Using SvelteKit server hooks as a reverse proxy

Warning

The following self-hosted proxy isn't provided by PostHog, so we can't take responsibility for it! If unsure, we recommend using our managed reverse proxy.
If you are using the EU cloud then use eu instead of us in all domains (e.g. us.i.posthog.com -> eu.i.posthog.com).
Avoid using generic or common path names like /analytics, /tracking, /ingest, or /posthog for your reverse proxy. They will most likely be blocked. Instead, use a non-obvious path name or something random and unique to your application that's unlikely to appear in a filter list.

For SvelteKit, you can use server hooks to proxy requests to PostHog. We've tested this (and it works) with Cloudflare Workers.

To do this, create a file named hooks.server.ts in your src directory (or the dir you configured to contain source files). In this file, set up code to match requests to a custom route, set a new host header, change the URL to point to PostHog, and rewrite the response.

Note: This only works in SSR mode. If your site is statically generated, SvelteKit ignores hooks.server.ts.

TypeScript

import type { Handle } from '@sveltejs/kit';

export const handle: Handle = async ({ event, resolve }) => {
  const { pathname } = event.url;

  if (pathname.startsWith('/<ph_proxy_path>')) {
    // Determine target hostname based on static or dynamic ingestion
    const hostname = pathname.startsWith('/<ph_proxy_path>/static/')
      ? 'us-assets.i.posthog.com' // change us to eu for EU Cloud
      : 'us.i.posthog.com';  // change us to eu for EU Cloud

    // Build external URL
    const url = new URL(event.request.url);
    url.protocol = 'https:';
    url.hostname = hostname;
    url.port = '443';
    url.pathname = pathname.replace('/<ph_proxy_path>/', '');

    // Clone and adjust headers
    const headers = new Headers(event.request.headers);
    headers.set('host', hostname);

    // Proxy the request to the external host
    const response = await fetch(url.toString(), {
      method: event.request.method,
      headers,
      body: event.request.body
    });

    return response;
  }

  const response = await resolve(event);
  return response;
};

Once done, configure the PostHog client to send requests via your rewrite like we do in this sample:

TypeScript

// src/routes/+layout.ts
import posthog from 'posthog-js';
import { browser } from '$app/environment';
import { PUBLIC_POSTHOG_KEY as POSTHOG_KEY } from '$env/static/public';

export function initTelemetry() {
  if (!browser) return;
  posthog.init(POSTHOG_KEY, {
    api_host: '/<ph_proxy_path>',
    ui_host: 'https://us.posthog.com', // change us to eu for EU Cloud
    person_profiles: 'always',
    persistence: 'localStorage'
    });
}

initTelemetry();

Questions? Ask Max AI.

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Edwin Lim

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