wp_script_add_data() │ WP 4.2.0

This example shows how to add a connection condition to a registered script. For example, let's add the condition <!--[if lt IE 9]>...<![endif]--> - for IE less than 9.

add_action( 'wp_enqueue_scripts', 'my_enqueue_scripts' );

function my_enqueue_scripts() {

	// Connect special scripts only for Internet Explorer 6
	wp_enqueue_script('pngfix', get_template_directory_uri() .'js/pngfix.js');

	wp_script_add_data('pngfix', 'conditional', 'lt IE 9');
}

As a result we get in the head of the document

<!--[if lt IE 9]>
<script type='text/javascript' src='http://example.com/wp-content/themes/theme/js/pngfix.js?ver=4.5.2'></script>
<![endif]-->

add_action( 'wp_enqueue_scripts', 'my_enqueue_scripts' );

function my_enqueue_scripts() {

	wp_enqueue_script( 'app', get_template_directory_uri() . 'js/app.js' );

	// Добавляем defer к зарегистрированному скрипту.
	wp_script_add_data( 'app', 'defer', true );
}

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

/**
 * Example of adding async|defer attribute for the script:
 *
 *     ```
 *     wp_register_script( $handle, ... )
 *
 *     // add async attribute
 *     wp_script_add_data( $handle, 'async', true );
 *
 *     // or add defer attribute
 *     wp_script_add_data( $handle, 'defer', true );
 *
 *     // unset the attr
 *     wp_script_add_data( $handle, 'async', false );
 *     ```
 */
function support_async_defer_script_attrs( $tag, $handle ) {

	foreach ( [ 'async', 'defer' ] as $attr ) {

		if ( ! wp_scripts()->get_data( $handle, $attr ) ) {
			continue;
		}

		// Prevent adding attribute twice.
		if ( ! preg_match( ":\s{$attr}[=>\s]:", $tag ) ) {
			$tag = preg_replace( ':(?=></script>):', " $attr", $tag, 1 );
		}

		break; // Only async or defer, not both.
	}

	return $tag;
}

As a result we get in the head of the document

<script type='text/javascript' src='http://example.com/wp-content/themes/theme/js/app.js?ver=4.5.2' defer></script>

add_action( 'wp_enqueue_scripts', 'twentytwenty_register_scripts' );
add_filter( 'script_loader_tag', 'filter_script_loader_tag', 10, 2 );

function twentytwenty_register_scripts() {

	wp_enqueue_script( 'twentytwenty-js', get_template_directory_uri() . '/assets/js/index.js', [], '1.0', false );

	wp_script_add_data( 'twentytwenty-js', 'async', true );
}

function filter_script_loader_tag( $tag, $handle ) {

	foreach ( array( 'async', 'defer' ) as $attr ) {

		if ( ! wp_scripts()->get_data( $handle, $attr ) ) {
			continue;
		}

		// Prevent adding attribute when already added in #12009.
		if ( ! preg_match( ":\s$attr(=|>|\s):", $tag ) ) {
			$tag = preg_replace( ':(?=></script>):', " $attr", $tag, 1 );
		}

		// Only allow async or defer, not both.
		break;
	}

	return $tag;
}