@Sgs this error seems to be happening with quite high frequency. From user agents seems like it might be some kind of custom (albeit popular build)
Would be great to use mw.storage (mediawiki.storage) rather than localStorage directly as it has various other benefits.

A variation of this error is seen on authentication pages, as reported via the MediaWiki Platform Team's client error dashboard in Logstash, e.g. on https://auth.wikimedia.org/frwiki/wiki/Special:Userlogin?…&usesul3=1&useformat=desktop

NS_ERROR_FAILURE: 
at shouldShowExpirationAlert
at initTempUserBannerTooltip
at HTMLDocument.<anonymous>
at mightThrow
at process

	function shouldShowExpirationAlert() {
		const tempUserExpirationAlertDismissed = localStorage.getItem( 'tempUserExpirationAlertDismissed' );

This is most likely because storage is full, unavailable, or denied for other reasons (i.e. private browsing, etc.).

As Jon says, localStorage should not be used directly in production code. Use mw.storage instead.

Change #1192868 had a related patch set uploaded (by Sergio Gimeno; author: Sergio Gimeno):

[mediawiki/core@master] tempUserBanner: make use of mw.storage instead of localStorage

https://gerrit.wikimedia.org/r/1192868

Change #1192868 merged by jenkins-bot:

[mediawiki/core@master] tempUserBanner: make use of mw.storage instead of localStorage

https://gerrit.wikimedia.org/r/1192868

(Not sure if that should be checked by Growth QA, or maybe better by Trust and Safety Product Team / Product Safety and Integrity QA as the primary owners of the code touched here. Maybe @Dreamy_Jazz has thoughts?)

In T401978#11233706, @Michael wrote:

(Not sure if that should be checked by Growth QA, or maybe better by Trust and Safety Product Team / Product Safety and Integrity QA as the primary owners of the code touched here. Maybe @Dreamy_Jazz has thoughts?)

We have very limited QA capacity, and have been generally moving all our tickets to Done once the patch has been merged. As such, is there is space for Growth QA to look at this then I would suggest going that route. Otherwise, I would recommend just closing the ticket.

@Krinkle and @Jdlrobson maybe we should have a mediawiki eslint rule for direct usage of localStorage ?

@TheDJ https://github.com/wikimedia/eslint-config-wikimedia/issues/632