worker.js

const MODRINTH_API = 'https://staging-api.modrinth.com/v2';
const MODRINTH_DESTINATION = 'https://staging.modrinth.com';

addEventListener("fetch", (event) => {
    event.respondWith(
        handleRequest(event.request).catch(
            (err) => new Response(err.stack, { status: 500 })
        )
    );
});

async function handleRequest(request) {
    const { pathname } = new URL(request.url);

    let statusCode = 301;
    let destination = MODRINTH_DESTINATION;

    const value = await MODLINK_KV.get(pathname);
    if (value === null) {
        const [ _, type, id ] = pathname.split('/');

        let notFound = true;

        switch (type) {
            case 'p':
                const projectString = await getProjectString(id);
                if (projectString != null) {
                    notFound = false;
                    destination += projectString;
                }
                break;
            case 'v':
                // Why vresp and uresp? Because JS is dumb and treats all "cases" as a single environment
                const vresp = await fetch(`${MODRINTH_API}/version/${id}`);
                if (vresp.ok) {
                    const version = await vresp.json();

                    const projectString = await getProjectString(version['project_id']);
                    if (projectString != null) {
                        notFound = false;
                        destination += `${projectString}/version/${id}`;
                    }
                }
                break;
            case 'u':
                const uresp = await fetch(`${MODRINTH_API}/user/${id}`);
                if (uresp.ok) {
                    notFound = false;
                    destination += `/user/${id}`;
                }
                break;
        }

        if (notFound) {
            statusCode = 302;
            destination += '/not-found';
        } else {
            MODLINK_KV.put(pathname, destination);
        }
    } else {
        destination = value;
    }

    return Response.redirect(destination, statusCode);
}

async function getProjectString(id) {
    const resp = await fetch(`${MODRINTH_API}/project/${id}`);
    if (resp.ok) {
        const project = await resp.json();
        return `/${project['project_type']}/${project['slug']}`;
    }
    return null;
}