ts
│   │   ├── next-env.d.ts
│   │   ├── package.json
│   │   └── tsconfig.json
│   └── my-lib
│       ├── featureA.js
│       ├── featureB.js
│       ├── index.js
│       └── package.json
├── package.json
├── package-lock.json
└── README.md

The Next.js app frontend depends on my-lib. When running npm install, a symlink is created in node_modules folder by npm like so: node_modules/my-lib -> ../packages/my-lib.

my-lib has 2 modules: featureA and featureB. The next.js app currently only uses featureA which is imported using the barrel file in index.js. my-lib looks like this:

// featureA.js
export function featureA() {
  console.log("featureA");
}

// featureB.js
export function featureB() {
  console.log("featureA");
}

// index.js (barrel file)
export * from "./featureA";
export * from "./featureB";

page.tsx looks like this:

"use client";
import { featureA } from "my-lib";

export default function Home() {
  featureA();
  return null;
}

// next.config.ts
import type { NextConfig } from "next";

const nextConfig: NextConfig = {
  /* config options here */
  reactStrictMode: true,

  experimental: {
    optimizePackageImports: ["my-lib"],
  }
};
export default nextConfig;

Current behavior

When inspecting which files are loaded, I can currently observe that featureA and featureB are being loaded:

Expected behavior

I would expect that specifying optimizePackageImports: ["my-lib"] would prevent the featureB module to be loaded by the browser.

The result should look like as if im importing from import { featureA } from "my-lib/featureA";:

Provide environment information

Operating System:
  Platform: linux
  Arch: x64
  Version: #14-Ubuntu SMP PREEMPT_DYNAMIC Sat Nov 30 23:51:51 UTC 2024
  Available memory (MB): 63439
  Available CPU cores: 16
Binaries:
  Node: 22.13.0
  npm: 10.9.2
  Yarn: N/A
  pnpm: 10.0.0
Relevant Packages:
  next: 15.2.0-canary.18 // Latest available version is detected (15.2.0-canary.18).
  eslint-config-next: N/A
  react: 19.0.0
  react-dom: 19.0.0
  typescript: 5.7.3
Next.js Config:
  output: N/A

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

Developer Experience, Performance, Turbopack

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

next dev (local)

Additional context

• My example is a very minimal reproduction of the repository I am working on. We have multiple apps and multiple (generated) libraries and we want to make use of optimizePackageImports because performance in dev mode is starting to be a pain.
• Everything seems to be working as expected when using webpack.
• The deprecated option modularizeImports seems to be working, too. But setting that up in my real repo seems to be complicated.
• I suspect symlinks to be the cause of the issue. When I manually copy my-lib/ into node_modules/my-lib, the optimization seems to work fine.
• I am having the same issue using pnpm which uses symlinks, too. I haven't tried yarn.

There are some workarounds. All of them are not satisfying. Are there better ones?

• Use webpack.
• Don't import from barrel files.
• Move away from the monorepo.
• Use deprecated modularizeImports.

Related issues:

• [NEXT-1143] Dev mode slow compilation #48748
• Next.js development high memory usage #54708

Thanks for looking into this and let me know if you need more information.