Modularize SvelteKit

[benmccann]

Describe the problem

A popular point of feedback from the other maintainers is that SvelteKit is not as modular as we had hoped we might make it at the beginning of the project

Describe the proposed solution

We could possible split out a few different pieces:

• The core router without file system support
• The file system router depending on the core router
• Kit itself with Vite integration. We've already split out vite-plugin-svelte and I'm not sure there's much more that makes sense to split out there
• Put optional features in plugins (e.g. AMP Move amp support to a plugin #2603)

I'd written a bit about this in sveltejs/rfcs#36 originally for Sapper and tried to update the proposal a bit for SvelteKit though further updates may be needed

I don't know whether it'd be possible to make the router implementation pluggable, but I think it'd be a cool goal. I also think it'd be nice to remove any globals which would help for testing (right now you can mostly only write tests if you use Jest and mock the globals) and allow you to have multiple router instances (@pngwn has long asked for microfrontend support and while that's not something I'm overly familiar with I could imagine multiple routers getting pulled in)

Alternatives considered

No response

Importance

nice to have

Additional Information

No response

[jarda-svoboda]

A pluggable router might be a way how to possibly create something like this #1251 🤔

[AlbertMarashi]

+1 on standalone router option. Being able to create more configurable routes would be great.

I've created a runtime router for svelte before, so if you want me to provide thoughts I can too.

I had this issue related to generating manifests with for the router though.
sveltejs/svelte#4854

[bertybot]

Do you think we could put in the option to override the default start.js file? The start.js file makes the system a whole lot less modular. We force an entry into the client with a bunch overhead even if the user doesn't use most of the features, and it also doesn't allow any configuration of the router.

It would be nice if we could write our own start.js or Sveltkit constructor like other JS frameworks. Vue allows you to set the entry with a Vue constructor and configure the features you want al a carte like a Router. We could fall back to the default one if the user don't supply one. It would also save on bundle size since right now the router is still loaded even if the user doesn't use it and ends up as a lot of dead code.