Reading a property of null in some cases in SW mode

[mossroy]

When doing some tests in SW mode on Chromium, I had the following error twice :
Cannot read property 'documentElement' of null
which happened on line 909 of app.js

It's a line we recently added. We also added a few null tests, but it looks like we need to add another one

[mossroy]

I don't remember the exact scenario, but I was switching from a git branch to another branch.
I suspect it might happen when the SW has been modified, and needs to be reloaded by the browser

[Jaifroid]

Unfortunately I don't think this issue is fixed. In trying to deduplicate code between app.js and service-worker.js in #556, I came across this error, and it seems reproducible on master in Chromium. I cannot reproduce the error on Edge classic or on Firefox Quantum. Therefore I believe it's a timing issue / race condition due to the way Chromium loads the Service Worker. Steps to reproduce on Chromium (running master from localhost):

Simple version

• Switch to Service Worker mode
• Open DevTools, load app.js if necessary
• Completely reload browser by pressing Ctrl-Shift-R inside DevTools, with DevTools open
• You should see error in screenshot below

Steps to reproduce while completely clearing and reloading Service Worker (to be ultra-sure)

• Switch to jQuery with DevTools shut
• Exit browser
• Open browser and go back to localhost/.../kwix-js/...
• Ensure you really are in jQuery mode, if not, repeat above, until browser opens from cold start in jQuery mode
• Open DevTools and unregister any Service Worker for kiwix-js on localhost
• Completely exit browser, re-open, open DevTools and be sure there is no SW registered
• Load app.js in DevTools if necessary, and leave it open
• Switch to Service Worker mode
• Reload browser completely with Ctrl-Shift-R while DevTools has the focus
• Error should be reproduced

[Jaifroid]

@mossroy Maybe this error could be solved with code from https://developer.mozilla.org/en-US/docs/Web/API/ServiceWorkerContainer/ready , e.g.

navigator.serviceWorker.ready.then(function(serviceWorkerRegistration) { ... });

[Jaifroid]

OK, on https://developer.mozilla.org/en-US/docs/Web/API/ServiceWorkerContainer/controller I find this:

The controller read-only property of the ServiceWorkerContainer interface returns a ServiceWorker object if its state is activated (the same object returned by ServiceWorkerRegistration.active). This property returns null if the request is a force refresh (Shift + refresh) or if there is no active worker.

Doh! I guess this is to force reload a Service Worker (?) when Devs are testing. Can we therefore ignore? It seems Chromium is the only browser written to spec here, or else I don't know how to issue a force refresh in the other browsers. I'll test a bit more to see if it's only on force refresh that the error occurs.

[mossroy]

I think this is a different problem, as it's not the same variable that is null. You should open another github issue.

[Jaifroid]

OK, but I think I found the cause of this error (as stated above) since it only occurs on force refresh and only in Chromium. Therefore I think it's an artefact, not a real error (or rather, it's "by design" in Chromium). If it appears in some meaningful way, I'll open an issue. For now, I'll just close this.