[@rollup/plugin-commonjs] MISSING_EXPORT when cjs re-export an other file

[dmail]

• Rollup Plugin Name: @rollup/plugin-commonjs
• Rollup Plugin Version: 20.0.0
• Rollup Version: 2.57.0
• Operating System (or Browser): Mac
• Node Version: 14.17
• Link to reproduction (⚠️ read below): https://github.com/dmail/rollup-issue-named-export-cjs

Some of my dependencies are using commonjs re-exporting exports from an other file

main.cjs:

module.exports = require("./helper.cjs");

helper.cjs:

exports.answer = 42

When using @rollup/plugin-commonjs to convert these files to esm, the named exports are lost. It practice answer is undefined in the following code where ./dist/main.cjs is generated by rollup.

import { answer } from './dist/main.mjs'

Expected Behavior

@rollup/plugin-commonjs generate code re-exporting the named exports.

export const answer = 42;

Actual Behavior

@rollup/plugin-commonjs is writing all named exports on a default export

var helper = {};

helper.answer = 42;

var main = helper;

export { main as default };

Additional Information

This issue replicates a pattern used by packages like react or react-is.

[dmail]

I assumed @rollup/plugin-commonjs was responsible of handling named exports but it seems to be in the scope of other tools like cjs-module-lexer

I just discovered esinstall. It is marvelous. They are doing a mix of static analysis + runtime analysis. Link to the code where they combine @rollup/plugin-commonjs with cjs-module-lexer:

https://github.com/snowpackjs/snowpack/blob/d004e827e0781144cc84c5d935c64996e4723116/esinstall/src/index.ts#L337-L344

Unfortunately I cannot reuse esinstall because my use case is different but I can borrow the concepts.

[stale[bot]]

Hey folks. This issue hasn't received any traction for 60 days, so we're going to close this for housekeeping. If this is still an ongoing issue, please do consider contributing a Pull Request to resolve it. Further discussion is always welcome even with the issue closed. If anything actionable is posted in the comments, we'll consider reopening it. ⓘ

[dmnsgn]

@dmail it might be useful to you: I have just published rollup-plugin-commonjs-named-exports which takes the cjs-module-lexer approach at exporting named re-exports.

[lukastaegert]

Nice one. I keep wondering if we should not add cjs-module-lexer to the commonjs plugin itself but cannot seem to find time for it as it would not be 100% trivial.

[dmail]

On my side I took the time to create something inspired from esinstall that is using cjs-module-lexer + many other rollup plugins to convert commonjs to esm. It works like a charm until now but it's not open source as it contains some specificities.