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

function isMobile(userAgent) {
  return userAgent.match(/iPhone|Android|webOS/i) ? 'Yes' : 'No';
}

async function fetchAndApply(request) {
  let isMobileResult = isMobile(request.headers.get('user-agent'))

  request = new Request(request)
  request.headers.set("Is-Mobile", isMobileResult)

  return fetch(request)
}