vercel__flags/examples/sveltekit-example/middleware.ts

import { normalizeUrl } from '@sveltejs/kit';
import { rewrite } from '@vercel/edge';
import { parse } from 'cookie';
import { marketingABTestManualApproach } from './src/lib/flags';
import {
  computeInternalRoute,
  createVisitorId,
} from './src/lib/precomputed-flags';

export const config = {
  // Either run middleware on all but the internal asset paths ...
  // matcher: '/((?!_app/|favicon.ico|favicon.png).*)'
  // ... or only run it where you actually need it (more performant).
  matcher: [
    '/examples/marketing-pages-manual-approach',
    '/examples/marketing-pages',
    // add more paths here if you want to run A/B tests on other pages, e.g.
    // '/something-else'
  ],
};

export default async function middleware(request: Request) {
  const { url, denormalize } = normalizeUrl(request.url);

  if (url.pathname === '/examples/marketing-pages-manual-approach') {
    // Retrieve cookies which contain the feature flags.
    let flag = parse(request.headers.get('cookie') ?? '').marketingManual || '';

    if (!flag) {
      flag = String(Math.random() < 0.5);
      request.headers.set('x-marketingManual', flag); // cookie is not available on the initial request
    }

    return rewrite(
      // Get destination URL based on the feature flag
      denormalize(
        (await marketingABTestManualApproach(request))
          ? '/examples/marketing-pages-variant-a'
          : '/examples/marketing-pages-variant-b',
      ),
      {
        headers: {
          'Set-Cookie': `marketingManual=${flag}; Path=/`,
        },
      },
    );
  }

  if (url.pathname === '/examples/marketing-pages') {
    // Retrieve cookies which contain the feature flags.
    let visitorId = parse(request.headers.get('cookie') ?? '').visitorId || '';

    if (!visitorId) {
      visitorId = createVisitorId();
      request.headers.set('x-visitorId', visitorId); // cookie is not available on the initial request
    }

    return rewrite(
      // Get destination URL based on the feature flag
      denormalize(await computeInternalRoute(url.pathname, request)),
      {
        headers: {
          'Set-Cookie': `visitorId=${visitorId}; Path=/`,
        },
      },
    );
  }
}