[ES Module but shipped in a CommonJS package] Lib importing another ESM-only library

Describe the bug

I was creating a library with unbuild using rollup and generating dist/*.cjs and dist/*.mjs files.
As I didn't want to import directly from lib/dist, I created the following package.json:

  "type": "module",
  "exports": {
    ".": {
      "require": "./dist/index.cjs",
      "node": "./dist/index.cjs",
      "import": "./dist/index.mjs",
      "types": "./dist/index.d.ts"
    },
    "./*": {
      "require": "./dist/*/index.cjs",
      "node": "./dist/*/index.cjs",
      "import": "./dist/*/index.mjs",
      "types": "./dist/*/index.d.ts"
    }
  },
  "files": [
    "dist"
  ],

This lib depends on another lib, esm-only.

esm-only is packaged as ESM but with .js extension and I can't control how this library bundled

My App project uses lib. When I ran vitest on my App project, I got this error:

 FAIL  src/lib.test.ts [ src/lib.test.ts ]
SyntaxError: Unexpected token 'export'
 ❯ ../lib/dist/utils/index.cjs:3:15
      1| 'use strict';
      2| 
      3| const utils = require('esm-only/dist/utils');
       |               ^
      4| 
      5| const multiply = (a, b) => {

Module node-packages/esm-only/dist/utils/index.js:1 seems to be an ES Module but shipped in a CommonJS package. To fix this issue, change the file extension to .mjs or add "type": "module" in your package.json.

The only thing I was able to make it work was to change lib/package.json to:

  "type": "module",
  "main": "dist/index.cjs",
  "module": "dist/index.mjs",
  "types": "dist/index.d.ts",
  "files": [
    "dist"
  ],

And then change src/lib.test.ts to:

import { multiply } from "lib/dist/utils";

But as you can see, I have lib/dist/utils, I want to be able to import lib/utils instead.

Another option is to change lib/build.config.ts to use mkdist:

import { defineBuildConfig } from "unbuild";

export default defineBuildConfig({
  entries: [
    {
      builder: "mkdist",
      input: "src/",
      distDir: "dist",
      ext: "js",
      format: "esm",
    },
  ],
  outDir: "dist",
  declaration: true,
});

And my lib/package.json to:

 "type": "module",
  "exports": {
    ".": {
      "require": "./dist/index.js",
      "node": "./dist/index.js",
      "import": "./dist/index.js",
      "types": "./dist/index.d.ts"
    },
    "./*": {
      "require": "./dist/*/index.js",
      "node": "./dist/*/index.js",
      "import": "./dist/*/index.js",
      "types": "./dist/*/index.d.ts"
    }
  },
  "files": [
    "dist"
  ],

Finally, this ONLY DOESN'T work on Vitest. On Vite I can normally run both in dev and production mode.

Related issues:

• https://github.com/vitest-dev/vitest/issues/5200: on my project, Vitest also suggest using server.deps.inline but it didn't work

Reproduction

I created this repository to reproduce the issue I was having in another project: https://github.com/varugasu/node-packages

• Go to esm-only, then pnpm build
• Go to lib, then pnpm build
• Go to app, then pnpm test

System Info

System:
    OS: macOS 15.1
    CPU: (10) arm64 Apple M1 Max
    Memory: 2.96 GB / 64.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 20.11.0 - ~/Library/Caches/fnm_multishells/50248_1730980063968/bin/node
    npm: 10.2.4 - ~/Library/Caches/fnm_multishells/50248_1730980063968/bin/npm
    pnpm: 9.9.0 - ~/Library/pnpm/pnpm
    bun: 1.1.26 - /opt/homebrew/bin/bun
    Watchman: 2024.10.21.00 - /opt/homebrew/bin/watchman
  Browsers:
    Safari: 18.1
  npmPackages:
    @vitejs/plugin-react: ^4.3.3 => 4.3.3 
    vite: ^5.4.10 => 5.4.10 
    vitest: ^2.1.4 => 2.1.4

Used Package Manager

pnpm

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion or join our Discord Chat Server.
• The provided reproduction is a minimal reproducible example of the bug.

Clone the linked node-packages reproduction, build esm-only and lib, then run pnpm test from app. Compare lib/package.json and lib/build.config.ts with Vitest's dependency handling, and confirm done when lib/utils imports work in Vitest without breaking the Vite dev and production cases.