Question around default exports in a dual ESM/CJS package

[Luke Bennett]

Sorry if this isn't the right place to raise this, but I'm having some troubles handling default exports in a dual CJS/ESM package.
I'm using tsup in a real project and am seeing this error:

Build tools:
- @arethetypeswrong/cli@^0.12.2
- typescript@^5.2.2

🤨 CommonJS module simulates a default export with exports.default and exports.__esModule, but does not also set module.exports for compatibility with Node. Node, and some bundlers under certain conditions, do not respect the __esModule marker, so accessing the intended default export will require a .default property access on the default import. https://github.com/arethetypeswrong/arethetypeswrong.github.io/blob/main/docs/problems/CJSOnlyExportsDefault.md


┌───────────────────┬───────────────────────┬───────────────────────────────┬──────────────────────────┐
│                   │ "package-a"           │ "package-a/second-entrypoint" │ "package-a/package.json" │
├───────────────────┼───────────────────────┼───────────────────────────────┼──────────────────────────┤
│ node10            │ 🤨 CJS default export │ 🟢                            │ 🟢 (JSON)                │
├───────────────────┼───────────────────────┼───────────────────────────────┼──────────────────────────┤
│ node16 (from CJS) │ 🤨 CJS default export │ 🤨 CJS default export         │ 🟢 (JSON)                │
├───────────────────┼───────────────────────┼───────────────────────────────┼──────────────────────────┤
│ node16 (from ESM) │ 🟢 (ESM)              │ 🟢 (ESM)                      │ 🟢 (JSON)                │
├───────────────────┼───────────────────────┼───────────────────────────────┼──────────────────────────┤
│ bundler           │ 🟢                    │ 🟢                            │ 🟢 (JSON)                │
└───────────────────┴───────────────────────┴───────────────────────────────┴──────────────────────────┘
 ELIFECYCLE  Command failed with exit code 1.

I've put together a reproduction here: https://github.com/lukebennett88/dual-package-example.
My question is — how do I change my code to support this. Am I using tsup incorrectly? Do I need to change my source code? Is everything configured correctly, do I need a custom esbuild plugin?

Hopefully this isn't too specific to tsup I'm happy to ask there instead if this is the wrong place 😅

[Andrew Branch]

I think I’m going to silence this error outside of node16 (from ESM), since that’s the only place it can cause a real issue. Since this is a dual package, real ESM in Node is going to see a real default export, while transpiled ESM will see a transpiled default export, and everything is consistent.