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A/B testing with middleware

Set up an A/B test by controlling what page is served based on cookies. This version supports passing the request through to test and control on the origin.
const cookieName = "ab-test-cookie"
const newHomepagePathName = "/test"
const abTest = async (context) => {
const url = new URL(context.request.url)
// if homepage
if (url.pathname === "/") {
// if cookie ab-test-cookie=new then change the request to go to /test
// if no cookie set, pass x% of traffic and set a cookie value to "current" or "new"
let cookie = request.headers.get("cookie")
// is cookie set?
if (cookie && cookie.includes(`${cookieName}=new`)) {
// pass the request to /test
url.pathname = newHomepagePathName
return context.env.ASSETS.fetch(url)
} else {
const percentage = Math.floor(Math.random() * 100)
let version = "current" // default version
// change pathname and version name for 50% of traffic
if (percentage < 50) {
url.pathname = newHomepagePathName
version = "new"
// get the static file from ASSETS, and attach a cookie
const asset = await context.env.ASSETS.fetch(url)
let response = new Response(asset.body, asset)
response.headers.append("Set-Cookie", `${cookieName}=${version}; path=/`)
return response
export const onRequest = [abTest];