I was still unable to find out that in Gutenberg. Specifically, i used:
wp.data.select("core/editor").getPostEdits();
to find out how many categories were checked. But it missed already checked categories… So, which function should I use to access in sum, how many categories are select at this moment? where is that data stored?
To get the categories from inside the editor of a post you can make use of the following selectors:
The categories the post has in the published version:
wp.data.select("core/editor").getCurrentPostAttribute("categories")
The current categories of the edit (for example if the user has selected a new category but hasn’t saved the post it will appear with this selector but not with the former):
wp.data.select("core/editor").getEditedPostAttribute("categories")
This will give you an array with the id of each category.