Add async or defer to a WordPress script

functions.php

//Enqueue JS like we normally do.
function add_test_js_files(){
   wp_enqueue_script('script1', '//script1.js?cid=6asdfsdf');
   wp_enqueue_script('script2', '//script2.js?cid=a213423423');
}
add_action('wp_enqueue_scripts','add_test_js_files');


function add_async_attribute($tag, $handle) {

   // add script handles to the array below
   $scripts_to_async = array('script1', 'script2');

   //Loop through each file and add the async value
   // You can switch async to defer here if needed.
   foreach($scripts_to_async as $async_script) {
      if ($async_script === $handle) {
         return str_replace(' src', ' async src', $tag);
      }
   }
   return $tag;
}
add_filter('script_loader_tag', 'add_async_attribute', 10, 2);