I am new to Drupal 8 theming and I've been long time trying the following:

I want to add classes to page-title in the template, page-title.html.twig. Two new classes:

1.- 'page_title'

2.- The real title of the page

The code I'm using is,:

{{ title_prefix }}

set classes = [

{% if title %}
<h1{{ title_attributes.addClass( classes ) }}>{{ title }}</h1>
{% endif %}

{{ title_suffix }}

it renders:

<h1 class="js-quickedit-page-title page-title array">FORMACIÓN: CURSOS Y  TALLERES</h1>

The string 'page-title' is working well but the variable title provoke an error:

Notice: Array to string conversion in Drupal\Component\Utility\Html::getClass() (line 72 of core\lib\Drupal\Component\Utility\Html.php).

And print the class, array.

Any help?

1 Answer 1


Seems title is a render array. You probably have to use title.value instead. But I'd recommend to simply preprocess that and leave the templates untouched.

You can implement hook_preprocess_hook and then use TitleResolver::getTitle and Html::cleanCssIdentifier to do what you want from inside your theme's *.theme file or from within a custom module's *.module file:

 * Implements hook_preprocess_HOOK().
function MYTHEME/MYMODULE_preprocess_page_title(&$variables) {

  // This won't work properly with Twig debugging enabled.
  // $title = $variables['title'];

  // Here is a safer method to get the current title.
  $request = \Drupal::request();
  $route_match = \Drupal::routeMatch();
  $title = \Drupal::service('title_resolver')->getTitle($request, $route_match->getRouteObject());

  $css_title = \Drupal\Component\Utility\Html::cleanCssIdentifier($title);
  $variables['title_attributes']['class'][] = 'page-title';
  $variables['title_attributes']['class'][] = strtolower($css_title);

You can use nearly the exact same snippet to add that CSS class to the <body> tag:

 * Implements hook_preprocess_HOOK().
function MYTHEME/MYMODULE_preprocess_html(&$variables) {

  $request = \Drupal::request();
  $route_match = \Drupal::routeMatch();
  $title = \Drupal::service('title_resolver')->getTitle($request, $route_match->getRouteObject());

  $css_title = \Drupal\Component\Utility\Html::cleanCssIdentifier($title);
  $variables['attributes']['class'][] = strtolower($css_title);
  • So, I implemented that function, but I'm getting errors when there is no page title available. Any adjustments that you'd recommend?
    – Kellen
    May 5, 2020 at 19:30
  • @Kellen – Depends on the errors and what pages exactly but I'd probably start with some basic if-else to catch that and provide a fallback?!
    – leymannx
    May 5, 2020 at 20:57
  • I'm not much of a programmer. I wouldn't know exactly what kind of if statement to apply. I expanded on my issue here if you can help: drupal.stackexchange.com/questions/293487/… Thank you!
    – Kellen
    May 7, 2020 at 13:50
  • @Kellen – "Expects string but array given" says all. Then you only add the CSS class if (!is_array($title)) { and check on } else { what exactly is inside this array. Maybe the title then is inside $title['value']. You could install the Devel submodule Kint. And then do ksm($title). Flush cache, flush cache and then you get the title variable pretty-printed on your page and you can inspect it.
    – leymannx
    May 7, 2020 at 14:25

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