cloudflare-workerooni.js

addEventListener('fetch', event => {
  event.respondWith(handleRequest(event.request))
})

async function handleRequest(request) {
  const originalResponse = await fetch(request)
  const originalHtml = await originalResponse.text()

  const url = new URL(request.url)
  const nftMetadata = await fetchNFTMetadata(url.pathname)
  const countryCode = nftMetadata.countryCode

  const modifiedHtml = injectHrefLang(originalHtml, countryCode)

  return new Response(modifiedHtml, {
    headers: { 'Content-Type': 'text/html' }
  })
}

async function fetchNFTMetadata(path) {
  return {
    countryCode: 'US'
  }
}

function injectHrefLang(html, countryCode) {
  const hrefLangTag = `<link rel="alternate" hreflang="${countryCode}" href="${request.url}">`
  const pos = html.indexOf('</head>')
  if (pos === -1) return html
  return html.slice(0, pos) + hrefLangTag + html.slice(pos)
}