function handler(event) {
  var request = event.request;
  var uri = request.uri;
  var last = uri.substring(uri.lastIndexOf("/") + 1);

  // S3 OAC has no directory index. default_root_object only covers "/".
  // Keep the browser URL; rewrite the origin key to …/index.html.
  if (uri.endsWith("/")) {
    request.uri = uri + "index.html";
  } else if (last.length > 0 && last.indexOf(".") === -1) {
    request.uri = uri + "/index.html";
  }

  return request;
}