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This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

The ExtendableEvent interface extends the lifetime of the install and activate events dispatched on the global scope as part of the service worker lifecycle. This ensures that any functional events (like FetchEvent) are not dispatched until it upgrades database schemas and deletes the outdated cache entries.

If waitUntil() is called outside of the ExtendableEvent handler, the browser should throw an InvalidStateError; note also that multiple calls will stack up, and the resulting promises will be added to the list of extend lifetime promises.

Note: The behaviour described in the above paragraph was fixed in Firefox 43 (see bug 1189644.)

This interface inherits from the Event interface.

Note: This interface is only available when the global scope is a ServiceWorkerGlobalScope. It is not available when it is a Window, or the scope of another kind of worker.


Creates a new ExtendableEvent object.


Doesn't implement any specific properties, but inherits properties from its parent, Event.


Inherits methods from its parent, Event.


Extends the lifetime of the event.  It is intended to be called in the install EventHandler for the installing worker and on the active EventHandler for the active worker.


This code snippet is from the service worker prefetch sample (see prefetch example live.) The code calls ExtendableEvent.waitUntil() in ServiceWorkerGlobalScope.oninstall, delaying treating the ServiceWorkerRegistration.installing worker as installed until the passed promise resolves successfully. The promise resolves when all resources have been fetched and cached, or else when any exception occurs.

The code snippet also shows a best practice for versioning caches used by the service worker. Though there's only one cache in this example, the same approach can be used for multiple caches. It maps a shorthand identifier for a cache to a specific, versioned cache name.

Note: In Chrome, logging statements are visible via the "Inspect" interface for the relevant service worker accessed via chrome://serviceworker-internals.

  prefetch: 'prefetch-cache-v' + CACHE_VERSION

self.addEventListener('install', function(event) {
  var urlsToPrefetch = [

  console.log('Handling install event. Resources to pre-fetch:', urlsToPrefetch);

    caches.open(CURRENT_CACHES['prefetch']).then(function(cache) {
      cache.addAll(urlsToPrefetch.map(function(urlToPrefetch) {
        return new Request(urlToPrefetch, {mode: 'no-cors'});
      })).then(function() {
        console.log('All resources have been fetched and cached.');
    }).catch(function(error) {
      console.error('Pre-fetching failed:', error);
Important: When fetching resources, it's very important to use {mode: 'no-cors'} if there is any chance that the resources are served off of a server that doesn't support CORS. In this example, www.chromium.org doesn't support CORS.


Specification Status Comment
Service Workers
The definition of 'ExtendableEvent' in that specification.
Working Draft Initial definition.

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support40 ?441 No24 No
ExtendableEvent() constructor40 ?441 No24 No
waitUntil40 ?44 No24 No
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidOpera AndroidiOS SafariSamsung Internet
Basic support4040 ?4424 No ?
ExtendableEvent() constructor4040 ?4424 No ?
waitUntil4040 ?4424 No ?

1. Service workers (and Push) have been disabled in the Firefox 45 & 52 Extended Support Releases (ESR).

See also

Document Tags and Contributors

 Contributors to this page: fscholz, chrisdavidmills, jpmedley, teoli, Meggin
 Last updated by: fscholz,