Set a checkmark in a category based on a URL-parameter

Question

I've made a site, where there's a category for each page. And now I'm trying to make it so, that if you on that given page press 'Create post', that it then sends a parameter to the URL, as such:

example.org/wp-admin/post-new.php?post_type=CPT&category=3

And it doesn't need to actually put the post in the category... It just needs to set the checkmark in the category with ID=3, so when it's posted, that it then does it then.

I imagined hooking onto something along the lines of 'create_post' or something like that and just do it with some JavaScript (find #categorydiv and .inside, and find the category by ID and mark it as checked). But it feels a bit 'hacky' to me.

Is it really the right way to do it, - or does WordPress offer a better way?

Answer 1

I figured it out myself, but by using several different small things. I'm sorry to 'steal the credit', - but I can't remember all the sources used:

This is the JavaScript:

jQuery(document).ready(function() {

  function GetURLParameter(sParam) { 
    var sPageURL = window.location.search.substring(1);
    var sURLVariables = sPageURL.split('&');
    for (var i = 0; i < sURLVariables.length; i++) {
      var sParameterName = sURLVariables[i].split('=');
      if (sParameterName[0] == sParam) {
        return sParameterName[1];
      } // Endif
    } // Endfor
  } // Endfor

  var just_the_category_ID = GetURLParameter('category'); // Get the category-parameter from the URL
  jQuery('#in-category-' + just_the_category_ID ).prop('checked', true);
});

And it it called with this function in functions.php:

function custom_enqueue_admin_scripts(  ) {

  global $page_category_mapping;
  $screen = get_current_screen(); // Which page we're on

  if ( in_array( $screen->id, array( 'page' ) ) && $screen->action == 'add' ) {
    wp_enqueue_script( 'mark-default-category', get_stylesheet_directory_uri() . '/assets/js/THE-LOCATION-OF-THE-JS-FROM-ABOV.js', array( 'jquery' ) );
  }
}
add_action( 'admin_footer', 'mss_enqueue_admin_scripts' );