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, Tedy Warsitha
 * Author URI: https://github.com/ChazUK
 * Version: 1.1.0
 * Plugin URI: https://github.com/ChazUK/wp-api-yoast-seo
 */
class WPAPIYoastMeta {

    protected $keys = array(
        'yoast_wpseo_focuskw',
        'yoast_wpseo_title',
        'yoast_wpseo_metadesc',
        'yoast_wpseo_linkdex',
        'yoast_wpseo_metakeywords',
        'yoast_wpseo_meta-robots-noindex',
        'yoast_wpseo_meta-robots-nofollow',
        'yoast_wpseo_meta-robots-adv',
        'yoast_wpseo_canonical',
        'yoast_wpseo_redirect',
        'yoast_wpseo_opengraph-title',
        'yoast_wpseo_opengraph-description',
        'yoast_wpseo_opengraph-image',
        'yoast_wpseo_twitter-title',
        'yoast_wpseo_twitter-description',
        'yoast_wpseo_twitter-image'
    );

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

    function add_yoast_data() {
        // Posts
        register_api_field( 'post',
            'yoast_meta',
            array(
                'get_callback'    => array( $this, 'wp_api_encode_yoast' ),
                'update_callback' => array( $this, 'wp_api_update_yoast' ),
                'schema'          => null,
            )
        );

        // Pages
        register_api_field( 'page',
            'yoast_meta',
            array(
                'get_callback'    => array( $this, 'wp_api_encode_yoast' ),
                'update_callback' => array( $this, 'wp_api_update_yoast' ),
                'schema'          => null,
            )
        );

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

    /**
     * Updates post meta with values from post/put request.
     * @param array $value
     * @param object $data
     * @param string $field_name
     *
     * @return array
     */
    function wp_api_update_yoast( $value, $data, $field_name ) {

        foreach ( $value as $k => $v ) {

            if ( in_array( $k, $this->keys ) ) {
                ! empty( $k ) ? update_post_meta( $data->ID, '_' . $k, $v ) : null;
            }
        }

        return $this->wp_api_encode_yoast( $data->ID, null, null );
    }

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

        foreach (array_keys(get_taxonomies(['public' => true, 'hierarchical' => true])) as $taxonomy) {
            $this->keys[] = 'yoast_wpseo_primary_' . $taxonomy;
        }

        $frontend = WPSEO_Frontend::get_instance();
        $object = get_post($post['id']);

        foreach ($this->keys as $key) {
            if ('yoast_wpseo_title' === $key) {
                $yoastMeta[$key] = $frontend->get_content_title($object);
            } else {
                $yoastMeta[$key] = get_post_meta($post['id'], '_' . $key, true);
            }
        }

        return (array) $yoastMeta;
    }

}

$WPAPIYoastMeta = new WPAPIYoastMeta();