Ruby 初心者スレッド Part 66

これも理解できない。
誰かに教えてほしい。

addEventListener('fetch', event => {
// Prevent the default, and handle the request ourselves.
event.respondWith(async function() {
// Try to get the response from a cache.
const cachedResponse = await caches.match(event.request);
// Return it if we found one.
if (cachedResponse) return cachedResponse;
// If we didn't find a match in the cache, use the network.
return fetch(event.request);
}());
});