How can this error be corrected?

Question

PHP

Good afternoon! How can I fix this error: Request failed: parsererror

var request = $.ajax({
      url: "ajax.php",
      type: "POST",
      data: { myData },
      dataType: "json",
      // headers: { 
     //        'Content-Type': 'application/json',
     //        Accept: 'application/json'
    //   	},
      cache: false,
      });

php code:

$data = $sth->fetchAll();
  //var_dump($data);
  //die();
  echo json_encode($data);

could not understand how it returns html format?

Answer 1

after returning json, it's better to exit
in devtools on the newtwork tab, you can find this request and see how the answer looks like