Script Modules API: Backport - Add import map polyfill

Gruntfile.js

@@ -1559,16 +1559,18 @@ module.exports = function(grunt) {
 	} );
 
 	/**
-	 * Build assertions for the lack of source maps in JavaScript files.
+	 * Compiled JavaScript files may link to sourcemaps. In some cases,
+	 * the source map may not be available, which can cause 404 errors when
+	 * browsers try to download the sourcemap from the referenced URLs.
+	 * Ensure that sourcemap links are not included in JavaScript files.
 	 *
 	 * @ticket 24994
 	 * @ticket 46218
+	 * @ticket 60348
 	 */
 	grunt.registerTask( 'verify:source-maps', function() {
 		const ignoredFiles = [
 			'build/wp-includes/js/dist/components.js',
-			'build/wp-includes/js/dist/block-editor.js',
-			'build/wp-includes/js/dist/block-editor.min.js'
 		];
 		const files = buildFiles.reduce( ( acc, path ) => {
 			// Skip excluded paths and any path that isn't a file.
@@ -1591,10 +1593,10 @@ module.exports = function(grunt) {
 					encoding: 'utf8',
 				} );
 				// `data:` URLs are allowed:
-				const match = contents.match( /sourceMappingURL=((?!data:).)/ );
+				const doesNotHaveSourceMap = ! /^\/\/# sourceMappingURL=((?!data:).)/m.test(contents);
 
 				assert(
-					match === null,
+					doesNotHaveSourceMap,
 					`The ${ file } file must not contain a sourceMappingURL.`
 				);
 			} );

package.json

@@ -150,6 +150,7 @@
 		"clipboard": "2.0.11",
 		"core-js-url-browser": "3.6.4",
 		"element-closest": "^3.0.2",
+		"es-module-shims": "1.8.2",
 		"formdata-polyfill": "4.0.10",
 		"framer-motion": "10.16.4",
 		"hoverintent": "2.2.1",

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

@@ -210,11 +210,26 @@ public function print_script_module_preloads() {
 	/**
 	 * Prints the import map using a script tag with a type="importmap" attribute.
 	 *
+	 * @global WP_Scripts $wp_scripts The WP_Scripts object for printing the polyfill.
 	 * @since 6.5.0
 	 */
 	public function print_import_map() {
 		$import_map = $this->get_import_map();
 		if ( ! empty( $import_map['imports'] ) ) {
+			global $wp_scripts;
+			if ( isset( $wp_scripts ) ) {
+				wp_print_inline_script_tag(
+					wp_get_script_polyfill(
+						$wp_scripts,
+						array(
+							'HTMLScriptElement.supports && HTMLScriptElement.supports("importmap")' => 'wp-polyfill-importmap',
+						)
+					),
+					array(
+						'id' => 'wp-load-polyfill-importmap',
+					)
+				);
+			}
 			wp_print_inline_script_tag(
 				wp_json_encode( $import_map, JSON_HEX_TAG | JSON_HEX_AMP ),
 				array(

src/wp-includes/script-loader.php

@@ -96,6 +96,7 @@ function wp_default_packages_vendor( $scripts ) {
 		'lodash',
 		'wp-polyfill-fetch',
 		'wp-polyfill-formdata',
+		'wp-polyfill-importmap',
 		'wp-polyfill-node-contains',
 		'wp-polyfill-url',
 		'wp-polyfill-dom-rect',
@@ -120,6 +121,7 @@ function wp_default_packages_vendor( $scripts ) {
 		'wp-polyfill-object-fit'      => '2.3.5',
 		'wp-polyfill-inert'           => '3.1.2',
 		'wp-polyfill'                 => '3.15.0',
+		'wp-polyfill-importmap'       => '1.8.2',
 	);
 
 	foreach ( $vendor_scripts as $handle => $dependencies ) {

tests/phpunit/tests/dependencies/scripts.php

@@ -3104,6 +3104,36 @@ public function test_wp_scripts_move_to_footer( $set_up, $expected_header, $expe
 		$this->assertEquals( $expected_groups, wp_scripts()->groups, 'Expected groups to match.' );
 	}
 
+	/**
+	 * Test that get_script_polyfill() returns the correct polyfill.
+	 *
+	 * @ticket 60348
+	 *
+	 * @covers ::wp_get_script_polyfill
+	 *
+	 * @global WP_Scripts $wp_scripts WP_Scripts instance.
+	 * @param string $script_name Script name.
+	 * @param string $test_script Conditional that checks browser compatibility.
+	 * @param string $script_url  Script source URL.
+	 */
+	public function test_wp_get_script_polyfill() {
+		global $wp_scripts;
+		$script_name = 'wp-polyfill-importmap';
+		$test_script = 'HTMLScriptElement.supports && HTMLScriptElement.supports("importmap")';
+		$script_url  = 'https://example.com/wp-polyfill-importmap.js';
+		wp_register_script( $script_name, $script_url, array(), null, true );
+
+		$polyfill = wp_get_script_polyfill(
+			$wp_scripts,
+			array(
+				$test_script => $script_name,
+			)
+		);
+		$expected = '( ' . $test_script . ' ) || document.write( \'<script src="' . $script_url . '"></scr\' + \'ipt>\' );';
+
+		$this->assertEquals( $expected, $polyfill );
+	}
+
 	/**
 	 * Data provider for test_wp_scripts_move_to_footer.
 	 *

tests/phpunit/tests/script-modules/wpScriptModules.php

@@ -11,7 +11,8 @@
  *
  * @coversDefaultClass WP_Script_Modules
  */
-class Tests_WP_Script_Modules extends WP_UnitTestCase {
+class Tests_Script_Modules_WpScriptModules extends WP_UnitTestCase {
+
 	/**
 	 * Instance of WP_Script_Modules.
 	 *
@@ -24,6 +25,7 @@ class Tests_WP_Script_Modules extends WP_UnitTestCase {
 	 */
 	public function set_up() {
 		parent::set_up();
+		// Set up the WP_Script_Modules instance.
 		$this->script_modules = new WP_Script_Modules();
 	}
 
@@ -600,4 +602,33 @@ public function test_wp_enqueue_script_module_registers_all_params() {
 		$this->assertCount( 1, $import_map );
 		$this->assertStringStartsWith( '/dep.js', $import_map['dep'] );
 	}
+
+	/**
+	 * Tests that the import map polyfill is not enqueued when there are no
+	 * modules registered.
+	 *
+	 * @ticket 60348
+	 *
+	 * @covers ::print_import_map_polyfill()
+	 */
+
+	public function test_wp_print_import_map_has_polyfill() {
+		// Polyfill is empty when no modules are registered.
+		$import_map_polyfill = get_echo( array( $this->script_modules, 'print_import_map' ) );
+
+		$this->assertEquals( '', $import_map_polyfill );
+
+		// Polyfill is not empty when a module is registered.
+		wp_register_script( 'wp-polyfill-importmap', '/wp-polyfill-importmap.js', array(), '1.0' );
+
+		$this->script_modules->enqueue( 'foo', '/foo.js', array( 'dep' ), '1.0' );
+		$this->script_modules->register( 'dep', '/dep.js' );
+		$import_map_polyfill = get_echo( array( $this->script_modules, 'print_import_map' ) );
+		$p                   = new WP_HTML_Tag_Processor( $import_map_polyfill );
+		$p->next_tag( array( 'tag' => 'SCRIPT' ) );
+		$id = $p->get_attribute( 'id' );
+
+		$this->assertEquals( 'wp-load-polyfill-importmap', $id );
+		wp_deregister_script( 'wp-polyfill-importmap' );
+	}
 }

tools/webpack/packages.js

@@ -93,6 +93,7 @@ module.exports = function (
 		'wp-polyfill-object-fit.js':
 			'objectFitPolyfill/src/objectFitPolyfill.js',
 		'wp-polyfill-inert.js': 'wicg-inert/dist/inert.js',
+		'wp-polyfill-importmap.js': 'es-module-shims/dist/es-module-shims.wasm.js',
 		'moment.js': 'moment/moment.js',
 		'react.js': 'react/umd/react.development.js',
 		'react-dom.js': 'react-dom/umd/react-dom.development.js',
@@ -121,6 +122,7 @@ module.exports = function (
 			'polyfill-library/polyfills/__dist/Node.prototype.contains/raw.js',
 		'wp-polyfill-dom-rect.min.js':
 			'polyfill-library/polyfills/__dist/DOMRect/raw.js',
+		'wp-polyfill-importmap.min.js': 'es-module-shims/dist/es-module-shims.wasm.js',
 	};
 
 	const phpFiles = {