Registering Menu, Custom Post Types, Custom Fields & Custom Plugins to REST API in Wordpress

functions.php

<?php

//Registering Menu Items to REST API (API URL : http://website.com/wp-json/menu/)
function get_menu() {
    # Change 'hamburger' to your own navigation slug.
    return wp_get_nav_menu_items('hamburger');
}

add_action( 'rest_api_init', function () {
        register_rest_route( 'spirit', '/menu', array(
        'methods' => 'GET',
        'callback' => 'get_menu',
    ) );
} );

/**
* Add REST API support to an already registered post type. (Custom Plugins or Custom Post Type)
* API URL : http://www.website.com/wp-json/wp/v2/<post_type>
*/
add_action( 'init', 'cp_recipe_rest_support', 25);
function cp_recipe_rest_support() {
    global $wp_post_types;

    //be sure to set this to the name of your post type!
    $post_type_name = 'cp_recipe'; 
    if( isset( $wp_post_types[ $post_type_name ] ) ) {
        $wp_post_types[$post_type_name]->show_in_rest = true;
        $wp_post_types[$post_type_name]->rest_base = $post_type_name;
        $wp_post_types[$post_type_name]->rest_controller_class = 'WP_REST_Posts_Controller';
    }
}

//Registering Custom Fields of (Custom Post Type or Custom Plugin) to Rest API
//Adding Custom Input Fields
add_action('rest_api_init', 'slug_register_short_desc');

function slug_register_short_desc() {
    register_rest_field('cp_recipe','_cp_recipe_short_description',
        array(
            'get_callback'  => 'slug_get_short_desc',
            'update_callback' => null,
            'schema'  => null
        )
    );
}

function slug_get_short_desc($object,$field_name,$request) {
    return get_post_meta($object['id'],$field_name,true);
}

//Addding Featured Image (Extracting URL and Adding it to REST API)
add_action('rest_api_init', 'slug_register_featured_image');

function slug_register_featured_image() {
    register_rest_field('cp_recipe','featured_image',
        array(
            'get_callback'  => 'slug_get_featured_image',
            'schema'    => null
        )
    );
}

function slug_get_featured_image($object,$field_name,$request) {
    // Only proceed if the post has a featured image.
    if ( ! empty( $object['featured_media'] ) ) {
        $image_id = (int)$object['featured_media'];
    } elseif ( ! empty( $object['featured_image'] ) ) {
        // This was added for backwards compatibility with < WP REST API v2 Beta 11.
        $image_id = (int)$object['featured_image'];
    } else {
        return '';
    }

    $image = get_post( $image_id );
    if ( ! $image ) {
        return '';
    }

    $featured_image = wp_get_attachment_url( $image_id );
    return apply_filters( 'slug_get_featured_image', $featured_image, $image_id );
}

?>