Extends WP REST API

extend_default_rest_fields.php

/**
 * Extends the default REST API response for posts to include custom fields such as 'category' and 'featured_img'.
 * This allows clients consuming the REST API to access additional information about posts without making extra requests.
 */

add_action( 'rest_api_init', 'rest_extended_default_rest_fields' );

function rest_extended_default_rest_fields() {
	$post_types = get_post_types( array( 'public' => true ), 'names' );
	$post_types = array_diff( $post_types, array( 'attachment' ) ); // Exclude 'attachment' post type
	$post_types = apply_filters( 'rest_extended_default_post_types', $post_types ); // Allow filtering the post types
	$taxonomies = apply_filters( 'rest_extended_default_taxonomies', array( 'category' ), $post_types ); // Allow filtering the taxonomies

	foreach ( $taxonomies as $taxonomy ) {
		if ( taxonomy_exists( $taxonomy ) ) {
			register_rest_field( $post_types, 
				$taxonomy, 
				array(
					'get_callback'    => 'get_rest_terms',
					'update_callback' => null,
					'schema'          => null,
				) 
			);
		}
	}
	
    register_rest_field( $post_types,
        'featured_img',
        array(
            'get_callback'    => 'get_rest_featured_image',
            'update_callback' => null,
            'schema'          => null,
        )
    );
}

function get_rest_terms( $object, $field_name, $request ) {
	$taxonmy = $field_name ?? 'category'; // use the field name as taxonomy, default to 'category' if not provided
	$terms = get_the_terms( $object['id'], $taxonmy ); 
	if ( ! empty( $terms ) && ! is_wp_error( $terms ) ) {
		return wp_list_pluck( $terms, 'name' ); // Return an array of term names
	}
	return false;
}

function get_rest_featured_image( $object, $field_name, $request ) {
    if( $object['featured_media'] ){
        $img_id = $object['featured_media'];

		$img_arr = array(
			'alt' => get_post_meta( $img_id, '_wp_attachment_image_alt', true ),
			'full' => wp_get_attachment_image_src( $img_id, 'full' )[0],
			'large' => wp_get_attachment_image_src( $img_id, 'large' )[0],
			'medium' => wp_get_attachment_image_src( $img_id, 'medium' )[0],
			'thumbnail' => wp_get_attachment_image_src( $img_id, 'thumbnail' )[0],
		);
		return $img_arr;

    }
    return false;
}