Can't add js to content creation pages
/**
 *  Implements hook_form_BASE_FORM_ID_alter.
 */
function mymodule_form_node_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  $form['#attached']['js'][] = drupal_get_path('module', 'mymodule') . '/js/custom.js';
}


In your case would be:

/**
 *  Implements hook_form_BASE_FORM_ID_alter.
 */
function add_js_form_node_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  $form['#attached']['js'][] = drupal_get_path('module', 'add_js') . '/js/custom.js';
}


Note: This will add the JS to all of your content types node edit pages. 



If you need to target a specific content type, all you gotta do is add an if statement, see:

How can I find the form ID of a form?

/**
*  Implements hook_form_BASE_FORM_ID_alter.
*/
  function add_js_form_node_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form_id == 'my_form_id') {
      $form['#attached']['js'][] = drupal_get_path('module', 'add_js') . '/js/custom.js';
    }
  }


and I believe you could alternatively do this, put the my_form_id directly in the hook:

/**
*  Implements hook_form_BASE_FORM_ID_alter.
*/
  function add_js_form_my_form_id_alter(&amp;$form, &amp;$form_state, $form_id) {

      $form['#attached']['js'][] = drupal_get_path('module', 'add_js') . '/js/custom.js';

  }