supabase/apps/ui-library/middleware.ts

import { negotiateMarkdown, type MarkdownDecision } from 'common/markdown-negotiation'
import { NextResponse, type NextRequest } from 'next/server'

import MARKDOWN_SLUGS from '@/public/markdown/manifest.json'

const BASE_PATH = process.env.NEXT_PUBLIC_BASE_PATH ?? '/library'
const DOCS_PATH = `${BASE_PATH}/docs`
const MARKDOWN_SLUG_SET = new Set(MARKDOWN_SLUGS)
// The homepage catalog, generated by scripts/build-markdown-index.ts.
const HOME_PATHS = new Set([BASE_PATH, `${BASE_PATH}/`, `${BASE_PATH}/index.md`])

export function middleware(request: NextRequest) {
  const url = new URL(request.url)
  const { pathname } = url

  // Server Actions POST to the page URL with `Accept: text/x-component`, which
  // matches none of the types these routes negotiate and would 406. Let Next.js
  // handle them.
  if (request.headers.get('next-action')) {
    return NextResponse.next()
  }

  const acceptHeader = request.headers.get('accept') ?? ''

  if (HOME_PATHS.has(pathname)) {
    return respond(
      negotiateMarkdown(
        { acceptHeader },
        { hasMarkdownVariant: true, isMarkdownSuffix: pathname.endsWith('.md') }
      ),
      url,
      `${BASE_PATH}/api/index-md`
    )
  }

  if (!pathname.startsWith(`${DOCS_PATH}/`)) {
    return NextResponse.next()
  }

  const isMdSuffix = pathname.endsWith('.md')
  const slug = pathname.replace(`${DOCS_PATH}/`, '').replace(/\.md$/, '')
  const decision = negotiateMarkdown(
    { acceptHeader },
    { hasMarkdownVariant: MARKDOWN_SLUG_SET.has(slug), isMarkdownSuffix: isMdSuffix }
  )

  return respond(decision, url, `${BASE_PATH}/api/docs-md/${slug}`)
}

function respond(decision: MarkdownDecision, url: URL, markdownPathname: string) {
  if (decision === 'not-acceptable') {
    return new NextResponse('Not Acceptable', {
      status: 406,
      headers: { 'Cache-Control': 'no-store', Vary: 'Accept' },
    })
  }

  if (decision === 'markdown') {
    const rewriteUrl = new URL(url)
    rewriteUrl.pathname = markdownPathname
    return NextResponse.rewrite(rewriteUrl)
  }

  return NextResponse.next()
}

export const config = {
  matcher: ['/', '/index.md', '/docs/:path*'],
}