next.js/test/production/standalone-mode/required-server-files/middleware.js

import { NextResponse } from 'next/server'
import { ImageResponse } from 'next/og'

export async function middleware(req) {
  console.log('middleware', req.url)
  if (req.nextUrl.pathname === '/a-non-existent-page/to-test-with-middleware') {
    return new ImageResponse(<div>Hello world</div>, {
      width: 1200,
      height: 600,
    })
  }
  return NextResponse.next()
}