rsnext/test/e2e/app-dir/app-middleware/middleware.js

import { NextResponse } from 'next/server'

// It should be able to import `headers` inside middleware
import { headers } from 'next/headers'
console.log(!!headers)

/**
 * @param {import('next/server').NextRequest} request
 */
export async function middleware(request) {
  const headers = new Headers(request.headers)
  headers.set('x-from-middleware', 'hello-from-middleware')

  const removeHeaders = request.nextUrl.searchParams.get('remove-headers')
  if (removeHeaders) {
    for (const key of removeHeaders.split(',')) {
      headers.delete(key)
    }
  }

  const updateHeader = request.nextUrl.searchParams.get('update-headers')
  if (updateHeader) {
    for (const kv of updateHeader.split(',')) {
      const [key, value] = kv.split('=')
      headers.set(key, value)
    }
  }

  if (request.nextUrl.pathname.includes('/rewrite-to-app')) {
    request.nextUrl.pathname = '/headers'
    return NextResponse.rewrite(request.nextUrl)
  }

  return NextResponse.next({
    request: {
      headers,
    },
  })
}