files/en-us/web/api/service_worker_api/index.md
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Service workers essentially act as proxy servers that sit between web applications, the browser, and the network (when available). They are intended, among other things, to enable the creation of effective offline experiences, intercept network requests, and take appropriate action based on whether the network is available, and update assets residing on the server. They will also allow access to push notifications and background sync APIs.
[!NOTE] Service workers are a type of web worker. See Web workers for general information about worker types and use cases.
A service worker is an event-driven worker registered against an origin and a path. It takes the form of a JavaScript file that can control the web page/site that it is associated with, intercepting and modifying navigation and resource requests, and caching resources in a very granular fashion to give you complete control over how your app behaves in certain situations (the most obvious one being when the network is not available).
Service workers run in a worker context: they therefore have no DOM access and run on a different thread to the main JavaScript that powers your app. They are non-blocking and designed to be fully asynchronous. As a consequence, APIs such as synchronous XHR and Web Storage can't be used inside a service worker.
Service workers can't import JavaScript modules dynamically, and import() will throw an error if it is called in a service worker global scope. Static imports using the import statement are allowed.
Service workers are only available in secure contexts: this means that their document is served over HTTPS, although browsers also treat http://localhost as a secure context, to facilitate local development. HTTP connections are susceptible to malicious code injection by {{Glossary("MitM", "man in the middle")}} attacks, and such attacks could be worse if allowed access to these powerful APIs.
[!NOTE] On Firefox, for testing you can run service workers over HTTP (insecurely); simply check the Enable Service Workers over HTTP (when toolbox is open) option in the Firefox DevTools options/gear menu.
[!NOTE] Unlike previous attempts in this area such as AppCache, service workers don't make assumptions about what you are trying to do, but then break when those assumptions are not exactly right. Instead, service workers give you much more granular control.
[!NOTE] Service workers make heavy use of promises, as generally they will wait for responses to come through, after which they will respond with a success or failure action. The promises architecture is ideal for this.
A service worker is first registered using the {{DOMxRef("ServiceWorkerContainer.register()")}} method. If successful, your service worker will be downloaded to the client and attempt installation/activation (see below) for URLs accessed by the user inside the whole origin, or a subset specified by you.
At this point, your service worker will observe the following lifecycle:
The service worker is immediately downloaded when a user first accesses a service worker–controlled site/page.
After that, it is updated when:
Installation is attempted when the downloaded file is found to be new — either different to an existing service worker (byte-wise compared), or the first service worker encountered for this page/site.
If this is the first time a service worker has been made available, installation is attempted, then after a successful installation, it is activated.
If there is an existing service worker available, the new version is installed in the background, but not yet activated — at this point it is called the worker in waiting. It is only activated when there are no longer any pages loaded that are still using the old service worker. As soon as there are no more pages to be loaded, the new service worker activates (becoming the active worker). Activation can happen sooner using {{DOMxRef("ServiceWorkerGlobalScope.skipWaiting()")}} and existing pages can be claimed by the active worker using {{DOMxRef("Clients.claim()")}}.
You can listen for the {{domxref("ServiceWorkerGlobalScope/install_event", "install")}} event; a standard action is to prepare your service worker for usage when this fires, for example by creating a cache using the built-in storage API, and placing assets inside it that you'll want for running your app offline.
There is also an {{domxref("ServiceWorkerGlobalScope/activate_event", "activate")}} event. The point where this event fires is generally a good time to clean up old caches and other things associated with the previous version of your service worker.
Your service worker can respond to requests using the {{DOMxRef("FetchEvent")}} event. You can modify the response to these requests in any way you want, using the {{DOMxRef("FetchEvent.respondWith()")}} method.
[!NOTE] Because
install/activateevents could take a while to complete, the service worker spec provides a {{domxref("ExtendableEvent.waitUntil", "waitUntil()")}} method. Once it is called oninstalloractivateevents with a promise, functional events such asfetchandpushwill wait until the promise is successfully resolved.
For a complete tutorial to show how to build up your first basic example, read Using Service Workers.
Service workers can incur an unnecessary performance cost — when a page is loaded for the first time in a while, the browser has to wait for the service worker to start up and run to know what content to load and whether it should come from a cache or the network.
If you already know ahead of time where certain content should be fetched from, you can bypass the service worker altogether and fetch resources immediately. The {{domxref("InstallEvent.addRoutes()")}} method can be used to implement this use case and more.
Service workers are also intended to be used for such things as:
In the future, service workers will be able to do several other useful things for the web platform that will bring it closer to native app viability. Interestingly, other specifications can and will start to make use of the service worker context, for example:
install and activate events dispatched on the {{DOMxRef("ServiceWorkerGlobalScope")}}, as part of the service worker lifecycle. This ensures that any functional events (like {{DOMxRef("FetchEvent")}}) are not dispatched to the {{DOMxRef("ServiceWorker")}}, until it upgrades database schemas, and deletes outdated cache entries, etc.FetchEvent represents a fetch action that is dispatched on the {{DOMxRef("ServiceWorkerGlobalScope")}} of a {{DOMxRef("ServiceWorker")}}. It contains information about the request and resulting response, and provides the {{DOMxRef("FetchEvent.respondWith", "FetchEvent.respondWith()")}} method, which allows us to provide an arbitrary response back to the controlled page.InstallEvent interface represents an install action that is dispatched on the {{DOMxRef("ServiceWorkerGlobalScope")}} of a {{DOMxRef("ServiceWorker")}}. As a child of {{DOMxRef("ExtendableEvent")}}, it ensures that functional events such as {{DOMxRef("FetchEvent")}} are not dispatched during installation.ServiceWorker object.{{Specifications}}