Preconstruct ignores "type": "module"

[steve-taylor]

Regardless of whether or not one of my package's package.json contains "type": "module", Preconstruct always makes "main" point to the CommonJS bundle. However, bundlers generally expect a dependency with "type": "module" to be an ESM package whose package.json "main" points to the ESM build. An ESM package shouldn't have a CommonJS bundle.

As an example, this is how Parcel works.

Library targets automatically output either native ES modules or CommonJS depending on the target.

main – by default, outputs CommonJS. If the .mjs extension is used, or the "type": "module" field is specified, then an ES module is output instead.
module – outputs an ES module.
browser – a browser-specific override of the main field. Outputs CommonJS.

I'm running into this issue because one of my packages uses react-markdown which is an ESM dependency, but in my unwanted CommonJS bundle, it's being required.

The following package.json snippet should be invalid:

"type": "module",
"main": "dist/my-package.cjs.js",

and should be fixed as

"type": "module",
"main": "dist/my-package.esm.js",

And

"type": "module",
"main": "dist/my-package.cjs.js",
"module": "dist/my-package.cjs.js",

should become

"type": "module",
"main": "dist/my-package.esm.js",

Conversely,

"main": "dist/my-package.esm.js",

should become

"main": "dist/my-package.cjs.js",

as it's not an ESM package.

If you fix this, not only will it make Preconstruct behave correctly, but it will also slash build times for packages with "type": "module" due to CommonJS bundles no longer being built for such packages.