Custom post type category not displaying category ID

Question

I'm trying to get the category ID so I can call a loop query on only that category ID. However the following code returns null. Any ideas why? In the admin bar it shows edit category with an ID of 3, however I need this ID to be generated dynamically.

$categories = get_the_category();
$category_id = $categories[0]->cat_ID;
var_dump($category_id);

This category is generated from a custom post type.

I used this to get the category ID:

$category = get_category( get_query_var( 'cat' ) );
$cat_id = $category->cat_ID; 

EDIT: I registered the custom post type like this:

// Register Custom Post Type
function custom_post_type() {

$labels = array(
    'name'                  => _x( 'Works', 'Post Type General Name', 'text_domain' ),
    'singular_name'         => _x( 'Work', 'Post Type Singular Name', 'text_domain' ),
    'menu_name'             => __( 'Works', 'text_domain' ),
    'name_admin_bar'        => __( 'Works', 'text_domain' ),
    'archives'              => __( 'Item Archives', 'text_domain' ),
    'attributes'            => __( 'Item Attributes', 'text_domain' ),
    'parent_item_colon'     => __( 'Parent Item:', 'text_domain' ),
    'all_items'             => __( 'All Items', 'text_domain' ),
    'add_new_item'          => __( 'Add New Item', 'text_domain' ),
    'add_new'               => __( 'Add New', 'text_domain' ),
    'new_item'              => __( 'New Item', 'text_domain' ),
    'edit_item'             => __( 'Edit Item', 'text_domain' ),
    'update_item'           => __( 'Update Item', 'text_domain' ),
    'view_item'             => __( 'View Item', 'text_domain' ),
    'view_items'            => __( 'View Items', 'text_domain' ),
    'search_items'          => __( 'Search Item', 'text_domain' ),
    'not_found'             => __( 'Not found', 'text_domain' ),
    'not_found_in_trash'    => __( 'Not found in Trash', 'text_domain' ),
    'featured_image'        => __( 'Featured Image', 'text_domain' ),
    'set_featured_image'    => __( 'Set featured image', 'text_domain' ),
    'remove_featured_image' => __( 'Remove featured image', 'text_domain' ),
    'use_featured_image'    => __( 'Use as featured image', 'text_domain' ),
    'insert_into_item'      => __( 'Insert into item', 'text_domain' ),
    'uploaded_to_this_item' => __( 'Uploaded to this item', 'text_domain' ),
    'items_list'            => __( 'Items list', 'text_domain' ),
    'items_list_navigation' => __( 'Items list navigation', 'text_domain' ),
    'filter_items_list'     => __( 'Filter items list', 'text_domain' ),
);
$args = array(
    'label'                 => __( 'Work', 'text_domain' ),
    'description'           => __( 'Work Description', 'text_domain' ),
    'labels'                => $labels,
    'supports'              => array( 'title', 'editor' ),
    'taxonomies'            => array( 'category', 'post_tag' ),
    'hierarchical'          => false,
    'public'                => true,
    'show_ui'               => true,
    'show_in_menu'          => true,
    'menu_position'         => 5,
    'show_in_admin_bar'     => true,
    'show_in_nav_menus'     => true,
    'can_export'            => true,
    'has_archive'           => true,
    'exclude_from_search'   => false,
    'publicly_queryable'    => true,
    'capability_type'       => 'page',
    'show_in_rest' => true,
);
register_post_type( 'works', $args );

}
add_action( 'init', 'custom_post_type', 0 );

Answer 1

get_the_category retrieve array of post categories.

This tag may be used outside the loop by passing a post id as the parameter.

The syntax is

$categories = get_the_category(  $post_id  ); // Outside the loop

// OR

$categories = get_the_category( ); // Inside the loop

Note: This function only returns results from the default “category” taxonomy. For custom taxonomies use get_the_terms() .

$categories = get_the_terms( $post_id, 'taxonomy' );

I hope this may help.

Answer 2

I used this to get the category ID.

$category = get_category( get_query_var( 'cat' ) );
$cat_id = $category->cat_ID;