Introduce fetchpriority for Scripts and Script Modules

src/wp-includes/blocks.php

@@ -175,11 +175,21 @@ function register_block_script_module_id( $metadata, $field_name, $index = 0 ) {
 	$block_version       = isset( $metadata['version'] ) ? $metadata['version'] : false;
 	$module_version      = isset( $module_asset['version'] ) ? $module_asset['version'] : $block_version;
 
+	$args = array();
+	if (
+		( isset( $metadata['supports']['interactivity'] ) && true === $metadata['supports']['interactivity'] ) ||
+		( isset( $metadata['supports']['interactivity']['interactive'] ) && true === $metadata['supports']['interactivity']['interactive'] )
+	) {
+		// TODO: Add ability for the fetchpriority to be specified in block.json for the viewScriptModule. In wp_default_script_modules() the fetchpriority defaults to low since server-side rendering is employed for core blocks, but there are no guarantees that this is the case for non-core blocks. That said, viewScriptModule entails Interactivity API, following the pattern from core it _should_ be SSR'ed and that is why this is the default for when the block supports interactivity.
+		$args['fetchpriority'] = 'low';
+	}
+
 	wp_register_script_module(
 		$module_id,
 		$module_uri,
 		$module_dependencies,
-		$module_version
+		$module_version,
+		$args
 	);
 
 	return $module_id;
@@ -243,6 +253,7 @@ function register_block_script_handle( $metadata, $field_name, $index = 0 ) {
 	$script_args         = array();
 	if ( 'viewScript' === $field_name && $script_uri ) {
 		$script_args['strategy'] = 'defer';
+		// TODO: There needs to be a way to specify that a script defined in a module is safe to use fetchpriority=low. Perhaps the viewScript should not only allow a handle string or a `file:./foo.js` string, but allow an an array of params like { "href": "./foo.js", "fetchpriority": "low" }. This would allow for more metadata to be supplied, like the script loading strategy. Related: <https://core.trac.wordpress.org/ticket/56408> and <https://core.trac.wordpress.org/ticket/54018>.
 	}
 
 	$result = wp_register_script(

src/wp-includes/class-wp-script-modules.php

@@ -46,6 +46,7 @@ class WP_Script_Modules {
 	 * identifier has already been registered.
 	 *
 	 * @since 6.5.0
+	 * @since n.e.x.t Added the $args parameter.
 	 *
 	 * @param string            $id       The identifier of the script module. Should be unique. It will be used in the
 	 *                                    final import map.
@@ -71,13 +72,18 @@ class WP_Script_Modules {
 	 *                                    It is added to the URL as a query string for cache busting purposes. If $version
 	 *                                    is set to false, the version number is the currently installed WordPress version.
 	 *                                    If $version is set to null, no version is added.
+	 * @param array             $args     {
+	 *     Optional. An array of additional args. Default empty array.
+	 *
+	 *     @type 'auto'|'low'|'high' $fetchpriority Fetch priority. Default 'auto'. Optional.
+	 * }
 	 */
-	public function register( string $id, string $src, array $deps = array(), $version = false ) {
+	public function register( string $id, string $src, array $deps = array(), $version = false, array $args = array() ) {
 		if ( ! isset( $this->registered[ $id ] ) ) {
 			$dependencies = array();
 			foreach ( $deps as $dependency ) {
 				if ( is_array( $dependency ) ) {
-					if ( ! isset( $dependency['id'] ) ) {
+					if ( ! isset( $dependency['id'] ) || ! is_string( $dependency['id'] ) ) {
 						_doing_it_wrong( __METHOD__, __( 'Missing required id key in entry among dependencies array.' ), '6.5.0' );
 						continue;
 					}
@@ -95,13 +101,66 @@ public function register( string $id, string $src, array $deps = array(), $versi
 				}
 			}
 
+			$fetchpriority = 'auto';
+			if ( isset( $args['fetchpriority'] ) ) {
+				if ( $this->is_valid_fetchpriority( $args['fetchpriority'] ) ) {
+					$fetchpriority = $args['fetchpriority'];
+				} else {
+					_doing_it_wrong(
+						__METHOD__,
+						/* translators: %s: Invalid fetchpriority. */
+						sprintf( __( 'Invalid fetchpriority: %s' ), $args['fetchpriority'] ),
+						'n.e.x.t'
+					);
+				}
+			}
+
 			$this->registered[ $id ] = array(
-				'src'          => $src,
-				'version'      => $version,
-				'enqueue'      => isset( $this->enqueued_before_registered[ $id ] ),
-				'dependencies' => $dependencies,
+				'src'           => $src,
+				'version'       => $version,
+				'enqueue'       => isset( $this->enqueued_before_registered[ $id ] ),
+				'dependencies'  => $dependencies,
+				'fetchpriority' => $fetchpriority,
+			);
+		}
+	}
+
+	/**
+	 * Checks if the provided fetchpriority is valid.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param mixed $priority Fetch priority.
+	 * @return bool Whether valid fetchpriority.
+	 */
+	private function is_valid_fetchpriority( $priority ): bool {
+		return in_array( $priority, array( 'auto', 'low', 'high' ), true );
+	}
+
+	/**
+	 * Sets the fetch priority for a script module.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param string              $id       Script module identifier.
+	 * @param 'auto'|'low'|'high' $priority Fetch priority for the script module.
+	 */
+	public function set_fetchpriority( string $id, string $priority ) {
+		if ( ! isset( $this->registered[ $id ] ) ) {
+			return;
+		}
+
+		if ( ! $this->is_valid_fetchpriority( $priority ) ) {
+			_doing_it_wrong(
+				__METHOD__,
+				/* translators: %s: Invalid fetchpriority. */
+				sprintf( __( 'Invalid fetchpriority: %s' ), $priority ),
+				'n.e.x.t'
 			);
+			return;
 		}
+
+		$this->registered[ $id ]['fetchpriority'] = $priority;
 	}
 
 	/**
@@ -111,6 +170,7 @@ public function register( string $id, string $src, array $deps = array(), $versi
 	 * will be registered.
 	 *
 	 * @since 6.5.0
+	 * @since n.e.x.t Added the $args parameter.
 	 *
 	 * @param string            $id       The identifier of the script module. Should be unique. It will be used in the
 	 *                                    final import map.
@@ -136,12 +196,17 @@ public function register( string $id, string $src, array $deps = array(), $versi
 	 *                                    It is added to the URL as a query string for cache busting purposes. If $version
 	 *                                    is set to false, the version number is the currently installed WordPress version.
 	 *                                    If $version is set to null, no version is added.
+	 * @param array             $args     {
+	 *     Optional. An array of additional args. Default empty array.
+	 *
+	 *     @type 'auto'|'low'|'high' $fetchpriority Fetch priority. Default 'auto'. Optional.
+	 * }
 	 */
-	public function enqueue( string $id, string $src = '', array $deps = array(), $version = false ) {
+	public function enqueue( string $id, string $src = '', array $deps = array(), $version = false, $args = array() ) {
 		if ( isset( $this->registered[ $id ] ) ) {
 			$this->registered[ $id ]['enqueue'] = true;
 		} elseif ( $src ) {
-			$this->register( $id, $src, $deps, $version );
+			$this->register( $id, $src, $deps, $version, $args );
 			$this->registered[ $id ]['enqueue'] = true;
 		} else {
 			$this->enqueued_before_registered[ $id ] = true;
@@ -208,13 +273,15 @@ public function add_hooks() {
 	 */
 	public function print_enqueued_script_modules() {
 		foreach ( $this->get_marked_for_enqueue() as $id => $script_module ) {
-			wp_print_script_tag(
-				array(
-					'type' => 'module',
-					'src'  => $this->get_src( $id ),
-					'id'   => $id . '-js-module',
-				)
+			$args = array(
+				'type' => 'module',
+				'src'  => $this->get_src( $id ),
+				'id'   => $id . '-js-module',
 			);
+			if ( 'auto' !== $script_module['fetchpriority'] ) {
+				$args['fetchpriority'] = $script_module['fetchpriority'];
+			}
+			wp_print_script_tag( $args );
 		}
 	}
 
@@ -231,9 +298,10 @@ public function print_script_module_preloads() {
 			// Don't preload if it's marked for enqueue.
 			if ( true !== $script_module['enqueue'] ) {
 				echo sprintf(
-					'<link rel="modulepreload" href="%s" id="%s">',
+					'<link rel="modulepreload" href="%s" id="%s"%s>',
 					esc_url( $this->get_src( $id ) ),
-					esc_attr( $id . '-js-modulepreload' )
+					esc_attr( $id . '-js-modulepreload' ),
+					'auto' !== $script_module['fetchpriority'] ? sprintf( ' fetchpriority="%s"', esc_attr( $script_module['fetchpriority'] ) ) : ''
 				);
 			}
 		}
@@ -278,7 +346,7 @@ private function get_import_map(): array {
 	 *
 	 * @since 6.5.0
 	 *
-	 * @return array[] Script modules marked for enqueue, keyed by script module identifier.
+	 * @return array<string, array> Script modules marked for enqueue, keyed by script module identifier.
 	 */
 	private function get_marked_for_enqueue(): array {
 		$enqueued = array();

src/wp-includes/class-wp-scripts.php

@@ -425,6 +425,9 @@ public function do_item( $handle, $group = false ) {
 		if ( $intended_strategy ) {
 			$attr['data-wp-strategy'] = $intended_strategy;
 		}
+		if ( isset( $obj->extra['fetchpriority'] ) && 'auto' !== $obj->extra['fetchpriority'] ) {
+			$attr['fetchpriority'] = $obj->extra['fetchpriority'];
+		}
 		$tag  = $translations . $ie_conditional_prefix . $before_script;
 		$tag .= wp_get_script_tag( $attr );
 		$tag .= $after_script . $ie_conditional_suffix;

src/wp-includes/functions.wp-scripts.php

@@ -158,6 +158,7 @@ function wp_add_inline_script( $handle, $data, $position = 'after' ) {
  * @since 2.1.0
  * @since 4.3.0 A return value was added.
  * @since 6.3.0 The $in_footer parameter of type boolean was overloaded to be an $args parameter of type array.
+ * @since n.e.x.t The $fetchpriority parameter of type string was added to the $args parameter of type array.
  *
  * @param string           $handle    Name of the script. Should be unique.
  * @param string|false     $src       Full URL of the script, or path of the script relative to the WordPress root directory.
@@ -171,8 +172,9 @@ function wp_add_inline_script( $handle, $data, $position = 'after' ) {
  *     Optional. An array of additional script loading strategies. Default empty array.
  *     Otherwise, it may be a boolean in which case it determines whether the script is printed in the footer. Default false.
  *
- *     @type string    $strategy     Optional. If provided, may be either 'defer' or 'async'.
- *     @type bool      $in_footer    Optional. Whether to print the script in the footer. Default 'false'.
+ *     @type string    $strategy      Optional. If provided, may be either 'defer' or 'async'.
+ *     @type bool      $in_footer     Optional. Whether to print the script in the footer. Default 'false'.
+ *     @type string    $fetchpriority Optional. The fetch priority for the script. Default 'auto'.
  * }
  * @return bool Whether the script has been registered. True on success, false on failure.
  */
@@ -193,6 +195,9 @@ function wp_register_script( $handle, $src, $deps = array(), $ver = false, $args
 	if ( ! empty( $args['strategy'] ) ) {
 		$wp_scripts->add_data( $handle, 'strategy', $args['strategy'] );
 	}
+	if ( ! empty( $args['fetchpriority'] ) ) {
+		$wp_scripts->add_data( $handle, 'fetchpriority', $args['fetchpriority'] );
+	}
 	return $registered;
 }
 
@@ -339,6 +344,7 @@ function wp_deregister_script( $handle ) {
  *
  * @since 2.1.0
  * @since 6.3.0 The $in_footer parameter of type boolean was overloaded to be an $args parameter of type array.
+ * @since n.e.x.t The $fetchpriority parameter of type string was added to the $args parameter of type array.
  *
  * @param string           $handle    Name of the script. Should be unique.
  * @param string           $src       Full URL of the script, or path of the script relative to the WordPress root directory.
@@ -352,8 +358,9 @@ function wp_deregister_script( $handle ) {
  *     Optional. An array of additional script loading strategies. Default empty array.
  *     Otherwise, it may be a boolean in which case it determines whether the script is printed in the footer. Default false.
  *
- *     @type string    $strategy     Optional. If provided, may be either 'defer' or 'async'.
- *     @type bool      $in_footer    Optional. Whether to print the script in the footer. Default 'false'.
+ *     @type string    $strategy      Optional. If provided, may be either 'defer' or 'async'.
+ *     @type bool      $in_footer     Optional. Whether to print the script in the footer. Default 'false'.
+ *     @type string    $fetchpriority Optional. The fetch priority for the script. Default 'auto'.
  * }
  */
 function wp_enqueue_script( $handle, $src = '', $deps = array(), $ver = false, $args = array() ) {
@@ -378,6 +385,9 @@ function wp_enqueue_script( $handle, $src = '', $deps = array(), $ver = false, $
 		if ( ! empty( $args['strategy'] ) ) {
 			$wp_scripts->add_data( $_handle[0], 'strategy', $args['strategy'] );
 		}
+		if ( ! empty( $args['fetchpriority'] ) ) {
+			$wp_scripts->add_data( $_handle[0], 'fetchpriority', $args['fetchpriority'] );
+		}
 	}
 
 	$wp_scripts->enqueue( $handle );

src/wp-includes/script-loader.php

@@ -1045,7 +1045,10 @@ function wp_default_scripts( $scripts ) {
 	did_action( 'init' ) && $scripts->localize( 'wp-plupload', 'pluploadL10n', $uploader_l10n );
 
 	$scripts->add( 'comment-reply', "/wp-includes/js/comment-reply$suffix.js", array(), false, 1 );
-	did_action( 'init' ) && $scripts->add_data( 'comment-reply', 'strategy', 'async' );
+	if ( did_action( 'init' ) ) {
+		$scripts->add_data( 'comment-reply', 'strategy', 'async' );
+		$scripts->add_data( 'comment-reply', 'fetchpriority', 'low' ); // In Chrome this is automatically low due to the async strategy, but in Firefox and Safari the priority is normal/medium.
+	}
 
 	$scripts->add( 'json2', "/wp-includes/js/json2$suffix.js", array(), '2015-05-03' );
 	did_action( 'init' ) && $scripts->add_data( 'json2', 'conditional', 'lt IE 8' );

src/wp-includes/script-modules.php

@@ -35,6 +35,7 @@ function wp_script_modules(): WP_Script_Modules {
  * identifier has already been registered.
  *
  * @since 6.5.0
+ * @since n.e.x.t Added the $args parameter.
  *
  * @param string            $id      The identifier of the script module. Should be unique. It will be used in the
  *                                   final import map.
@@ -60,9 +61,14 @@ function wp_script_modules(): WP_Script_Modules {
  *                                   It is added to the URL as a query string for cache busting purposes. If $version
  *                                   is set to false, the version number is the currently installed WordPress version.
  *                                   If $version is set to null, no version is added.
+ * @param array             $args    {
+ *     Optional. An array of additional args. Default empty array.
+ *
+ *     @type 'auto'|'low'|'high' $fetchpriority Fetch priority. Default 'auto'. Optional.
+ * }
  */
-function wp_register_script_module( string $id, string $src, array $deps = array(), $version = false ) {
-	wp_script_modules()->register( $id, $src, $deps, $version );
+function wp_register_script_module( string $id, string $src, array $deps = array(), $version = false, array $args = array() ) {
+	wp_script_modules()->register( $id, $src, $deps, $version, $args );
 }
 
 /**
@@ -72,6 +78,7 @@ function wp_register_script_module( string $id, string $src, array $deps = array
  * will be registered.
  *
  * @since 6.5.0
+ * @since n.e.x.t Added the $args parameter.
  *
  * @param string            $id      The identifier of the script module. Should be unique. It will be used in the
  *                                   final import map.
@@ -97,9 +104,14 @@ function wp_register_script_module( string $id, string $src, array $deps = array
  *                                   It is added to the URL as a query string for cache busting purposes. If $version
  *                                   is set to false, the version number is the currently installed WordPress version.
  *                                   If $version is set to null, no version is added.
+ * @param array             $args    {
+ *     Optional. An array of additional args. Default empty array.
+ *
+ *     @type 'auto'|'low'|'high' $fetchpriority Fetch priority. Default 'auto'. Optional.
+ * }
  */
-function wp_enqueue_script_module( string $id, string $src = '', array $deps = array(), $version = false ) {
-	wp_script_modules()->enqueue( $id, $src, $deps, $version );
+function wp_enqueue_script_module( string $id, string $src = '', array $deps = array(), $version = false, array $args = array() ) {
+	wp_script_modules()->enqueue( $id, $src, $deps, $version, $args );
 }
 
 /**
@@ -169,7 +181,13 @@ function wp_default_script_modules() {
 				break;
 		}
 
+		// The Interactivity API is designed with server-side rendering as its primary goal, so all of its script modules should be loaded with low fetch priority since they should not be needed in the critical rendering path.
+		$args = array();
+		if ( str_starts_with( $script_module_id, '@wordpress/interactivity' ) || str_starts_with( $script_module_id, '@wordpress/block-library' ) ) {
+			$args['fetchpriority'] = 'low';
+		}
+
 		$path = includes_url( "js/dist/script-modules/{$file_name}" );
-		wp_register_script_module( $script_module_id, $path, $script_module_data['dependencies'], $script_module_data['version'] );
+		wp_register_script_module( $script_module_id, $path, $script_module_data['dependencies'], $script_module_data['version'], $args );
 	}
 }