install.js

if (typeof navigator.serviceWorker !== 'undefined')
    navigator.serviceWorker.register('/sw.js');

let deferredPrompt;
const addBtn = document.querySelector("#install-button");

window.addEventListener("beforeinstallprompt", (e) => {
    e.preventDefault();
    deferredPrompt = e;
    addBtn.addEventListener("click", (e) => {
        deferredPrompt.prompt();
        deferredPrompt.userChoice.then((choiceResult) => {
            if (choiceResult.outcome === "accepted") {
                console.log("User accepted the A2HS prompt");
            } else {
                console.log("User dismissed the A2HS prompt");
            }
            deferredPrompt = null;
        });
    });
});

sw.js

// This is the "Offline page" service worker

importScripts('https://storage.googleapis.com/workbox-cdn/releases/5.1.2/workbox-sw.js');

const CACHE = "pwabuilder-page";

// TODO: replace the following with the correct offline fallback page i.e.: const offlineFallbackPage = "offline.html";
const offlineFallbackPage = "/index.html";

self.addEventListener("message", (event) => {
  if (event.data && event.data.type === "SKIP_WAITING") {
    self.skipWaiting();
  }
});

self.addEventListener('install', async (event) => {
  event.waitUntil(
      caches.open(CACHE)
          .then((cache) => cache.add(offlineFallbackPage))
  );
});

if (workbox.navigationPreload.isSupported()) {
  workbox.navigationPreload.enable();
}

self.addEventListener('fetch', (event) => {
  if (event.request.mode === 'navigate') {
    event.respondWith((async () => {
      try {
        const preloadResp = await event.preloadResponse;

        if (preloadResp) {
          return preloadResp;
        }

        const networkResp = await fetch(event.request);
        return networkResp;
      } catch (error) {

        const cache = await caches.open(CACHE);
        const cachedResp = await cache.match(offlineFallbackPage);
        return cachedResp;
      }
    })());
  }
});