Example "no-op" navigation preload service worker

sw.js

self.addEventListener('fetch', evt => {
  // Fallback for subresource requests or in browsers that do not
  // support navigation preload.
  if (evt.request.mode !== "navigate" || !evt.preloadResponse)
    return;
 
  evt.respondWith(async _ => {
    // Try to get the navigation preload response.
    let r = await evt.preloadResponse;
 
    // Navigation preload worked, so we are done.
    if (r) return r;
 
    // Otherwise we got an unexpected error, so switch to a pass-through
    // fetch.
    return fetch(evt.request);
  }());
});

s/self.addEventListener(evt => {/self.addEventListener('fetch', evt => {

Fixed. Thanks.