The URL in the error message is misleading. The error almost certainly comes from your site JavaScript (the pages 'MediaWiki:Common.js' and 'MediaWiki:Vector.js'), especially if you copied them from Wikipedia a long time ago. Vector doesn't use the hookEvent function.

@matmarex @Aklapper The same warning appears throughout the wiki (http://www.hypoverse.com). Removing everything from MediaWiki:Common.js, resulted in same above error message. I already checked if there was something missing from load.php, there was none compared to MediaWiki 1.28.2. Is it possible that something is corrupted, the domain was canceled and later reactivated a few weeks later?

I don't see any warnings when viewing the site.

In T165552#3272078, @matmarex wrote:

I don't see any warnings when viewing the site.

ok, now that the cache finally cleared away; I don't see the warning either. The original JavaScript was copied from www.dkwiki.com or http://wow.gamepedia.com and there are no errors there. The only extra code i added was to append a template to the bottom of each page:

$(document).ready(function() {
    var newSection = '<div id="disclaimer">' + '</div>';
    $('ul#footer-places').after(newSection);
    $.getJSON('/w/api.php?action=parse&text={{Disclaimer}}&format=json', function(data) {
        var code = data.parse.text['*'];
        $('ul#footer-places').append(code);
    });
});

It appears the warning is triggered due to that and the current count down timer. Despite no call to hookEvent.

I wanted to check it out again but your wiki (http://www.hypoverse.com/) seems to be down…

In T165552#3275954, @matmarex wrote:

I wanted to check it out again but your wiki (http://www.hypoverse.com/) seems to be down…

Try this link: http://www.hypoverse.com/wiki/Main_Page

In T165552#3272760, @Aklapper wrote:

Please see T165552#3271282 and/or please provide a public testcase.

Ok, I have allowed the creation of new accounts for temporary measures so you can see for yourself. I will be removing this in a few days.

The hookEvent call and the warning is coming from the OnlineStatus extension you have installed.

In T165552#3277469, @matmarex wrote:

The hookEvent call and the warning is coming from the OnlineStatus extension you have installed.

Thanks and I can confirm this by disabling OnlineStatus extension and the warning went away. Is there another extension that could be substituted for now? Seems to be generated by the following in OnlineStatus.js:

hookEvent( 'load', OnlineStatus );

Now under MediaWiki 1.29.2, FireFox console tells me

Exception in module-execute in module ext.onlineStatus:
ReferenceError: hookEvent is not defined ReferenceError: hookEvent is not defined

If I disable Extension:OnlineStatus, it goes away