Load the import map polyfill only when there is an import map (#56699)

* Load polyfill only when there is an import map

* Remove the polyfill to check the performance impact

* Revert "Remove the polyfill to check the performance impact"

This reverts commit af5eaad.

lib/experimental/interactivity-api/modules.php

@@ -16,18 +16,6 @@ function gutenberg_register_interactivity_module() {
 		array(),
 		defined( 'GUTENBERG_VERSION' ) ? GUTENBERG_VERSION : get_bloginfo( 'version' )
 	);
-
-	// TODO: Replace with a simpler version that only provides support for import maps.
-	// TODO: Load only if the browser doesn't support import maps (https://github.com/guybedford/es-module-shims/issues/371).
-	wp_enqueue_script(
-		'es-module-shims',
-		gutenberg_url( '/build/modules/importmap-polyfill.min.js' ),
-		array(),
-		null,
-		array(
-			'strategy' => 'defer',
-		)
-	);
 }
 
 add_action( 'wp_enqueue_scripts', 'gutenberg_register_interactivity_module' );

lib/experimental/modules/class-gutenberg-modules.php

@@ -114,6 +114,26 @@ public static function print_module_preloads() {
 		}
 	}
 
+	/**
+	 * Prints the necessary script to load import map polyfill for browsers that
+	 * do not support import maps.
+	 *
+	 * TODO: Replace the polyfill with a simpler version that only provides
+	 * support for import maps and load it only when the browser doesn't support
+	 * import maps (https://github.com/guybedford/es-module-shims/issues/371).
+	 */
+	public static function print_import_map_polyfill() {
+		$import_map = self::get_import_map();
+		if ( ! empty( $import_map['imports'] ) ) {
+			wp_print_script_tag(
+				array(
+					'src'   => gutenberg_url( '/build/modules/importmap-polyfill.min.js' ),
+					'defer' => true,
+				)
+			);
+		}
+	}
+
 	/**
 	 * Gets the module's version. It either returns a timestamp (if SCRIPT_DEBUG
 	 * is true), the explicit version of the module if it is set and not false, or
@@ -193,3 +213,6 @@ function gutenberg_enqueue_module( $module_identifier ) {
 
 // Prints the preloaded modules in the head tag.
 add_action( 'wp_head', array( 'Gutenberg_Modules', 'print_module_preloads' ) );
+
+// Prints the script that loads the import map polyfill in the footer.
+add_action( 'wp_footer', array( 'Gutenberg_Modules', 'print_import_map_polyfill' ), 11 );