A/B testing

Split traffic between two responses in a function, picking a group at random and recording it in a cookie.

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Serve one of two responses from a function on Azion’s global network, choosing the group at random and recording it in a cookie. Use this pattern to run randomized experiments, such as comparing two page variants, without sending traffic to an origin to decide which version to show.

JavaScript
01 function handleRequest(request) {
02 const NAME = "TestA/B"
03 const TEST_RESPONSE = new Response("Cookie A")
04 const CONTROL_RESPONSE = new Response("Cookie B")
05
06 const cookie = request.headers.get("cookie")
07 if (cookie && cookie.includes(`${NAME}=a`)) {
08 return CONTROL_RESPONSE
09 }
10 else if (cookie && cookie.includes(`${NAME}=b`)) {
11 return TEST_RESPONSE
12 }
13 else {
14 const group = Math.random() < 0.5 ? "test" : "control"
15 const response = group === "control" ? CONTROL_RESPONSE : TEST_RESPONSE
16 response.headers.append("Set-Cookie", `${NAME}=${group}; path=/`)
17
18 return response
19 }
20 }
21
22 addEventListener("fetch", event => {
23 event.respondWith(handleRequest(event.request))
24 })

How it works

The fetch handler reads the incoming Cookie header with request.headers.get("cookie"). When the header contains TestA/B=a the function returns the control response, and when it contains TestA/B=b it returns the test response. When neither is present, Math.random() picks the test or control group at roughly 50/50, and response.headers.append("Set-Cookie", ...) writes the chosen group into the TestA/B cookie so the browser sends it on the next request. event.respondWith() then returns that Response to the client.

JavaScript examples

Browse the other ready-to-use examples you can copy into a function.

FetchEvent

Reference for the fetch event and the request the handler receives.

Response

Reference for the response object this example builds and sets a cookie on.

Functions limits

The ceilings a single invocation of this function runs inside, and the ones that vary by plan.

Quickstart

What an account needs before a function can run, and the steps that put one in the path of a request.

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