I've been trying to get rid of the ESLint warnings for the JavaScript code in some of my extensions, when they go through Jenkins validation. One warning that appears fairly often is this one:

Where possible, maintain application state in JS to avoid slower DOM queries

no-jquery/no-class-state

I'm not sure if this is a warning that's specific to Wikimedia code, but doing a web search on it brings up this Wikimedia help page as the only real result:

This page is rather confusing. It says that the warning comes when calling either hasClass() or toggleClass() on a jQuery element. That part makes sense, but then the suggested alternatives are strange. The page says that the following are some examples of bad code:

$( 'div' ).hasClass();
$div.hasClass();

In their place, it suggests the following:

hasClass();
[].hasClass();
div.hasClass();

Can anyone explain this? As far as I'm aware, hasClass() is only defined in jQuery - and there, only as a method for variables representing jQuery elements. So what do those "recommended" lines mean? And in general, what is the best way to determine whether some element on the page has a certain class?

-Yaron

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