plugin.php

<?php
/**
 * Plugin Name: WP REST API Yoast SEO
 * Description: Adds Yoast fields to page and post metadata to WP REST API responses
 * Author: Charlie Francis
 * Author URI: https://github.com/ChazUK
 * Version: 1.2.0
 * Plugin URI: https://github.com/ChazUK/wp-api-yoast-seo
 */

class WPAPIYoastMeta {

    function __construct() {
        add_action('rest_api_init', array($this, 'add_yoast_data'));
	}

    function add_yoast_data() {
		// Posts
		register_rest_field( 'post',
	        'yoast',
	        array(
	            'get_callback'    => array( $this, 'wp_api_encode_yoast_post' ),
	            'update_callback' => null,
	            'schema'          => null,
	        )
	    );

		// Pages
		register_rest_field( 'page',
	        'yoast',
	        array(
	            'get_callback'    => array( $this, 'wp_api_encode_yoast_post' ),
	            'update_callback' => null,
	            'schema'          => null,
	        )
	    );

		// Public custom post types
		$types = get_post_types(array(
			'public' => true,
			'_builtin' => false
		));
		foreach($types as $key => $type) {
			register_rest_field( $type,
		        'yoast',
		        array(
		            'get_callback'    => array( $this, 'wp_api_encode_yoast_post' ),
		            'update_callback' => null,
		            'schema'          => null,
		        )
		    );
		}
	}

    function wp_api_encode_yoast_post($post, $field_name, $request) {

        include __DIR__ . '/classes/class-frontend.php';
        include __DIR__ . '/classes/class-opengraph.php';
        include __DIR__ . '/classes/class-twitter.php';

        wp_reset_query();
        query_posts([
            'p' => $post['id'],
            'post_type' => $post['type']
        ]);



        $wpseo_frontend = WPAPI_WPSEO_Frontend::get_instance();
        $wpseo_opengraph = new WPAPI_WPSEO_OpenGraph();
        $wpseo_twitter = new WPAPI_WPSEO_Twitter();

        $yoastMeta = array(
            'metas' => array(
                'focuskw' => get_post_meta($post['id'],'_yoast_wpseo_focuskw', true),
                'title' => get_post_meta($post['id'], '_yoast_wpseo_title', true),
                'metadesc' => get_post_meta($post['id'], '_yoast_wpseo_metadesc', true),
                'linkdex' => get_post_meta($post['id'], '_yoast_wpseo_linkdex', true),
                'metakeywords' => get_post_meta($post['id'], '_yoast_wpseo_metakeywords', true),
                'meta-robots-noindex' => get_post_meta($post['id'], '_yoast_wpseo_meta-robots-noindex', true),
                'meta-robots-nofollow' => get_post_meta($post['id'], '_yoast_wpseo_meta-robots-nofollow', true),
                'meta-robots-adv' => get_post_meta($post['id'], '_yoast_wpseo_meta-robots-adv', true),
                'canonical' => get_post_meta($post['id'], '_yoast_wpseo_canonical', true),
                'redirect' => get_post_meta($post['id'], '_yoast_wpseo_redirect', true),
                'opengraph-title' => get_post_meta($post['id'], '_yoast_wpseo_opengraph-title', true),
                'opengraph-description' => get_post_meta($post['id'], '_yoast_wpseo_opengraph-description', true),
                'opengraph-image' => get_post_meta($post['id'], '_yoast_wpseo_opengraph-image', true),
                'twitter-title' => get_post_meta($post['id'], '_yoast_wpseo_twitter-title', true),
                'twitter-description' => get_post_meta($post['id'], '_yoast_wpseo_twitter-description', true),
                'twitter-image' => get_post_meta($post['id'], '_yoast_wpseo_twitter-image', true)
            ),
            'title' => $wpseo_frontend->title(wp_get_document_title()),
            'canonical' => $wpseo_frontend->canonical(false),
            'metatags' => array(
                'description' => $wpseo_frontend->metadesc(false),
                'keywords' => $wpseo_frontend->metakeywords(),
                'robots' => $wpseo_frontend->robots(),
                'og:title' => $wpseo_opengraph->og_title(false),
                'og:description' => $wpseo_opengraph->description(false),
                'twitter:title' => $wpseo_twitter->title(),
                'twitter:description' => $wpseo_twitter->description(),
                'twitter:image' => $wpseo_twitter->image()
            )
        );

        $ogimages = $wpseo_opengraph->image();
        if (is_array($ogimages) && !empty($ogimages)) {
            foreach($ogimages as $tag) {
                $yoastMeta['metatags'][$tag['name']] = $tag['value'];
            }
        }

        return (array) $yoastMeta;
    }
}

$WPAPIYoastMeta = new WPAPIYoastMeta();