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An attempt at a minimal viable service worker.
// Licensed under a CC0 1.0 Universal (CC0 1.0) Public Domain Dedication
// HTML files: try the network first, then the cache.
// Other files: try the cache first, then the network.
// Both: cache a fresh version if possible.
// (beware: the cache will grow and grow; there's no cleanup)
const cacheName = 'files';
addEventListener('fetch', fetchEvent => {
const request = fetchEvent.request;
if (request.method !== 'GET') {
fetchEvent.respondWith(async function() {
const fetchPromise = fetch(request);
fetchEvent.waitUntil(async function() {
const responseFromFetch = await fetchPromise;
const responseCopy = responseFromFetch.clone();
const myCache = await;
return myCache.put(request, responseCopy);
if (request.headers.get('Accept').includes('text/html')) {
try {
return await fetchPromise;
catch(error) {
return caches.match(request);
} else {
const responseFromCache = await caches.match(request);
return responseFromCache || fetchPromise;
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paulyabsley commented Jun 30, 2018

Ah cool, thanks!

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dracos commented Dec 18, 2019

Just to note that the main gist here is missing an await in the "return fetchPromise" line (as given in @jakearchibald's update), which means as-is it won't respond from the cache for HTML pages - as the promise can always be returned so will be, regardless of whether it resolves or rejects.

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