What's controlled by the module type setting in package.json Next project? should it ever be set at all?

[djechlin]

Summary

• In a Node.js javascript project, it's clear what type: 'module' does, as all the Javascript files will either be cjs style with 'require' or mjs style with 'import.'
• In a typescript project - and my understanding gets weaker here - that field doesn't really affect how programmers write typescript. The tsconfig.json will include a setting like module: esnext that affects the .ts code, and whatever TS compiler is used, like tsc, will consult that value to read in the ts and consult package.json type to output to the correct cjs or mjs.
• For a next project, build is more complicated, as any given .ts code can run in several environments: build-time, request-time, server or client. So next owns all its own compilation with its own compiler built on swc.
• Which raises the question, why would a programmer of a next project ever touch the type field?

I see various issues related to it, like some users apparently set it to module and break their build, but I'm missing the conceptual 'why'. To me this just looks like fiddling with a nextjs build internal and it would affect the code in a haphazard way at best (forex it shouldn't affect the client bundle at all).

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