Get the ID of category page with or without any posts

by

Make use of get_queried_object_id() in your category page. This will return the ID of the category.

This is really useful little function. It will return the the:

  • author ID on an author archive page

  • term ID on a taxonomy, tag and category archive pages

  • post ID on a single post page

  • page ID of a page

EDIT

From your answer, you can simplify your code to something like this

if ( is_category() ) {

    $catid = get_queried_object_id();

} elseif ( is_single() ) {

    $cats = wp_get_post_terms( get_queried_object_id(), 'category', array( 'fields' => 'ids' ) ); // category object
    $catid = $cats[0];
}

The above code is faster and is more reliable 🙂