Prevent posting when no category is selected

Hello,

I’m trying to trigger an action if, when a user tries to publish a post, no category is selected:
– It could either be just an alert message (and the post is still published);
– Or actually alter message + preventing the post from being published.

Searching on this forum, I’ve found the code:

jQuery('#submitdiv').on('click','#publish',function(e){
            var $checked = jQuery('#category-all li input:checked');
            if ( $checked.length <= 0 ) { //Checks if cat is selected
            alert("Please Select atleast one category");
            return false;
            }else{ //Else continue
                return true;
            }
            });

But I have no idea where to put it.

Thanks!