<?php add_action( 'rest_api_init', function() { register_rest_field( 'comment', 'karma', array( 'get_callback' => function( $comment_arr ) {
            $comment_obj = get_comment( $comment_arr['id'] ); 
            return (int) $comment_obj->comment_karma;
        },
        'update_callback' => function( $karma, $comment_obj ) {
            $ret = wp_update_comment( array(
                'comment_ID'    => $comment_obj->comment_ID,
                'comment_karma' => $karma
            ) );
            if ( false === $ret ) {
                return new WP_Error( 'rest_comment_karma_failed', __( 'Failed to update comment karma.' ), array( 'status' => 500 ) );
            }
            return true;
        },
        'schema' => array(
            'description' => __( 'Comment karma.' ),
            'type'        => 'integer'
        ),
    ) );
} );