FetchEvent

Baseline Widely available *

This feature is well established and works across many devices and browser versions. It’s been available across browsers since April 2018.

* Some parts of this feature may have varying levels of support.

Note: This feature is only available in Service Workers.

This is the event type for fetch events dispatched on the service worker global scope. It contains information about the fetch, including the request and how the receiver will treat the response. It provides the event.respondWith() method, which allows us to provide a response to this fetch.

Event ExtendableEvent FetchEvent

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

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.

Examples

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.

js
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);
    })(),
  );
});