Background Fetch next to Workbox

In Chromium, Offlibrary downloads large files with Background Fetch. When one finishes, the browser wakes the service worker with a backgroundfetchsuccess event, and something has to store the files and tell the page. In a Workbox worker, that’s initOfflibrary():

import { initOfflibrary } from 'offlibrary/workbox';

initOfflibrary();

It handles:

Event	What happens
backgroundfetchsuccess	Files are checked (status, CORS, integrity), written to the collection’s cache, the item becomes completed, the notification shows ui.successTitle, and pages get a complete event
backgroundfetchfail	The item becomes failed with a mapped error code; the notification shows ui.failTitle
backgroundfetchabort	The user cancelled from the browser UI; the item is removed and pages get an abort event
backgroundfetchclick	Focuses an open app window, or opens the item’s clickUrl (same-origin only)

Events for Background Fetches that aren’t Offlibrary’s are ignored, so your own Background Fetch code can live next to it.

It adds no fetch listener. Serving stored files is left to your Workbox routes (the route or the plugin), so it never competes with Workbox’s router. It’s exactly offlibrarySW({ ...options, serve: false }).

Options

The same callbacks as offlibrarySW():

initOfflibrary({
  onComplete: (item, event) => self.registration.showNotification(`${item.title} is ready offline`),
  onFail: (item, event) => console.warn('download failed', item.id, item.error?.code),
  onClick: (item, event) => false, // return true to skip the default focus/open
});

Do I need it?

• If any collection can use Background Fetch (the default strategy: 'auto' does in Chromium): yes. Without it, finished Background Fetches are never stored.
• If every collection uses strategy: 'fetch', downloads run in the page and you can skip it. Calling it anyway is harmless and keeps the routing index fresh.

Workbox has no Background Fetch module of its own (workbox-background-sync replays failed requests, a different job), so there’s nothing to configure on the Workbox side.