NextJS Middleware Javascript

middleware.js

import { NextResponse } from 'next/server';

export const config = {
  matcher: ['/'],
};

const THRESHOLD = 0.5; 
const COOKIE_NAME = 'tm_var';

export function middleware(req) {
  const variant = req.cookies.get(COOKIE_NAME) || (Math.random() < THRESHOLD ? 'new' : 'old');
  const url = req.nextUrl.clone();

  if (variant.value === 'new' || variant === 'new') {
    url.pathname = '/index-new';
  }

  const res = NextResponse.rewrite(url);

  if (!req.cookies.get(COOKIE_NAME)) {
    res.cookies.set(COOKIE_NAME, variant, { path: '/' });
  }

  return res;
}