Why does $page['content']['#attached']['js'] works , while $page['#attached']['js'] does not?

Question

Trying to attach js by means of #attached by means of hook_page_alter (see https://drupal.stackexchange.com/questions/171918/how-to-add-javascript-with-attached-and-hook-page-alter), I found that the following code

 function MODULE_page_alter(&$page) {
  $my_path = drupal_get_path('module', 'MODULE' );
  $page ['content']['#attached']['js'][] = array(
    'js' => array(
    'type'=>'file',
    'data' => $my_path . '/module_test.js'),
  );
}

works properly - it attaches the file - while

function MODULE_page_alter(&$page) {
  $my_path = drupal_get_path('module', 'MODULE' );
  $page ['#attached']['js'][] = array(
    'js' => array(
    'type'=>'file',
    'data' => $my_path . '/module_test.js'),
  );
}

does not. Why is it so?

Answer 1

For #attached elements to be processed, the array needs to be run through drupal_render(). That never happens for the $page array.

Parts of it are rendered, e.g. in page.tpl.php:

<?php print render($page['content']); ?>

But never the $page array itself.