Can you post a test case showing when this happens? Also can you use the latest jQuery 3 in your test case?

Be aware that version 2.x is no longer supported. If there is a bug to fix we'd only be fixing it in jQuery 3.x, or 4.x if it requires a breaking change.

I'am afraid if I present my use case, the attention will move away from my finding. Which is:
jQuery 3.3.1 returns a different outcome for the same function and argument then 3.5.1

jQuery 3.3.1 returns: false
jQuery 3.5.1 returns: Uncaught TypeError: Cannot read property 'namespaceURI' of undefined
For this line of code: jQuery.isXMLDoc(undefined)

jQuery 3.5.1 doesn't check the argument, 3.3.1 does. undefined is not an XML node, so in my interpretation it should return false, as version 3.3.1 does.

I'am talking about jQuery 3.x, not 2.x, only 2.x I mentioned is Sizzle update from 2.3.3 to 2.3.4, where this isXML() function is changed and causing this new TypeError since 3.4.0. Linked under Additional info in my original post.

As indicated in the signature of that method on its API page, only arguments of Element type are allowed as input to that method. Behavior for all other inputs is undefined as jQuery generally doesn't validate whether input is of a correct type.

If you want to make sure you only use allowed types, using TypeScript with jQuery typings is a good idea.

I'll let it open for discussion for now but generally we've been leaning towards not handling undefined inputs in APIs where it's not expected.

We're deliberating whether we want to add back the falsey check.

I think we're willing to add back the falsey check, but we'd all feel better if there was a valid use case presented.

the attention will move away from my finding

Just the opposite btw. It will grant some validity to your finding.

Sorry for the late reply. Here is a part of that code working with ajax responses.

mainComplete = function(xhr, status) {
    var result
    var resultText

    if ($.isXMLDoc(xhr.responseXML) && xhr.responseXML.documentElement.nodeName === 'result') {
        result = processCoreResponse(xhr.responseXML)
    }

    resultText = result ? (result.content[''] || '') : xhr.responseText

    ...
}

The $.isXMLDoc(xhr.responseXML) is failing because of the lack of the falsey check as before.

Could you reopen this issue please?

Thanks for the use case, reopening.

PR to jQuery (master branch, for jQuery 4.0): #4814
PR to Sizzle (for jQuery 3.6.0): jquery/sizzle#474

Landed in jquery/sizzle#474 & #4814 but we still need to update Sizzle on 3.x-stable.

Reopening as it awaits a Sizzle update.

Fixed on 3.x-stable (jQuery 3.x) by #4846.
Previously, it was only fixed on main (jQuery 4.0) by #4814.

Closing.