There are going to be a few PHP 8 incompatibilties - so this is probably the place to note them down.
Please add to this list if you find any ... JCook is not being developed anymore - so we need to help each other to keep our older websites running.

If you upgrade to PHP 8 and your whole website goes down... its is probably line 384 in libraries/jdom.php (which is loaded for eevry page).

If you swicth on the Joomla Degug mode, you will see this error...
"count(): Argument #1 ($value) must be of type Countable|array, null given"
This is because a emplty variable, a non-array, is being used with the count function.

In libraries/jdom edit file jdom.php line 384

Change

to

Now, after this first hurdle... when you go to create a New item, you'll get the same error. This is another bug in the item controller.

In /public_html/administrator/components/com_yourcomponent/classes/controller/item.php Line 240

Change:
To

Good work

Hi Chris,
Thanks a lot for this.
I've also a warning (error report : Dev) in the next line 385.
Did you change anything, or never mind ?Warning: Trying to access array offset on value of type null in /home2/user_xxx/public_html/my_site/libraries/jdom/dom.php on line 385
change to

To be continue...
Please find below a few code corrections to upgrade PHP8 version with your J-Cook component:

libraries/jdom/dom.php:384
administrator/components/com_mycomponent/classes/controller/item.php:240
administrator/components/com_mycomponent/classes/model/list.php:328
administrator/components/com_mycomponent/helpers/menu.php:61