<?php
/* Async and defer script */
function my_theme_async_defer_script( $tag , $handle , $src )
{
  $async = ( $handle == 'my-theme-scripts' ) ? 'async defer src' : 'src';
  return str_replace( "src" , $async , $tag );
}

add_action( 'script_loader_tag' , 'my_theme_async_defer_script' , 10 , 4 );
?>