You can debug with chrom dev tool:

1. Use console to debug service worker:

Swith to sw.js context then you can access 'self' object

2. Set breakpoint:

You can see all the event object.

3. You can see the active service worker:

4. Once you change the code, you can see the waiting servie worker:

4. After you close and reopen the page, the waiting service work become the active one.