function sendIdToFrontend(){
  global $post;
  $postId = array('post_id' => $post->ID);
  die(json_encode($postId));
}

This is my ajax function, I am at single post page. Where I've button which triggers this function. But I'm unable to get the post id. Is there anything wrong with this function ? I'm getting "null" in response.

jQuery.ajax({
  url: ajax_object.ajaxurl,     
  type: 'POST',
  dataType: 'json',
  data: {'test': 'test', action: 'checkCorrectAnswersViaAjax'},
  success: function(response){
    console.log(response);
  },
  error: function(err){
    console.log(err);
  } 
});