Make adapters smaller and easier

[bobTheBuilder7]

Right now the node adapter forces everyone to use either polka or express. I think that it might be a good idea to make SvelteKit runtime agnostic. The current approach with different adapters is supposed to do that but the burden is on library maintainers to keep them updated for each framework and that makes using SvelteKit with Deno or Bun an unpleasant experience.

I think it might be a good approach that after building the project, build/index.js exported something like an app object with a fetch function that has the following type (Request) => Response | Promise<Response>. This would make SvelteKit easy to integrate with any current or future runtimes.

It would help end users of different runtimes to use the built project like this:

Deno:

import { app } from "./build/index.js"`

Deno.serve(app.fetch)

Bun:

import { app } from "./build/index.js"

Bun.serve(app.fetch)

A few things to consider:

1. The server code generated by the node adapter uses some node:* packages that are either not available or not the recommended to be used because of better alternatives.

A possible solution might be for the end user to bring the fs interaction, that way SvelteKit stays agnostic.

/** @type {import('@sveltejs/kit').Config} */
const config = {
	preprocess: vitePreprocess(),
	kit: {
		adapter: adapter({
			readFile: async (path) => {
                            return await Deno.file(path)
                            or
                            return await Bun.file(path)
                            or
                            return readFileSync(path)
                            or
                            return whateverThatRuntimeHas(path)
                        }
		}),
	}
};

2. How would things like Vercel and Cloudflare workers work with this approach.

As far as I know they already support this kind of behavior. But I might be wrong.

Why I think this is important?

1. If this ever gets implemented it would make it much easier to make a new adapter because any runtime can implement a simple (Request) => Response | Promise<Response>.
2. Right now all the adapters have some duplicate code, with this approach that code is the same for all adapters and the adapters only define how are the files read and the underlying http server.

I would love to hear everyone's feedback on this!

[bobTheBuilder7]

Hono already uses the same approach (I do understand that Hono and SvelteKit do completely separate things but still)
Deno

import { Hono } from 'hono'

const app = new Hono()

app.get('/', (c) => c.text('Hello Deno!'))

Deno.serve(app.fetch)

Bun

import { Hono } from 'hono'

const app = new Hono()

app.get('/', (c) => c.text('Hello Bun!'))

Bun.serve(app.fetch)

Vercel

import { Hono } from 'hono'

const app = new Hono()

const welcomeStrings = [
  'Hello Hono!',
  'To learn more about Hono on Vercel, visit https://vercel.com/docs/frameworks/backend/hono',
]

app.get('/', (c) => {
  return c.text(welcomeStrings.join('\n\n'))
})

export default app

Cloudflare Workers

import { Hono } from 'hono'
const app = new Hono()

app.get('/', (c) => c.text('Hello Cloudflare Workers!'))

export default app