LayoutServerLoad return type is not compatible with simple interface object

[zlepper]

Describe the bug

If you explicitly declare the return type of the load function in +layout.server.ts, then you will be told by typescript that it doesn't match the expected return type. For example:

import type { LayoutServerLoad } from './$types';

export interface Session {
	authToken: string;
}

export const load: LayoutServerLoad = ({ locals }): { session: Session } => {
	return {
		session: {
			authToken: locals.authToken,
		},
	};
};

Specifically you get this nice error:

Error: Type '({ locals }: ServerLoadEvent<LayoutParams, null>) => { session: Session; }' is not assignable to type 'LayoutServerLoad'.
  Type '{ session: Session; }' is not assignable to type 'MaybePromise<void | JSONObject>'.
    Type '{ session: Session; }' is not assignable to type 'JSONObject'.
      Property 'session' is incompatible with index signature.
        Type 'Session' is not assignable to type 'JSONValue'.
          Type 'Session' is not assignable to type 'JSONObject'.
            Index signature for type 'string' is missing in type 'Session'.

Running npm run check will output the above error if you don't get it directly in your development environment

Reproduction

https://stackblitz.com/edit/sveltejs-kit-template-default-o1ssqr?file=src%2Froutes%2F%2Blayout.server.ts,package.json&terminal=dev

Stackblitz can't find the ./$types so it doesn't actually reproduce the error...

Logs

No response

System Info

System:
    OS: Windows 10 10.0.19044
    CPU: (64) x64 AMD Ryzen Threadripper PRO 3975WX 32-Cores
    Memory: 31.36 GB / 127.85 GB
  Binaries:
    Node: 16.13.1 - C:\Program Files\nodejs\node.EXE
    Yarn: 1.22.15 - C:\Program Files\nodejs\yarn.CMD
    npm: 8.1.2 - C:\Program Files\nodejs\npm.CMD
  Browsers:
    Chrome: 104.0.5112.81
    Edge: Spartan (44.19041.1266.0), Chromium (104.0.1293.54)
    Internet Explorer: 11.0.19041.1566
  npmPackages:
    @sveltejs/adapter-auto: next => 1.0.0-next.57
    @sveltejs/adapter-cloudflare: next => 1.0.0-next.31
    @sveltejs/kit: 1.0.0-next.415 => 1.0.0-next.415
    svelte: ^3.44.0 => 3.49.0
    vite: ^3.0.0 => 3.0.8

Severity

annoyance

Additional Information

No response

[elliott-with-the-longest-name-on-github]

https://github.com/tcc-sejohnson/kit-broken-layout-server-typings

Here's another repro. Turns out this can bite you even if you just have an explicitly typed variable as part of your return signature.

[Rich-Harris]

Does changing it to a type help? I seem to recall JSONObject working with types but not interfaces:

-export interface Session {
+export type Session = {
	authToken: string;
}

Not that that's a solution — ideally it'd work either way — but it'd be useful information, and is a possible workaround

[dummdidumm]

It works that way, yes, but it sucks to do that. This is essentially the reincarnation of that problem we had with the old types, which is why we introduced the BodyValidator type, only this time we probably can't use that.

[abdo643-HULK]

For deep interfaces type aliasing on the root like this type GetBody = { product: Product; jsonLd: Schema; } & { [key: string]: JSONValue }; works for now

[dummdidumm]

Ok so here's my summary of the issue(s):
Right now the OutputData generic (which represent's the load return value) is typed as extending JSONObject. JSONObject contains an index signature. This fucks up TS when you pass in an interface (in this example: Index signature for type 'string' is missing in type 'Section'). I also found no way to relax the type signature in a way that the generic ServerLoad type accepts more types but errors on a invalid types. This means we can't type "this needs to be a JSON object" (if you do const foo: JSONObject = {session: null as Session}, that will fail with the same index signature error). I therefore think we should get rid of that JSONObject entirely and instead rely on runtime checks like in #5635 - this would also be more future proof if we in the future decide to allow more sophisticated object<->string converters.