ServiceWorker

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.

Secure context: This feature is available only in secure contexts (HTTPS), in some or all supporting browsers.

Note: This feature is available in Web Workers.

The ServiceWorker interface of the Service Worker API provides a reference to a service worker. Multiple browsing contexts (e.g., pages, workers, etc.) can be associated with the same service worker, each through a unique ServiceWorker object.

A ServiceWorker object is available via a number of properties:

ServiceWorkerRegistration.active
ServiceWorkerGlobalScope.serviceWorker
ServiceWorkerContainer.controller — when the service worker is in activating or activated state
ServiceWorkerRegistration.installing — when the service worker is in installing state
ServiceWorkerRegistration.waiting — when the service worker is in installed state

The ServiceWorker.state property and statechange event can be used to check and observe changes in the lifecycle-state of the object's associated service worker. Related lifecycle events, such as install and activate are fired at the service worker itself.

Service workers allow static import of ECMAScript modules, if supported, using import. Dynamic import is disallowed by the specification — calling import() will throw.

Service workers can only be registered in the Window scope in some or all browsers, because the ServiceWorker object is not exposed to DedicatedWorkerGlobalScope and SharedWorkerGlobalScope. Check the browser compatibility for information.

Instance properties

The ServiceWorker interface inherits properties from its parent, EventTarget.

ServiceWorker.scriptURL Read only: Returns the ServiceWorker serialized script URL defined as part of ServiceWorkerRegistration. The URL must be on the same origin as the document that registers the ServiceWorker.
ServiceWorker.state Read only: Returns the state of the service worker. It returns one of the following values: parsed, installing, installed, activating, activated, or redundant.

Instance methods

The ServiceWorker interface inherits methods from its parent, EventTarget.

ServiceWorker.postMessage(): Sends a message — consisting of any structured-cloneable JavaScript object — to the service worker. The message is transmitted to the service worker using a message event on its global scope.

Events

statechange: Fired when ServiceWorker.state changes.
error: Fired when an error happens inside the ServiceWorker object.

Examples

This code snippet is from the service worker registration-events sample (live demo). The code listens for any change in the ServiceWorker.state and returns its value.

if ("serviceWorker" in navigator) {
 navigator.serviceWorker
 .register("service-worker.js", {
 scope: "./",
 })
 .then((registration) => {
 let serviceWorker;
 if (registration.installing) {
 serviceWorker = registration.installing;
 document.querySelector("#kind").textContent = "installing";
 } else if (registration.waiting) {
 serviceWorker = registration.waiting;
 document.querySelector("#kind").textContent = "waiting";
 } else if (registration.active) {
 serviceWorker = registration.active;
 document.querySelector("#kind").textContent = "active";
 }
 if (serviceWorker) {
 // logState(serviceWorker.state);
 serviceWorker.addEventListener("statechange", (e) => {
 // logState(e.target.state);
 });
 }
 })
 .catch((error) => {
 // Something went wrong during registration. The service-worker.js file
 // might be unavailable or contain a syntax error.
 });
} else {
 // The current browser doesn't support service workers.
 // Perhaps it is too old or we are not in a Secure Context.
}