Add generic type for session

[seanlail]

Previously, in Sapper, we could use generics to ensure our session type was used correctly:

E.g. if I had a session type that was { foo: string; }

import { stores } from "@sapper/app";

const { session } = stores<MySessionType>();

$session.foo; // correct usage
$session.bar; // invalid, compiler shows error

This PR adds the same generic support in getStores as well as a 3rd param in the generic for LoadInput

SvelteKit:

import { getStores } from "$app/stores";

const { session } = getStores<MySessionType>();

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[seanlail]

@dummdidumm GrygrFlzr pointed out on Discord that this PR doesn't help with this:

import { session } from '$app/stores';

I wonder if there's anything we could do here?
I had a quick search and found this https://stackoverflow.com/questions/42986950/how-to-define-import-variable-type

i.e. update your own ambient type per project, something like this:

declare module "$app/stores" {
    const session : MySessionType;
    export = session;
}

If that's the only solution, perhaps it should be documented?

[dummdidumm]

That seems like a useful addition, but I wouldn't add it to the site docs, only to the jsdoc in the ambient-modules.d.ts file

[seanlail]

Odd. I just tested this in the example project.

In a component (with a typo: fo instead of foo)

<script lang="ts">
  import { session } from "$app/stores";
</script>

<div>{$session.fo}</div>

This doesn't work, VSCode and build / lint scripts think session in the component is Writable<any>

/// <reference types="@sveltejs/kit" />

declare module "$app/stores" {
  import type { Writable } from "svelte/store";

  export const session: Writable<{ foo: string; bar: string }>;
}

However, if I remove the reference it all works and VSCode picks up the error.

Does this line actually need to be included?

/// <reference types="@sveltejs/kit" />

[dummdidumm]

Yes it needs to be included, else other imports from $.. and .svelte imports in TS components are no longer recognized by TypeScript (giving false positives). My guess is that the broader type wins here, since both definitions are loaded, so this approach won't work. If people really want session to be a certain writable, I guess they have to do something like export const myTypedSession: Writable<SomeInterface> = session. But I wouldn't add this to the docs then.

[janosh]

I'm prob missing something. Why doesn't SvelteKit propagate the return type of getSession from src/hooks.ts?

I have

// src/hooks.ts

export function getSession(): Record<string, string> {
  // ...
  return session
}

yet when I import

import { session } from '$app/stores'

it has type

(alias) const session: Writable<any>