Twenty Nineteen - JS error on mobile menu when no visible items

[anevins]

Looks like there's an issue with the JS that populates the 'more' (...) menu on mobile. If there are no visible menu links inside the main menu on mobile and all of them are inside the 'more' menu, then the JS will break. I do not think this is an issue on a Child Theme.

Line 128 priority-menu.js:

prependElement( hiddenList, ! visibleList.lastChild || null === visibleList.lastChild ? visibleList.previousElementSibling : visibleList.lastChild );

Regarding .lastChild.

The visibleList element:

// Adds the necessary UI to operate the menu.
var visibleList  = container.parentNode.querySelector('.main-menu[id]');

If no items are inside the visible list (and all items are inside the more menu) then visibleList will indeed return as a Node, however lastChild will return null.

This means the value will not be catched properly by this inline condition:

visibleList.previousElementSibling : visibleList.lastChild 

And null will be passed to the second argument of the prependElement function when it is not a node - and cause a JS error.

_​https://wordpress.org/support/topic/javascript-error-on-child-theme/#post-11433882_

[desrosj]

@anevins can you provide some more steps to recreate this? The error noted on the support ticket is

Uncaught TypeError: Failed to execute ‘insertBefore’ on ‘Node’: parameter 1 is not of type ‘Node’. at prependElement (priority-menu.js:47)

I am only able to trigger the following similar but different error when the screen size is less than about 100 pixels wide: TypeError: Argument 1 of Node.insertBefore is not an object.

[anevins]

If you visit the URL provided by the second participant in that thread, you'll see on their site they have zero items visible in the menu at 767px or below.

[markbanf]

Has there been any progress on this bug?

To see it in action simply resize the following website to less than 768px and check the JS errors in priority-menu.js.

​https://gardener.markbanf.co.uk/

[anevins]

@markbanf I think it's not being considered as a wide use case and not prioritised to work on.

[markbanf]

Ok, thanks Andrew. Looks like I'll have to create my own navigation and give up on the built in menus.

Regardless, your help is much appreciated.

Mark