I've got this snippet that should set the cookie in the middleware (Next.js).

const growthbook = new GrowthBook({
    attributes: { id: visitor_id },
    features: await getFeatures(),
    trackingCallback: (experiment, result) => {
      console.log("EXPERIMENT", experiment)
      console.log("RESULT", result)
      res.cookie("GB_EXPERIMENTS", JSON.stringify([
        {
          key: experiment.key,
          variationId: String(result.variationId)
        }
      ]))
    }
  });

are the console.log's firing?

the console logs are firing

make sure you're changing the visitor_id.

visitor ID does change

I'm not sure what you're using to set the cookie, but perhaps that's not working

what do you mean what I'm using to set the cookie?

try this - find a visitor ID that returns variation 1

changed it, and it doesn't set it

possibly

well, it could be something different about the callback on variants

fwiw, you're continuously setting the same cookie here - you'd need to aggregate all the experiments into one array before setting, or it'll keep overwriting

I'm only calling

trackingCallback

once, in the middleware

@future-teacher-7046 might have some ideas when he's up in a few hours

I think I know what's going on. Are you doing a redirect for variation 1? If so, you have to make sure to set the cookie on the redirect response in addition to the normal response. https://github.com/vercel/next.js/discussions/34822

hmmm...

// Pick which page to render depending on a feature flag
  let res = NextResponse.next();
  if (growthbook.feature("prorb").on) {
    const url = req.nextUrl.clone();
    url.pathname = "/lp/practical-ruby-on-rails-for-beginners-20220618";
    res = NextResponse.rewrite(url);
  }

Yes, it's that

res = NextResponse.rewrite

part that's causing the issue. When you create a new response object it loses everything you did earlier such as setting cookies

I see... ok, let me try and fix that.

To fix it, I would have the tracking callback update a variable instead of setting a cookie directly. Then before you return a response object, set the cookie on it.

yeah, thank you

Yes. isOn will be false if the flag is "off" for the user

ah, ok... so it's going to be "on" just for people who see the variant?

Correct

Cool, so it seems to work now. Does this code look right to you? Is there anything I'm missing?

let visitor_id = req.cookies[COOKIE] || crypto.randomUUID()
  let experimentKey = null;
  let variantId = null;

  const growthbook = new GrowthBook({
    attributes: { id: visitor_id },
    features: await getFeatures(),
    trackingCallback: (experiment, result) => {
      experimentKey = experiment.key;
      variantId = result.variationId;
    }
  });

  // Pick which page to render depending on a feature flag
  let res = NextResponse.next();

  if (growthbook.feature("prorb").on) {
    const url = req.nextUrl.clone();
    url.pathname = "/lp/practical-ruby-on-rails-for-beginners-20220618";
    res = NextResponse.rewrite(url);

  }
  res.cookie("GB_EXPERIMENTS", JSON.stringify([
    {
      key: experimentKey,
      variationId: variantId
    }
  ]))

  // Store the visitor cookie if not already there
  if (!req.cookies[COOKIE]) {
    res.cookie(COOKIE, visitor_id)
  }

  return res

yes, that looks ok to me, as long as your front-end code can handle when the key and variationId in the cookie are null (e.g. if you turn off the experiment)

Awesome, thank you for your help.