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A/B testing with same-URL direct access

Set up an A/B test by controlling what response is served based on cookies. This version supports passing the request through to test and control on the origin, bypassing random assignment.
const NAME = "myExampleWorkersABTest";
export default {
async fetch(req) {
const url = new URL(req.url);
// Enable Passthrough to allow direct access to control and test routes.
if (url.pathname.startsWith("/control") || url.pathname.startsWith("/test")) return fetch(req);
// Determine which group this requester is in.
const cookie = req.headers.get("cookie");
if (cookie && cookie.includes(`${NAME}=control`)) {
url.pathname = "/control" + url.pathname;
} else if (cookie && cookie.includes(`${NAME}=test`)) {
url.pathname = "/test" + url.pathname;
} else {
// If there is no cookie, this is a new client. Choose a group and set the cookie.
const group = Math.random() < 0.5 ? "test" : "control"; // 50/50 split
if (group === "control") {
url.pathname = "/control" + url.pathname;
} else {
url.pathname = "/test" + url.pathname;
}
// Reconstruct response to avoid immutability
let res = await fetch(url);
res = new Response(res.body, res);
// Set cookie to enable persistent A/B sessions.
res.headers.append("Set-Cookie", `${NAME}=${group}; path=/`);
return res;
}
return fetch(url);
}
}
const NAME = "myExampleWorkersABTest";
const handler: ExportedHandler = {
async fetch(req) {
const url = new URL(req.url);
// Enable Passthrough to allow direct access to control and test routes.
if (url.pathname.startsWith("/control") || url.pathname.startsWith("/test")) return fetch(req);
// Determine which group this requester is in.
const cookie = req.headers.get("cookie");
if (cookie && cookie.includes(`${NAME}=control`)) {
url.pathname = "/control" + url.pathname;
} else if (cookie && cookie.includes(`${NAME}=test`)) {
url.pathname = "/test" + url.pathname;
} else {
// If there is no cookie, this is a new client. Choose a group and set the cookie.
const group = Math.random() < 0.5 ? "test" : "control"; // 50/50 split
if (group === "control") {
url.pathname = "/control" + url.pathname;
} else {
url.pathname = "/test" + url.pathname;
}
// Reconstruct response to avoid immutability
let res = await fetch(url);
res = new Response(res.body, res);
// Set cookie to enable persistent A/B sessions.
res.headers.append("Set-Cookie", `${NAME}=${group}; path=/`);
return res;
}
return fetch(url);
}
};
export default handler;