I have a requirement to use absolute URLs for the canonical URL element generated by Drupal 8.

hook_page_attachments_alter doesn't seem to have access the HTML element. Is there any other way I can access it?


True, for nodes you can't use hook_page_attachments_alter. You have to use hook_ENTITY_TYPE_view_alter!

And very likely you also find all of them in the end in hook_preprocess_html.

Here's how I got it working:

 * Implements hook_ENTITY_TYPE_view_alter().
function MYMODULE_node_view_alter(array &$build, Drupal\Core\Entity\EntityInterface $entity, \Drupal\Core\Entity\Display\EntityViewDisplayInterface $display) {
  if (isset($build['#attached']['html_head_link'])) {
    foreach ($build['#attached']['html_head_link'] as $key => $head) {
      if ((isset($head[0]['rel']) ? $head[0]['rel'] : FALSE) == 'canonical') {

        $url = \Drupal\Core\Url::fromRoute('<current>', [], ['absolute' => 'true'])

        $build['#attached']['html_head_link'][$key][0]['href'] = $url;

Source: How to remove specific meta-tag from header section of page?

Alternatively you can use Metatag. All you have to do is to set the [node:url:absolute] token into the Canonical URL form field in the Advanced section of the Metatag configuration for all Content.

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And only then, all of sudden, they become available to be preprocessed in hook_page_attachments_alter. There I removed their trailing slash which gets added by default.

 * Implements hook_page_attachments_alter().
function MYTHEME/MYMODULE_page_attachments_alter(array &$attachments) {

  // Remove trailing slash from canonicals.
  foreach ($attachments['#attached']['html_head'] as $key => $head) {

    if ((isset($head[1]) ? $head[1] : FALSE) == 'canonical_url') {

      $href = isset($head[0]['#attributes']['href']) ? $head[0]['#attributes']['href'] : FALSE;

      if ($href) {
        $attachments['#attached']['html_head'][$key][0]['#attributes']['href'] = rtrim($href, '/');

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