WordPress function to accurately retrieve the current category ID outside the loop.

current_category_id.php

<?php

/* current_category_ID()
 *
 * Get the currently-being-viewed category's ID.
 *
 * THE PROBLEM:
 * WordPress offers no easy way to get the id of the category that is currently
 * being viewd in an index.php. It offers nice conventions to get the title and
 * description (within the loop), but I needed this information outside the loop.
 *
 * THE SOLUTION:
 * After searching WordPress' documentation, get_queried_object() finally emerged
 * as a way to functionally get this information.
 */
function current_category_ID()
{
  $category = get_queried_object();
  return $category->term_id;
}