Prioritize ES5 (main) over ES6 (module) in webpack resolve.mainFields config [build failures are irrelevant]

[jedwards1211]

Since Webpack by default loads from module instead of main in package.json, any library that publishes both ES5 and ES6 this way will trigger minification errors with create-react-app.
But if we change the Webpack config to try main first, libraries that publish both ES5 and ES6 won't have this problem, and create-react-app will still do its best on libraries that only have ES6.

I'm really surprised no one has made this change already. Others have suggested this change yet I thought of it independently, so I would think it's obvious.

Here's how I came upon this, devised my fix and verified that it works:

1. A user of my material-ui-render-props-styles package, which publishes both ES5 and ES6, reported problems with create-react-app and gave me an example repo: https://github.com/solayao/dizzy_comic_fe
2. I cloned that repo and installed deps in it, confirmed yarn build has the minification error
3. Then I ran npm edit react-scripts (I tried linking in my fork, but a few too many things had changed from the version the project was using)
4. I added mainFields: ['browser', 'main', 'module'] to its webpack.config.prod.js resolve configuration
5. retried yarn build -- it worked!

[Timer]

This will be resolved in v2, but thank you for the PR!

We want webpack to resolve module, then main. Jest should resolve straight to main.

[jedwards1211]

I'm confused, why do you want to resolve module first, given the minification issues?

[Timer]

These issues only exist in v1 -- they're fixed in v2.

[jedwards1211]

I see. Would it be helpful to make this a patch release to v1 though?

[Timer]

It would be a breaking change so it can't land in v1.

[jedwards1211]

I get what you mean by that but,

• as far as I can imagine, anything that this change would affect was already broken
• what working projects would it actually break? In any package that has both main and module, the main should just be the transpiled form of module and hence should work in its place.