pkg.main, pkg.module are incompatible, so require("quickselect") behaves differently under Rollup/Webpack

[andersk]

In Node, require("quickselect") returns the function quickselect, but in Webpack, require("quickselect") returns a module { default: quickselect }. This means a quickselect dependent can’t be compatible with both environments without contortions.

$ echo 'console.log(require("quickselect"))' > src.js
$ npm i quickselect webpack webpack-cli
$ node src.js
[Function: quickselect]
$ npx webpack -d
$ node dist/main.js 
Object [Module] { default: [Getter] }

This is because Node is using pkg.main (quickselect.js) and Webpack is using pkg.module (index.js), and the two files do not provide compatible interfaces.

This problem is called out in the Rollup documentation:

Note: There are some tools such as Babel, TypeScript, Webpack, and @rollup/plugin-commonjs that are capable of resolving a CommonJS require(...) call with an ES module. If you are generating CommonJS output that is meant to be interchangeable with ESM output for those tools, you should always use named export mode. The reason is that most of those tools will by default return the namespace of an ES module on require where the default export is the .default property.

This was previously touched on in #6, but only in the context of the current quickselect in Webpack being incompatible with an old quickselect in Webpack (resolved by bumping the major version), not the current quickselect in Webpack being incompatible with the current quickselect in Node (still an issue).

Three potential solutions are:

• use named export mode, so require("quickselect").default would work everywhere; or
• add quickselect.default = quickselect for compatibility; or
• remove pkg.module.

[mourner]

To be honest, my preferred solution would be to not support the "require in Webpack" case at all, and leave things as is. I still strongly believe this is a major mistake on the Webpack side (using the module entry point for modules that are loaded with require) which the maintainers frustratingly refused to fix, putting the burden for this mess on maintainers of all other open source projects with dual exports.

[andersk]

Annoyance at Webpack is understandable, but I don’t think it had a better option. Using pkg.module for import and pkg.main for require could result in two independent copies of that module being bundled, which at best leads to bloat, and at worst unexpectedly bifurcates its mutable state. Using pkg.module for require but auto-resolving default would break modules with both default and non-default exports.

Perhaps this problem could have been anticipated at the specification level when ES6 modules were being designed, but that ship has sailed. :neutral_face:

Rollup does the same thing as Webpack.

$ npm i quickselect rollup @rollup/plugin-commonjs @rollup/plugin-node-resolve
$ echo 'console.log(require("quickselect"))' > src.js
$ npx rollup src.js -p node-resolve -p commonjs -f cjs -o out.js
$ node out.js
[Object: null prototype] { default: [Function: quickselect] }