In earlier versions of the service worker spec,
event.waitUntil had to be called synchronously, as in during the initial execution of the event handler. Meaning this would fail:
This fails because
waitUntil was called after the execution of the event handler. But we need to use
waitUntil here because the browser needs to know to keep the service worker alive after we've sent the response back.
We fixed this in the spec, so you can call
waitUntil as long as the promises already passed to
respondWith haven't settled yet.
The browsers haven't caught up yet, but this polyfill makes it work as expected.
In your service worker:
That's it! The above example now works.