WordPress: Helper functions to get first category's metadata.

functions.php

<?php

function your_theme_prefix_get_first_cat_object($page_id, $category_metadata) {
  if ( empty($page_id) ) {
    return new WP_Error('404', __('Missing $page_id parameter', 'your_theme_prefix'), array());
  }
  
  $categories = get_the_category($page_id);
  
  return !empty($categories) ? $categories[0]->{$category_metadata} : null;
}

function your_theme_prefix_get_category_slug($page_id) {
  return your_theme_prefix_get_first_cat_object($page_id, 'slug');
}

function your_theme_prefix_get_category_name($page_id) {
  return your_theme_prefix_get_first_cat_object($page_id, 'name');
}

?>