Getting children of <object> tag with contents() not working correctly

[patocallaghan]

Description

Version: jQuery 3.4.1

Since 0ba8e38 it appears that using contents() to get the children of an <object> is not working as expected.

Taking this example HTML

<object type="application/x-shockwave-flash" width="200" height="300" id="penguin">
    <param name="movie" value="flash/penguin.swf">
    <param name="quality" value="high">
    <img src="images/penguin.jpg" width="200" height="300" alt="Penguin">
</object>

I'd expect the following to happen

jQuery(objectHtml).contents().length // => 3

But what actually happens is:

jQuery(objectHtml).contents().length // => 0

This happens because on this line

jquery/src/traversing.js

Lines 148 to 149 in 110802c

	if ( typeof elem.contentDocument !== "undefined" ) {
	return elem.contentDocument;

it only checks if elem.contentDocument is not undefined. But in this scenario elem.contentDocument is null so the conditional is truthy and enters the if block and returns null for the contents.

If we change the conditional to be elem.contentDocument != null it works correctly.

Link to test case

https://codepen.io/patocallaghan/pen/bybaWv?editors=0011

I have a PR to fix it but I wanted to open an issue first.

[mgol]

Thanks for the report! This is a regression from jQuery 3.3.1 so I added the 3.4.2 milestone.

[mgol]

PR: #4385