Properly handle new NodePackageImporter() with an ESM entrypoint

CHANGELOG.md

@@ -8,6 +8,14 @@
 
 * Export the `NodePackageImporter` class in ESM mode.
 
+* Allow `NodePackageImporter` to locate a default directory even when the
+  entrypoint is an ESM module.
+
+### Dart API
+
+* Make passing a null argument to `NodePackageImporter()` a static error rather
+  than just a runtime error.
+
 ### Embedded Sass
 
 * In the JS Embedded Host, properly install the musl Linux embedded compiler

lib/src/importer/node_package.dart

@@ -19,12 +19,8 @@ class NodePackageImporter extends Importer {
   late final String _entryPointDirectory;
 
   /// Creates a Node package importer with the associated entry point.
-  NodePackageImporter(String? entryPointDirectory) {
-    if (entryPointDirectory == null) {
-      throw "The Node package importer cannot determine an entry point "
-          "because `require.main.filename` is not defined. "
-          "Please provide an `entryPointDirectory` to the `NodePackageImporter`.";
-    } else if (isBrowser) {
+  NodePackageImporter(String entryPointDirectory) {
+    if (isBrowser) {
       throw "The Node package importer cannot be used without a filesystem.";
     }
     _entryPointDirectory = p.absolute(entryPointDirectory);

lib/src/js/compile.dart

@@ -334,10 +334,13 @@ List<AsyncCallable> _parseFunctions(Object? functions, {bool asynch = false}) {
 final JSClass nodePackageImporterClass = () {
   var jsClass = createJSClass(
       'sass.NodePackageImporter',
-      (Object self, [String? entryPointDirectory]) => NodePackageImporter(
-          entryPointDirectory ??
-              (requireMainFilename != null
-                  ? p.dirname(requireMainFilename!)
-                  : null)));
+      (Object self, [String? entrypointDirectory]) => NodePackageImporter(
+              switch ((entrypointDirectory, entrypointFilename)) {
+            ((var directory?, _)) => directory,
+            (_, var filename?) => p.dirname(filename),
+            _ => throw "The Node package importer cannot determine an entry "
+                "point because `require.main.filename` is not defined. Please "
+                "provide an `entryPointDirectory` to the `NodePackageImporter`."
+          }));
   return jsClass;
 }();

lib/src/js/module.dart

@@ -0,0 +1,26 @@
+// Copyright 2024 Google Inc. Use of this source code is governed by an
+// MIT-style license that can be found in the LICENSE file or at
+// https://opensource.org/licenses/MIT.
+
+import 'package:js/js.dart';
+
+@JS('nodeModule')
+external JSModule get module;
+
+/// A Dart API for the [`node:module`] module.
+///
+/// [`node:module`]: https://nodejs.org/api/module.html#modules-nodemodule-api
+@JS()
+@anonymous
+class JSModule {
+  /// See https://nodejs.org/api/module.html#modulecreaterequirefilename.
+  external JSModuleRequire createRequire(String filename);
+}
+
+/// A `require` function returned by `module.createRequire()`.
+@JS()
+@anonymous
+class JSModuleRequire {
+  /// See https://nodejs.org/api/modules.html#requireresolverequest-options.
+  external String resolve(String filename);
+}

lib/src/js/utils.dart

@@ -5,7 +5,7 @@
 import 'dart:js_util';
 import 'dart:typed_data';
 
-import 'package:node_interop/js.dart';
+import 'package:node_interop/node.dart' hide module;
 import 'package:js/js.dart';
 import 'package:js/js_util.dart';
 
@@ -14,6 +14,7 @@ import '../utils.dart';
 import '../value.dart';
 import 'array.dart';
 import 'function.dart';
+import 'module.dart';
 import 'reflection.dart';
 import 'url.dart';
 
@@ -234,6 +235,22 @@ Syntax parseSyntax(String? syntax) => switch (syntax) {
       _ => jsThrow(JsError('Unknown syntax "$syntax".'))
     };
 
-/// The value of require.main.filename
-@JS("require.main.filename")
-external String? get requireMainFilename;
+/// The path to the Node.js entrypoint, if one can be located.
+String? get entrypointFilename {
+  if (_requireMain?.filename case var filename?) {
+    return filename;
+  } else if (process.argv case [_, String path]) {
+    return module.createRequire(path).resolve(path);
+  } else {
+    return null;
+  }
+}
+
+@JS("require.main")
+external _RequireMain? get _requireMain;
+
+@JS()
+@anonymous
+class _RequireMain {
+  external String? get filename;
+}

tool/grind.dart

@@ -35,6 +35,8 @@ void main(List<String> args) {
     pkg.JSRequire("chokidar", target: pkg.JSRequireTarget.cli),
     pkg.JSRequire("readline", target: pkg.JSRequireTarget.cli),
     pkg.JSRequire("fs", target: pkg.JSRequireTarget.node),
+    pkg.JSRequire("module",
+        target: pkg.JSRequireTarget.node, identifier: 'nodeModule'),
     pkg.JSRequire("stream", target: pkg.JSRequireTarget.node),
     pkg.JSRequire("util", target: pkg.JSRequireTarget.node),
   ];