SvelteKit shared load

+page.ts

import { getApiV1UserOptions } from '$lib/api/client/@tanstack/svelte-query.gen.js';
import { getLoadContext } from '$lib/load-context.js';

export const load = async (event) => {
	const { queryClient, fetch } = getLoadContext(event);

	await queryClient.prefetchQuery(
		getApiV1UserOptions({
			fetch,
		}),
	);
};

load-context.ts

import { keepPreviousData, QueryClient } from '@tanstack/svelte-query';
import { createLoadContext } from './load-singleton';
import { browser } from '$app/environment';
import { getApiV1UserOptions } from './api/client/@tanstack/svelte-query.gen';
import type { GetApiV1UserResponse } from './api/client';

export const getLoadContext = createLoadContext(({ fetch }) => {
	const queryClient = new QueryClient({
		defaultOptions: {
			queries: {
				enabled: browser,
				refetchOnMount: false,
				gcTime: 1000 * 60,
				placeholderData: keepPreviousData,
			},
		},
	});

	return {
		queryClient,
	};
});

load-singleton.ts

import { browser } from '$app/environment';
import type { LoadEvent } from '@sveltejs/kit';

export function createLoadSingleton<T>(
	factory: (event: LoadEvent) => T,
): (event: LoadEvent) => T {
	if (browser) {
		let value: T;
		return (event: LoadEvent) => {
			return (value ??= factory(event));
		};
	} else {
		const map = new WeakMap<URL, T>();

		return (event: LoadEvent) => {
			if (map.has(event.url)) {
				return map.get(event.url)!;
			}
			const value = factory(event);
			map.set(event.url, value);
			return value;
		};
	}
}

type GenericLoadContext = Record<string, any>;

interface GetLoadContext<Context extends GenericLoadContext> {
	<T extends LoadEvent>(event: T): Omit<Context, keyof T> & T;
}

export function createLoadContext<Context extends GenericLoadContext>(
	factory: (event: LoadEvent) => Context,
): GetLoadContext<Context> {
	const getter = createLoadSingleton(factory);

	return (event) => {
		return {
			...getter(event),
			...event,
		};
	};
}