self.addEventListener('activate', (event) => {
  if (self.registration && self.registration.navigationPreload) {
    event.waitUntil(
      self.registration.navigationPreload.enable().then(() => {
        // Defaults to Service-Worker-Navigation-Preload: true if not set.
        if (headerValue) {
          self.registration.navigationPreload.setHeaderValue(headerValue);
        }
      })
    );
  }
});