designtopack/public/site/plugins/comments/routes/reply.php

<?php

use adrienpayet\comments\Reply;

return [
  'pattern' => '(:all)reply-comment.json',
  'method' => 'POST',
  'action'  => function () {
      $json = file_get_contents('php://input');
      $data = json_decode($json);

      $page = page($data->pageUri);
      $file = $page->file($data->fileName);
      $user = kirby()->user($data->userUuid);


      $comments = $file->comments()->isEmpty() == true ? [] : Yaml::decode($file->comments()->value());

      $data = [
        'page' => $page,
        'parentId' => $data->parentId,
        'file' => $file,
        'filePageIndex' => $data->filePageIndex,
        'text' => $data->text,
        'author' => $user,
        'date' => (string) $data->date,
        'id' => $data->id,
        'type' => 'comment'
      ];

      $newReply = new Reply($data);

      foreach ($comments as &$comment) {
        if ($comment['id'] === $newReply->parentId()) {
            $comment['replies'][] = $newReply->toArray();
        }
      }
      
      $newFile = $file->update([
        'comments' => $comments
      ]);

      $user->sendNotification($newReply);

      return getFileData($newFile);
  }
];