/**
 * Add Async & Defer to certain scripts.
 *
 * @param string $tag      Script Tags to display.
 * @param string $handle   Name of file specified above.
 */
function add_async_defer_attribute( $tag, $handle ) {

	$scripts = array( 'script-handle' );
	foreach ( $scripts as $script ) {
		if ( $script === $handle ) {
			return str_replace( ' src', ' async="async" defer="defer" src', $tag );
		}
	}
	return $tag;

}
add_filter( 'script_loader_tag', 'add_defer_attribute', 10, 2 );