An EventListener property called whenever an event of type statechange is fired; it is basically fired anytime the ServiceWorker.state changes.


ServiceWorker.onstatechange = function(statechangeevent) { ... }
ServiceWorker.addEventListener('statechange', function(statechangeevent) { ... } )


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.

var 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 ( {
  serviceWorker =;
  document.querySelector('#kind').textContent = 'active';

if (serviceWorker) {
  serviceWorker.addEventListener('statechange', function(e) {

Note that when statechange fires, the service worker's references may have changed. For example:

navigator.serviceWorker.register(..).then(function(swr) {
  swr.installing.state == "installing"
  swr.installing.onstatechange = function() {
    swr.installing == null;
    // At this point, swr.waiting OR might be true. This is because the statechange
    // event gets queued, meanwhile the underlying worker may have gone into the waiting
    // state and will be immediately activated if possible.


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

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