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---
title: FetchEvent
slug: Web/API/FetchEvent
tags:
- API
- FetchEvent
- Interface
- NeedsTranslation
- Offline
- Reference
- Service Workers
- TopicStub
- Workers
translation_of: Web/API/FetchEvent
---
<p>{{SeeCompatTable}}{{APIRef("Service Workers API")}}</p>
<p><span class="seoSummary">This is the event type for <code>fetch</code> events dispatched on the {{domxref("ServiceWorkerGlobalScope", "service worker global scope", "", 1)}}. It contains information about the fetch, including the request and how the receiver will treat the response. It provides the {{domxref("FetchEvent.respondWith", "event.respondWith()")}} method, which allows us to provide a response to this fetch.</span></p>
<h2 id="Constructor">Constructor</h2>
<dl>
<dt>{{domxref("FetchEvent.FetchEvent()", "FetchEvent()")}}</dt>
<dd>Creates a new <code>FetchEvent</code> object. This constructor is not typically used. The browser creates these objects itself and provides them to <code>fetch</code> event callbacks.</dd>
</dl>
<h2 id="Properties">Properties</h2>
<p><em>Inherits properties from its ancestor, {{domxref("Event")}}</em>.</p>
<dl>
<dt>{{domxref("fetchEvent.clientId")}} {{readonlyInline}}</dt>
<dd>The {{domxref("Client.id", "id")}} of the same-origin {{domxref("Client", "client")}} that initiated the fetch.</dd>
<dt>{{domxref("fetchEvent.preloadResponse")}} {{readonlyinline}}</dt>
<dd>A {{jsxref("Promise")}} for a {{domxref("Response")}}, or void if this is not a navigation, or {{domxref("NavigationPreloadManager", "navigation preload", "", 1)}} is not enabled.</dd>
<dt>{{domxref("fetchEvent.request")}} {{readonlyInline}}</dt>
<dd>The {{domxref("Request")}} the browser intends to make.</dd>
</dl>
<h2 id="Methods">Methods</h2>
<p><em>Inherits methods from its parent, </em><em>{{domxref("ExtendableEvent")}}</em>.</p>
<dl>
<dt>{{domxref("fetchEvent.respondWith()")}}</dt>
<dd>Prevent the browser's default fetch handling, and provide (a promise for) a response yourself.</dd>
<dt>{{domxref("extendableEvent.waitUntil()")}}</dt>
<dd>
<p>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.</p>
</dd>
</dl>
<h2 id="Examples">Examples</h2>
<p>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.</p>
<pre class="brush: js">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 function() {
// 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);
}());
});</pre>
<h2 id="Specifications">Specifications</h2>
<table class="standard-table">
<tbody>
<tr>
<th scope="col">Specification</th>
<th scope="col">Status</th>
<th scope="col">Comment</th>
</tr>
<tr>
<td>{{SpecName('Service Workers', '#fetch-event-section', 'FetchEvent')}}</td>
<td>{{Spec2('Service Workers')}}</td>
<td>Initial definition.</td>
</tr>
</tbody>
</table>
<h2 id="Browser_compatibility">Browser compatibility</h2>
<div>
<p>{{Compat("api.FetchEvent")}}</p>
</div>
<h2 id="See_also">See also</h2>
<ul>
<li>{{jsxref("Promise")}}</li>
<li><a href="/en-US/docs/Web/API/Fetch_API">Fetch API</a></li>
</ul>
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