ServiceWorker.onstatechange

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

Syntax

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

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.

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 (registration.active) {
  serviceWorker = registration.active;
  document.querySelector('#kind').textContent = 'active';
}
if (serviceWorker) {
  logState(serviceWorker.state);
  serviceWorker.addEventListener('statechange', function(e) {
  logState(e.target.state);
  });
}

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 swr.active 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.
  }
})

Specifications

Browser compatibility

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