Proxy with SvelteKit

// src/routes/sp/[...path]/+server.ts
// Serves Spreelytics from /sp/ on your site: /sp/js/ is the script, everything else is tracking.
import type { RequestHandler } from "./$types";

const SCRIPTS = "https://cdn.spreelytics.com/cdn/sdk/";
const API = "https://sdk.spreelytics.com/1.0/";
const FORWARD = ["content-type", "user-agent", "origin", "referer", "accept", "accept-language"];

const proxy: RequestHandler = async ({ request, params, url, getClientAddress }) => {
  const path = params.path;
  const target = (path.startsWith("js/") ? SCRIPTS + path.slice(3) : API + path) + url.search;

  const headers = new Headers();
  for (const name of FORWARD) {
    const value = request.headers.get(name);
    if (value) headers.set(name, value);
  }
  // The visitor's IP address.
  headers.set("x-forwarded-for", getClientAddress());

  const upstream = await fetch(target, {
    method: request.method,
    headers,
    body: request.method === "GET" || request.method === "HEAD" ? undefined : await request.arrayBuffer(),
  });

  // fetch has already unzipped the body, so these headers no longer apply.
  const responseHeaders = new Headers(upstream.headers);
  responseHeaders.delete("content-encoding");
  responseHeaders.delete("content-length");
  return new Response(upstream.body, { status: upstream.status, headers: responseHeaders });
};

export { proxy as GET, proxy as POST };

<script src="/sp/js/spreelytics-loader.js"></script>
<script>
  Spreelytics.init({
    client_key: "YOUR_CLIENT_KEY",
    api_host: "https://www.example.com/sp",
  });
</script>