Drupal: How to correctly add js in a form_alter()

gistfile1.php

<?php

function MODULE_NAME_form_alter($form, &$form_state, $form_id) {
...
  $form['#after_build'] = array('MODULE_NAME_add_assets');
...
}

function MODULE_NAME_add_assets($element) {
  drupal_add_js(......);

  return $element;
}