sw.js

"use strict";

var cacheVersion = 4.9;
var currentCache = "offline" + cacheVersion;
const offlineUrl = "oof.html";

//sw install
self.addEventListener("install", function(event) {
  // Put `offline.html` page into cache
  var offlineRequest = new Request("oof.html");
  event.waitUntil(
    fetch(offlineRequest).then(function(response) {
      return caches.open("offline").then(function(cache) {
        return cache.put(offlineRequest, response);
      });
    })
  );
});

//fetch event
self.addEventListener("fetch", function(event) {
  // Only fall back for HTML documents.
  var request = event.request;
  // && request.headers.get('accept').includes('text/html')
  if (request.method === "GET") {
    // `fetch()` will use the cache when possible, to this examples
    // depends on cache-busting URL parameter to avoid the cache.
    event.respondWith(
      fetch(request).catch(function(error) {
        // `fetch()` throws an exception when the server is unreachable but not
        // for valid HTTP responses, even `4xx` or `5xx` range.
        return caches.open("offline").then(function(cache) {
          return cache.match("oof.html");
        });
      })
    );
  }
  // Any other handlers come here. Without calls to `event.respondWith()` the
  // request will be handled without the ServiceWorker.
});