Adds async/defer capability to JavaScript loading

functions.php

@@ -117,7 +117,8 @@ function twentynineteen_content_width() {
 function twentynineteen_scripts() {
 	wp_enqueue_style( 'twentynineteen-style', get_stylesheet_uri() );
 
-	wp_enqueue_script( 'twentynineteen-skip-link-focus-fix', get_template_directory_uri() . '/js/skip-link-focus-fix.js', array(), '20151215', true );
+	wp_enqueue_script( 'twentynineteen-skip-link-focus-fix', get_template_directory_uri() . '/js/skip-link-focus-fix.js', array(), '20151215', false );
+	wp_script_add_data( 'twentynineteen-skip-link-focus-fix', 'defer', true );
 
 	if ( is_singular() && twentynineteen_can_show_post_thumbnail() ) {
 		wp_add_inline_style( 'twentynineteen-style', twentynineteen_header_featured_image_css() );

inc/template-functions.php

@@ -31,6 +31,37 @@ function twentynineteen_body_classes( $classes ) {
 }
 add_filter( 'body_class', 'twentynineteen_body_classes' );
 
+/**
+ * Adds async/defer attributes to enqueued / registered scripts.
+ *
+ * If #12009 lands in WordPress, this function can no-op since it would be handled in core.
+ *
+ * @link https://core.trac.wordpress.org/ticket/12009
+ * @param string $tag    The script tag.
+ * @param string $handle The script handle.
+ * @return array
+ */
+function twentynineteen_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;
+}
+
+add_filter( 'script_loader_tag', 'twentynineteen_filter_script_loader_tag', 10, 2 );
+
 /**
  * Add a pingback url auto-discovery header for single posts, pages, or attachments.
  */
@@ -187,4 +218,4 @@ function twentynineteen_get_discussion_data() {
 		'responses'  => $responses,                    /* Number of responses, excluding responses from current user. */
 	);
 	return $discussion;
-}
+}