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

import { NextResponse } from 'next/server'

import { headers as nextHeaders } from 'next/headers'

/**
 * @param {import('next/server').NextRequest} request
 */
export async function proxy(request) {
  const headersFromRequest = new Headers(request.headers)
  // It should be able to import and use `headers` inside proxy
  const headersFromNext = await nextHeaders()
  headersFromRequest.set('x-from-proxy', 'hello-from-proxy')

  // make sure headers() from `next/headers` is behaving properly
  if (
    headersFromRequest.get('x-from-client') &&
    headersFromNext.get('x-from-client') !==
      headersFromRequest.get('x-from-client')
  ) {
    throw new Error('Expected headers from client to match')
  }

  // Cannot set draftMode in nodejs runtime
  // TODO: Investigate https://github.com/vercel/next.js/pull/85174
  // if (request.nextUrl.searchParams.get('draft')) {
  //   ;(await draftMode()).enable()
  // }

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

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

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

  if (request.nextUrl.pathname === '/rsc-cookies') {
    const res = NextResponse.next()
    res.cookies.set('rsc-cookie-value-1', `${Math.random()}`)
    res.cookies.set('rsc-cookie-value-2', `${Math.random()}`)

    return res
  }

  if (request.nextUrl.pathname === '/rsc-cookies/cookie-options') {
    const res = NextResponse.next()
    res.cookies.set('rsc-secure-cookie', `${Math.random()}`, {
      secure: true,
      httpOnly: true,
    })

    return res
  }

  if (request.nextUrl.pathname === '/rsc-cookies-delete') {
    const res = NextResponse.next()
    res.cookies.delete('rsc-cookie-value-1')

    return res
  }

  if (request.nextUrl.pathname === '/preloads') {
    const res = NextResponse.next({
      headers: {
        link: '<https://example.com/page>; rel="alternate"; hreflang="en"',
      },
    })
    return res
  }

  if (request.nextUrl.pathname === '/test-location-header') {
    return NextResponse.json(
      { foo: 'bar' },
      {
        headers: {
          location: 'https://next-data-api-endpoint.vercel.app/api/random',
        },
      }
    )
  }

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