Add Defer & Async Attributes to WordPress Scripts

WP-Defer-&-Async.php

<?php
/**
 * Add Defer & Async Attributes to WordPress Scripts
 * @author Matthew Horne - http://matthewhorne.me/defer-async-wordpress-scripts/
 */


/**
 * Defer
 */
function add_defer_attribute($tag, $handle) {
   $scripts_to_defer = array('my-js-handle', 'another-handle');
   foreach($scripts_to_defer as $defer_script) {
      if ($async_script !== $handle) return $tag;
      return str_replace(' src', ' defer="defer" src', $tag);
   }
   return $tag;
}
add_filter('script_loader_tag', 'add_defer_attribute', 10, 2);

/**
 * Async
 */
function add_async_attribute($tag, $handle) {
   $scripts_to_async = array('my-js-handle', 'another-handle');
   foreach($scripts_to_async as $async_script) {
      if ($async_script !== $handle) return $tag;
      return str_replace(' src', ' async="async" src', $tag);
   }
   return $tag;
}
add_filter('script_loader_tag', 'add_async_attribute', 10, 2);

You've made a mistake on line 14. $defer_script not $async_script :)