By A/B testing directly on the server-side, you'll reduce layout shift from client-loaded experiments and improving your site's performance with smaller JavaScript bundles.

https://edge-functions-ab-testing-simple.vercel.sh

Since the different variants are generated statically on the server side, it mitigates any potential layout shift that could happen when a variant is inserted into the DOM client side, hence improving your site's performance.

The magic happens in the _middleware.ts file:

import { NextRequest, NextResponse } from 'next/server'
import { getBucket } from '@lib/ab-testing'
import { HOME_BUCKETS } from '@lib/buckets'

const COOKIE_NAME = 'bucket-home'

export function middleware(req: NextRequest) {
  // Get the bucket cookie
  const bucket = req.cookies[COOKIE_NAME] || getBucket(HOME_BUCKETS)
  const res = NextResponse.rewrite(`/home/${bucket}`)

  // Add the bucket to cookies if it's not there
  if (!req.cookies[COOKIE_NAME]) {
    res.cookie(COOKIE_NAME, bucket)
  }

  return res
}

Deploy the example using Vercel:

Execute create-next-app with npm or Yarn to bootstrap the example:

npx create-next-app --example https://github.com/vercel/examples/tree/main/edge-functions/ab-testing-simple ab-testing-simple
# or
yarn create next-app --example https://github.com/vercel/examples/tree/main/edge-functions/ab-testing-simple ab-testing-simple

Next, run Next.js in development mode:

npm install
npm run dev

# or

yarn
yarn dev

Deploy it to the cloud with Vercel (Documentation).