WordPress: add defer and async to selected scripts via script_loader_tag

functions.php

<?php

/**
 *
 * Add defer and async to selected scripts
 *
 * @author Matthew Horne
 * @link   https://matthewhorne.me/defer-async-wordpress-scripts/
 */

add_filter('script_loader_tag', 'prefix_add_defer_attribute', 10, 2);

add_filter('script_loader_tag', 'prefix_add_async_attribute', 10, 2);


function prefix_add_defer_attribute($tag, $handle) {
   // add script handles to the array below
   $scripts_to_defer = array('my-js-handle', 'another-handle');
   
   foreach($scripts_to_defer as $defer_script) {
      if ($defer_script === $handle) {
         return str_replace(' src', ' defer="defer" src', $tag);
      }
   }
   return $tag;
}

function prefix_add_async_attribute($tag, $handle) {
   // add script handles to the array below
   $scripts_to_async = array('my-js-handle', 'another-handle');
   
   foreach($scripts_to_async as $async_script) {
      if ($async_script === $handle) {
         return str_replace(' src', ' async="async" src', $tag);
      }
   }
   return $tag;
}