Add configuration options for SvelteKit Adapter handler resolve function

[scott181182]

Goals

1. Allow developers to specify configuration values they want the Auth.js handler to pass to SvelteKit's resolve function, like transformPageChunk, filterSerializedResponseHeaders, and preload

Non-Goals

No response

Background

I'm using Auth.js in my SvelteKit project, in which I use load functions to fetch data from my API so I can Server-Side Render (SSR) my views. The API client that I'm using (ts-rest) needs access to the "Content-Type" header on the API responses in order to parse the response body. For pre-rendering pages, this works wonderfully, but I'm now integrating with TanStack which requires the API queries to be available on the client-side (and therefore during hydration). This makes my API client send fetch requests during hydration on the client-side, but SvelteKit caches these responses so it doesn't actually need to send a network request when SvelteKit's fetch function takes over. Fantastic, we love to see SSR working.

However, SvelteKit doesn't expose any of the headers on these cached requests by default. They even have a nice configuration option to allow some headers, like "Content-Type", to be passed through. Now I'm following the Auth.js documentation to setup my hooks.server.ts, but I can't configure it at all to pass those headers through that my API client needs to parse cached responses during client-side hydration.

The workaround is fairly straightforward: mask the handle invocation and give it your own resolve function that does apply the wanted options:

import { handle as authHandle } from "./auth";

export function handle({ event, resolve }) {
    return authHandle({
        event,
        resolve(event, opts) {
            return resolve(event, {
                ...opts,
                filterSerializedResponseHeaders: (name, _value) => name === "content-type",
            });
        },
    });
}

I think this is unnecessarily verbose compared to being able to specify this in the SvelteKitAuth function, especially if there were multiple places I want to use the handle function it returns.

Proposal

I think @auth/sveltekit should add an additional field to the AuthConfig interface, along the lines of:

import { ResolveOptions } from "@sveltejs/kit";

export interface AuthConfig {
    // ...existing fields...
    resolveOptions?: ResolveOptions;
}

This can then be passed to the resolve invocation in the SvelteKitAuth function:

export function SvelteKitAuth(config) {
    return {
        signIn: async (event) => { /* stuff */ },
        signOut: async (event) => { /* stuff */ },
        async handle({ event, resolve }) {
            // ...existing code...
            return resolve(event, _config.resolveOptions);
        },
    };
}

This passthrough option would allow developers more control over their server handlers as SvelteKit changes.