feat: allow generated tsconfig to be modified

[Rich-Harris]

closes #6868, closes #8237

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[benmccann]

I believe this won't be necessary with TypeScript 5 due out in March because if I'm remembering correctly it will allow you to extend from multiple tsconfigs. I feel like that might be the preferable solution though it would require making users wait two months.

[Rich-Harris]

Interesting. I guess it comes down to whether there are other things beyond extends that people might need to change (though if the answer is 'no' then maybe we should have just merged #8237 instead...)

[benmccann]

If the user has to change things I think that's fine. They can decide whether they want to upgrade and if they care about extending multiple tsconfigs then they can do that.

I think it would become more problematic if there were things in SvelteKit that needed to be changed to allow you to run TypeScript 5. As far as I can tell, thus far we're probably safe and SvelteKit should work with TS 4 or 5: https://github.com/microsoft/TypeScript/wiki/API-Breaking-Changes. npm run check runs successfully against the demo project with TypeScript 5.0.0-dev.20230119

[Rich-Harris]

The question is 'aside from extends, is there something in the generated tsconfig.json that someone might have a reason to change?' — unrelated to TypeScript 4 vs 5

[Rich-Harris]

Marked this as draft because it probably makes sense to wait until TypeScript 5 if extends is the only reason to need this ability, rather than expanding configuration surface area. If we discover other use cases in the meantime, we can undraft it.

Until then, there's a workaround, so no-one is blocked on this even if it's a little inconvenient: manually copy the monorepo config into packages.

[baisong]

Clarifying question, is this related to this typescript note from the (experimental) svelte-package docs?

Setting "moduleResolution": "NodeNext" in your tsconfig.json or jsconfig.json will help you with [adhering to Node's ESM algorithm].

Mentioning because I've recently been modifying the default jsconfig.json, tsconfig.json and .svelte-kit/tsconfig.json while troubleshooting using svelte-package to publish a component library npm module.

I noticed that, even when "moduleResolution": "NodeNext" is present in all three files, svelte-package always overwrites the autogenerated .svelte-kit/tsconfig.json to be "moduleResolution": "node".

[Rich-Harris]

@baisong that's unrelated to this PR/issue — this is purely about the contents of .svelte-kit/tsconfig.json

[anthonator]

I'd be curious if this feature would solve an issue we're currently having.

When extending the @typescript-eslint/recommended-requiring-type-checking eslint config we get the following error.

Parsing error: ESLint was configured to run on `<tsconfigRootDir>/svelte.config.ts` using `parserOptions.project`: <tsconfigRootDir>/../../../../../users/tyranthosaur/projects/imable/ui/tsconfig.json

We've tried adding ./svelte.config.ts to the include property in the project root's tsconfig.json but unfortunately this performs an override instead of a merge of the include from ./.svelte-kit/tsconfig.json. We got around this issue by creating a tsconfig.eslint.json with an include property we copy/pasted from ./.svelte-kit/tsconfig.json. Definitely not ideal.

It seems like this feature would allow people to get around issues like this in a more elegant way.

[Rich-Harris]

Yep, that seems like a good enough reason! Marking this PR ready