Fetch event of service worker

sw.js

// Runs whenever there is a fetch request
self.addEventListener("fetch", (event) => {
    // Checks the cache to find matching request. 
    // If there is a cached version of the request, it returns the cached version, otherwise it makes a new request.
    event.respondWith(
        caches.match(event.request).then((res) => {
            return res || fetch(event.request);
        })
    );
});