add-custom-fields-to-json.php

add_action( 'rest_api_init', 'create_api_posts_meta_field' );

function create_api_posts_meta_field() {
    register_rest_field( 'post', 'custom_fields', array(
           'get_callback' => 'get_post_meta_for_api',
           'schema'       => null,
        )
    );
}

function get_post_meta_for_api( $object ) {
    $post_id = $object['id'];
    return get_post_meta( $post_id );
}