FetchEvent - Web APIs

FetchEvent - Web APIs | MDN

Constructor

FetchEvent(): Creates a new FetchEvent object. This constructor is not typically used. The browser creates these objects and provides them to fetch event callbacks.

Instance properties

Inherits properties from its ancestor, Event.

FetchEvent.clientId Read only: The id of the same-origin client that initiated the fetch.
FetchEvent.handled Read only: A promise that is pending while the event has not been handled, and fulfilled once it has.
FetchEvent.isReload Read only Deprecated Non-standard: Returns true if the event was dispatched by the user attempting to reload the page, and false otherwise.
FetchEvent.preloadResponse Read only: A Promise for a Response, or undefined if this fetch is not a navigation, or navigation preload is not enabled.
FetchEvent.replacesClientId Read only: The id of the client that is being replaced during a page navigation.
FetchEvent.resultingClientId Read only: The id of the client that replaces the previous client during a page navigation.
FetchEvent.request Read only: The Request the browser intends to make.

Instance methods

Inherits methods from its parent, ExtendableEvent.

FetchEvent.respondWith(): Prevent the browser's default fetch handling, and provide (a promise for) a response yourself.
ExtendableEvent.waitUntil(): Extends the lifetime of the event. Used to notify the browser of tasks that extend beyond the returning of a response, such as streaming and caching.

This fetch event uses the browser default for non-GET requests. For GET requests it tries to return a match in the cache, and falls back to the network. If it finds a match in the cache, it asynchronously updates the cache for next time.

self.addEventListener("fetch", (event) => {
  // Let the browser do its default thing
  // for non-GET requests.
  if (event.request.method !== "GET") return;

  // Prevent the default, and handle the request ourselves.
  event.respondWith(
    (async () => {
      // Try to get the response from a cache.
      const cache = await caches.open("dynamic-v1");
      const cachedResponse = await cache.match(event.request);

      if (cachedResponse) {
        // If we found a match in the cache, return it, but also
        // update the entry in the cache in the background.
        event.waitUntil(cache.add(event.request));
        return cachedResponse;
      }

      // If we didn't find a match in the cache, use the network.
      return fetch(event.request);
    })(),
  );
});