designtopack/public/site/config/routes/add-comment.php

<?php

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

      // return json_encode($data->pageUri);

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


      $comments = $file->comments()->isEmpty() == true ? [] : Data::decode($file->comments()->toArray()['comments'], 'yaml');

      $newComment = [
        'text' => $data->text,
        'username' => $user->name()->isNotEmpty() ? (string) $user->name() : (string) $user->email(),
        'date' => (string) $data->date,
        'id' => $data->id
      ];

      $comments[$data->targetPage][$data->id] = $newComment;
      
      $newFile = $file->update([
        'comments' => $comments
      ]);

      foreach (kirby()->users()->without($user) as $otherUser) {
        try {
          $notifications = $otherUser->notifications()->isNotEmpty() ? $otherUser->notifications()->toArray() : ["comments" => []];
          $notifications['comments'][$data->id] = $newComment;
          $otherUser->update([
            "notifications" => $notifications
          ]);
        } catch (\Throwable $th) {
          throw new Exception($th->getMessage(), 1);
          
        }
      }

      return getFileData($newFile);
  }
];