add #ajax property to email field in node add form usion hook_form_alter

Question

I've tried to add the property "#ajax" to email field but It dosen't work, I've tried all this :

$form["field_user_email"]['#ajax'] = array(
      'callback' => 'user_ajax_check',
      'wrapper' => 'usercheck',
      'effect' => 'fade',
  );

$form["field_user_email"]['und']['#ajax'] = array(
      'callback' => 'user_ajax_check',
      'wrapper' => 'usercheck',
      'effect' => 'fade',
  );
$form["field_user_email"]['und'][0]['#ajax'] = array(
      'callback' => 'user_ajax_check',
      'wrapper' => 'usercheck',
      'effect' => 'fade',
  );

Answer 1

If the field is provided by the Email Field module it'll probably be:

$form["field_user_email"]['und'][0]['email']['#ajax'] = array(...

If it's just a standard text field it will be:

$form["field_user_email"]['und'][0]['value']['#ajax'] = array(...