files/en-us/web/api/serviceworkerglobalscope/backgroundfetchfail_event/index.md
{{APIRef("Background Fetch API")}}{{SeeCompatTable}}{{SecureContext_Header}}{{AvailableInWorkers("service")}}
The backgroundfetchfail event of the {{domxref("ServiceWorkerGlobalScope")}} interface is fired when a background fetch operation has failed: that is, when at least one network request in the fetch has failed to complete successfully.
This event is not cancelable and does not bubble.
Use the event name in methods like {{domxref("EventTarget.addEventListener", "addEventListener()")}}, or set an event handler property.
addEventListener("backgroundfetchfail", (event) => { })
onbackgroundfetchfail = (event) => { }
A {{domxref("BackgroundFetchUpdateUIEvent")}}.
{{InheritanceDiagram("BackgroundFetchUpdateUIEvent")}}
Inherits properties from its parent, {{domxref("BackgroundFetchEvent")}}.
When a background fetch operation fails (meaning that at least one of the individual network requests has not completed successfully), the browser starts the service worker, if necessary, and fires the backgroundfetchfail event in the service worker's global scope.
In the background fetch API, the browser shows a UI element to the user to indicate the progress of the operation. In the backgroundfetchfail handler, the service worker can update that UI to show that the operation has failed. To do this, the handler calls the event's {{domxref("BackgroundFetchUpdateUIEvent/updateUI", "updateUI()")}} method, passing in a new title and/or icons.
In the handler for this backgroundfetchfail, the service worker can also clean up any related data for the operation. It can also retrieve and store any successful responses (for example, using the {{domxref("Cache")}} API). To access the response data, the service worker uses the event's {{domxref("BackgroundFetchEvent/registration", "registration")}} property.
This event handler updates the UI to let the user know that the operation failed.
addEventListener("backgroundfetchfail", (event) => {
event.updateUI({ title: "Could not complete download" });
});
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