Add Javascript to a specific 'Basic page' or node based on its title

The complete solution here:

1. My theme is called themex.

2. Inside the file themex.libraries.yml define an asset library containing the Javascript code that you want to include in your specific page.

3. In the file themex.theme implement the hook hook_page_attachments_alter as defined below. The hook checks if the requested page (or node) has some specific attributes and when the condition is true it associates the asset library trickyjs with the requested page.

    /* Implements hook_page_attachments_alter() */
   
    1 function themex_page_attachments_alter(&$page)
    2 {
    3    $rm = \Drupal::routeMatch();
    4    $node = $rm->getParameter('node');
    5    
    6    if( $node instanceof \Drupal\node\NodeInterface )
    7    {
    8        if( $node->getTitle() == "policy" )
    9        {
   10            $page['#attached']['library'][] = 'themex/trickyjs';
   11        }
   12    }
   13 }
   

4. The lines 3, 4 and 6 are necessary to check if the requested URL is for a Node. The code in the line 8 checks if the page meets the special condition and the code in the line 10 associates the asset library with your page.

5. In the output of your page you will find a <script> tag including your library.