Ship stdlib es6 format in .mjs extension

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Note we are going to land a PR which allows user to customize its file extension: https://github.com/rescript-lang/rescript-compiler/issues/4687

With this customization, so we don’t need fight which one is better, so user can just pick what they like.

For the stdlib js files which is shipped with bs-platform, currently it uses .js extension, in such case, it is hard to use such es6 format under JS directly(without jumping hoops), so I am considering changing such files from bs-platform/lib/es6/list.js to bs-platform/lib/es6/list.mjs, would it potentially break anything?

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Yeah it would potentially break 99% of the projects, because webpack is usually not set up to handle mjs (in my understanding). Especially it will be higher friction to use ReScript for for instance React-Native where I’ve never seen a mjs extension.

Directly serving files is a bit of fringe use case (what would be a real world use-case?). For performance you usually still do some bundling, even on HTTP2. For node, it can handle .js perfectly fine.

I have never seen any major project use mjs, so if we align with what is most used in the ecosystem why would this be the default?

BTW I don’t think mjs is required, it is just recommended (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Modules). Tools like TypeScript (important for interop) may never support mjs.

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