How do I call a method on my ServiceWorker from within my page?

Question

I have a ServiceWorker registered on my page and want to pass some data to it so it can be stored in an IndexedDB and used later for network requests (it's an access token).

Is the correct thing just to use network requests and catch them on the SW side using fetch, or is there something more clever?

Note for future readers wondering similar things to me:

Setting properties on the SW registration object, e.g. setting self.registration.foo to a function within the service worker and doing the following in the page:

navigator.serviceWorker.getRegistration().then(function(reg) { reg.foo; })

Results in TypeError: reg.foo is not a function. I presume this is something to do with the lifecycle of a ServiceWorker meaning you can't modify it and expect those modification to be accessible in the future, so any interface with a SW likely has to be postMessage style, so perhaps just using fetch is the best way to go...?

Answer 1

So it turns out that you can't actually call a method within a SW from your app (due to lifecycle issues), so you have to use a postMessage API to pass serialized JSON messages around (so no passing callbacks etc).

You can send a message to the controlling SW with the following app code:

navigator.serviceWorker.controller.postMessage({'hello': 'world'})

Combined with the following in the SW code:

self.addEventListener('message', function (evt) {
  console.log('postMessage received', evt.data);
})

Which results in the following in my SW's console:

postMessage received Object {hello: "world"}

So by passing in a message (JS object) which indicates the function and arguments I want to call my event listener can receive it and call the right function in the SW. To return a result to the app code you will need to also pass a port of a MessageChannel in to the SW and then respond via postMessage, for example in the app you'd create and send over a MessageChannel with the data:

var messageChannel = new MessageChannel();
messageChannel.port1.onmessage = function(event) {
  console.log(event.data);
};
// This sends the message data as well as transferring messageChannel.port2 to the service worker.
// The service worker can then use the transferred port to reply via postMessage(), which
// will in turn trigger the onmessage handler on messageChannel.port1.
// See https://html.spec.whatwg.org/multipage/workers.html#dom-worker-postmessage
navigator.serviceWorker.controller.postMessage(message, [messageChannel.port2]);

and then you can respond via it in your Service Worker within the message handler:

evt.ports[0].postMessage({'hello': 'world'});