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

/**
 * When we receive a request, fetch it from our S3 bucket
 * 
 * For example, a request for:
 *   https://mydomain.com/images/castle01.jpg
 * will be fetched from:
 *   https://s3-de-central.profitbricks.com/my-images/test-folder/castle01.jpg
 */
async function handleRequest(request) {
  // map our request folder to the s3 folder
  let requestFolder = '/images'
  let s3Folder = '/my-images/test-folder'
  
  let url = new URL(request.url)
  let origPathname = url.pathname

  // fetch from the folder on our s3 bucket
  url.hostname = 's3-de-central.profitbricks.com'
  url.pathname = origPathname.replace(new RegExp('^'+escapeRegExp(requestFolder)), s3Folder)

  return await fetch(url, request)
}
function escapeRegExp(string) {
  return string.replace(/[.*+?^${}()|[\]\\\/]/g, '\\$&'); // $& means the whole matched string
}