Google Analytics for Cloudflare Workers

ga_cfw.js

/********************************
 * GA data
 ********************************/
let data = {
  v: 1,
}
/********************************
 * Initializes GA data
 * @param {string} tid your tracking id for GA
 * @param {object} req the request object from event.request
 * @param {string} dp the default path you want GA to use (can override in pageview)
 ********************************/
export const init = (tid, { url: dl, headers }, dp) => {
  const uip = headers.get('cf-connecting-ip') //calling user ip (for geo)
  const ua = headers.get('user-agent')
  const dr = headers.get('referer')
  let cid = headers.get('cookie') //use the cloudflare cookie as a cid for ga
  if (!cid) cid = `anon_${Date.now()}`
  else cid = `cf_${cid.split('__cfduid=')[1]}`
  data = { ...data, tid, cid, uip, ua, dr, dl, dp }
  console.log('ga init', data)
}
/********************************
 * Sends a page view to GA
 * @param {string} dt the page title
 * @param {string} dp the path
 * @returns a promise (should be waited on by caller)
 ********************************/
export const pageview = async (dt, dp = data.dp) => {
  await send({ ...data, t: 'pageview', dt, dp, })
}
/********************************
 * Sends an event to GA
 * @param {string} ec event category
 * @param {string} ea event action
 * @param {string} el event label
 * @param {number} ev event value
 * @returns a promise (should be waited on by caller)
 ********************************/
export const event = async (ec, ea, el = "", ev = 0) => {
  await send({ ...data, t: 'event', ec, ea, el, ev, })
}
/********************************
 * Sends the data to GA
 * @returns a promise
 ********************************/
const send = async (data) => {
  const uri = Object.keys(data).map((k) => `${k}=${data[k]}`).join('&')
  await fetch('http://www.google-analytics.com/collect?' + uri)
}

You need to use wrangler from cloudflare and in your wrangler.toml set type = webpack because it's a module.
Save this gist as it's own JS file, in my case util/ga.js
Then in your request handler file, use it something like this:

import { init as initGA, pageview, event } from './util/ga'
addEventListener('fetch', (e) => {
	e.respondWith(handleRequest(e.request, e))
})
async function handleRequest(req, e) {
	const path = req.url.split('.com')[1] //or whatever your tld is
	//GA
	initGA(UA-XXXXXXXXXXX-X, req, path)
	await pageview(`hello from my cloudflare worker`)
...
}

Fill in your UA-XXXXXXXXXXX-X