Simple helper you can use to communicate with Payload CMS from your application via their REST API. Authorization via API key needs to be enabled in your users collection. More on enabling API keys here: https://payloadcms.com/docs/authentication/config#api-keys

+page.server.ts

import { getDocumentByID } from "./PayloadCommunicator.ts";
import type { PageServerLoad } from './$types';
 
export const load = (async ({ params }) => {
    return await getDocumentByID('collection', params.id);
}) satisfies PageServerLoad;

PayloadCommunicator.ts

import qs from 'qs';
import { error } from '@sveltejs/kit';

export const cmsURL = "cms URL without the / at the end";
export const apiKey = "your api key";

export async function getDocuments (collectionName: string, whereQuery?) {
    if (whereQuery) {
        const stringifiedQuery = qs.stringify({
            where: whereQuery 
        }, { addQueryPrefix: true });
    
        const potentialFoundDocument = await (await fetch(`${cmsURL}/api/${collectionName}${stringifiedQuery}`, {
            method: 'GET',
            headers: {
                "Content-Type": "application/json",
                Authorization: `users API-Key ${apiKey}`
            },
        })).json()

        if (potentialFoundDocument.errors) {
            throw error(
                404,
                potentialFoundDocument.errors[0]
            )
        }

        return potentialFoundDocument;
    } else {
        const potentialFoundDocument = await (await fetch(`${cmsURL}/api/${collectionName}`, {
            method: 'GET',
            headers: {
                "Content-Type": "application/json",
                Authorization: `users API-Key ${apiKey}`
            },
        })).json()

        if (potentialFoundDocument.errors) {
            throw error(
                404,
                potentialFoundDocument.errors[0]
            )
        }

        return potentialFoundDocument;
    }
}

export async function getDocumentByID (collectionName: string, id: string) {
    const potentialFoundDocument  = await (await fetch(`${cmsURL}/api/${collectionName}/${id}`, {
        method: 'GET',
        headers: {
            "Content-Type": "application/json",
            Authorization: `users API-Key ${apiKey}`
        },
    })).json()

    if (potentialFoundDocument.errors) {
        throw error(
            404,
            potentialFoundDocument.errors[0]
        )
    }

    return potentialFoundDocument;
}

export async function newDocument (collectionName: string, initialFields) {
    const requestResponse = await (await fetch(`${cmsURL}/api/${collectionName}`, {
        method: 'POST',
        headers: {
            "Content-Type": "application/json",
            Authorization: `users API-Key ${apiKey}`
        },
        body: JSON.stringify(initialFields),
    })).json();

    if (requestResponse.errors) {
        throw error(
            500,
            requestResponse.errors[0]
        )
    }

    return requestResponse;
}

export async function updateDocument (collectionName: string, documentToUpdateID: string, updatedFields) {
    const requestResponse = await (await fetch(`${cmsURL}/api/${collectionName}/${documentToUpdateID}`, {
        method: 'PATCH',
        headers: {
            "Content-Type": "application/json",
            Authorization: `users API-Key ${apiKey}`
        },

        body: JSON.stringify(updatedFields),
    })).json();

    if (requestResponse.errors) {
        throw error(
            500,
            requestResponse.errors[0]
        )
    }

    return requestResponse;
}