Implement sass --embedded in pure JS mode

.github/workflows/test.yml

@@ -128,24 +128,40 @@ jobs:
         working-directory: sass-spec
 
   sass_spec_js_embedded:
-    name: 'JS API Tests | Embedded | Node ${{ matrix.node-version }} | ${{ matrix.os }}'
+    name: "JS API Tests | Embedded ${{ matrix.js && 'Pure JS' || 'Dart' }} | Node ${{ matrix.node-version }} | ${{ matrix.os }}"
     runs-on: ${{ matrix.os }}
     if: "github.event_name != 'pull_request' || !contains(github.event.pull_request.body, 'skip sass-embedded')"
 
     strategy:
       fail-fast: false
       matrix:
+        js: [true, false]
         os: [ubuntu-latest, windows-latest, macos-latest]
         node-version: ['lts/*']
         include:
           # Test older LTS versions
-          - os: ubuntu-latest
+          - js: true
+            os: ubuntu-latest
             dart_channel: stable
             node-version: lts/-1
-          - os: ubuntu-latest
+          - js: true
+            os: ubuntu-latest
             dart_channel: stable
             node-version: lts/-2
-          - os: ubuntu-latest
+          - js: true
+            os: ubuntu-latest
+            dart_channel: stable
+            node-version: lts/-3
+          - js: false
+            os: ubuntu-latest
+            dart_channel: stable
+            node-version: lts/-1
+          - js: false
+            os: ubuntu-latest
+            dart_channel: stable
+            node-version: lts/-2
+          - js: false
+            os: ubuntu-latest
             dart_channel: stable
             node-version: lts/-3
 
@@ -168,19 +184,13 @@ jobs:
       - name: Initialize embedded host
         run: |
           npm install
-          npm run init -- --compiler-path=.. --language-path=../build/language
+          npm run init -- --compiler-path=.. --language-path=../build/language ${{ matrix.js && '--compiler-js' || '' }}
           npm run compile
-          mv {`pwd`/,dist/}lib/src/vendor/dart-sass
         working-directory: embedded-host-node
 
       - name: Version info
-        run: |
-          path=embedded-host-node/dist/lib/src/vendor/dart-sass/sass
-          if [[ -f "$path.cmd" ]]; then "./$path.cmd" --version
-          elif [[ -f "$path.bat" ]]; then "./$path.bat" --version
-          elif [[ -f "$path.exe" ]]; then "./$path.exe" --version
-          else "./$path" --version
-          fi
+        run: node dist/bin/sass.js --version
+        working-directory: embedded-host-node
 
       - name: Run tests
         run: npm run js-api-spec -- --sassPackage ../embedded-host-node --sassSassRepo ../build/language

README.md

@@ -434,9 +434,7 @@ an API for users to invoke Sass and define custom functions and importers.
 * `sass --embedded --version` prints `versionResponse` with `id = 0` in JSON and
   exits.
 
-The `--embedded` command-line flag is not available when you install Dart Sass
-as an [npm package]. No other command-line flags are supported with
-`--embedded`.
+No other command-line flags are supported with `--embedded`.
 
 [npm package]: #from-npm
 

bin/sass.dart

@@ -17,10 +17,7 @@ import 'package:sass/src/importer/filesystem.dart';
 import 'package:sass/src/io.dart';
 import 'package:sass/src/stylesheet_graph.dart';
 import 'package:sass/src/utils.dart';
-import 'package:sass/src/embedded/executable.dart'
-    // Never load the embedded protocol when compiling to JS.
-    if (dart.library.js) 'package:sass/src/embedded/unavailable.dart'
-    as embedded;
+import 'package:sass/src/embedded/executable.dart' as embedded;
 
 Future<void> main(List<String> args) async {
   if (args case ['--embedded', ...var rest]) {

lib/src/embedded/compilation_dispatcher.dart

@@ -3,17 +3,17 @@
 // https://opensource.org/licenses/MIT.
 
 import 'dart:convert';
-import 'dart:io';
-import 'dart:isolate';
+import 'dart:io' if (dart.library.js) 'js/io.dart';
+import 'dart:isolate' if (dart.library.js) 'js/isolate.dart';
 import 'dart:typed_data';
 
-import 'package:native_synchronization/mailbox.dart';
 import 'package:path/path.dart' as p;
 import 'package:protobuf/protobuf.dart';
 import 'package:pub_semver/pub_semver.dart';
 import 'package:sass/sass.dart' as sass;
 import 'package:sass/src/importer/node_package.dart' as npi;
 
+import '../io.dart' show FileSystemException;
 import '../logger.dart';
 import '../value/function.dart';
 import '../value/mixin.dart';
@@ -23,6 +23,7 @@ import 'host_callable.dart';
 import 'importer/file.dart';
 import 'importer/host.dart';
 import 'logger.dart';
+import 'sync_receive_port.dart';
 import 'util/proto_extensions.dart';
 import 'utils.dart';
 
@@ -35,8 +36,8 @@ final _outboundRequestId = 0;
 /// A class that dispatches messages to and from the host for a single
 /// compilation.
 final class CompilationDispatcher {
-  /// The mailbox for receiving messages from the host.
-  final Mailbox _mailbox;
+  /// The synchronous receive port for receiving messages from the host.
+  final SyncReceivePort _receivePort;
 
   /// The send port for sending messages to the host.
   final SendPort _sendPort;
@@ -52,8 +53,8 @@ final class CompilationDispatcher {
   late Uint8List _compilationIdVarint;
 
   /// Creates a [CompilationDispatcher] that receives encoded protocol buffers
-  /// through [_mailbox] and sends them through [_sendPort].
-  CompilationDispatcher(this._mailbox, this._sendPort);
+  /// through [_receivePort] and sends them through [_sendPort].
+  CompilationDispatcher(this._receivePort, this._sendPort);
 
   /// Listens for incoming `CompileRequests` and runs their compilations.
   void listen() {
@@ -408,16 +409,16 @@ final class CompilationDispatcher {
     message.writeToCodedBufferWriter(protobufWriter);
 
     // Add one additional byte to the beginning to indicate whether or not the
-    // compilation has finished (1) or encountered a fatal error (2), so the
-    // [IsolateDispatcher] knows whether to treat this isolate as inactive or
-    // close out entirely.
+    // compilation has finished (1) or encountered a fatal error (exitCode), so
+    // the [IsolateDispatcher] knows whether to treat this isolate as inactive
+    // or close out entirely.
     var packet = Uint8List(
       1 + _compilationIdVarint.length + protobufWriter.lengthInBytes,
     );
     packet[0] = switch (message.whichMessage()) {
       OutboundMessage_Message.compileResponse => 1,
-      OutboundMessage_Message.error => 2,
-      _ => 0,
+      OutboundMessage_Message.error => exitCode,
+      _ => 0
     };
     packet.setAll(1, _compilationIdVarint);
     protobufWriter.writeTo(packet, 1 + _compilationIdVarint.length);
@@ -427,9 +428,9 @@ final class CompilationDispatcher {
   /// Receive a packet from the host.
   Uint8List _receive() {
     try {
-      return _mailbox.take();
+      return _receivePort.receive();
     } on StateError catch (_) {
-      // The [_mailbox] has been closed, exit the current isolate immediately
+      // The [SyncReceivePort] has been closed, exit the current isolate immediately
       // to avoid bubble the error up as [SassException] during [_sendRequest].
       Isolate.exit();
     }

lib/src/embedded/executable.dart

@@ -1,42 +1,5 @@
-// Copyright 2019 Google Inc. Use of this source code is governed by an
+// Copyright 2025 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 'dart:io';
-import 'dart:convert';
-
-import 'package:stream_channel/stream_channel.dart';
-
-import 'isolate_dispatcher.dart';
-import 'util/length_delimited_transformer.dart';
-
-void main(List<String> args) {
-  switch (args) {
-    case ["--version", ...]:
-      var response = IsolateDispatcher.versionResponse();
-      response.id = 0;
-      stdout.writeln(
-        JsonEncoder.withIndent("  ").convert(response.toProto3Json()),
-      );
-      return;
-
-    case [_, ...]:
-      stderr.writeln(
-        "sass --embedded is not intended to be executed with additional "
-        "arguments.\n"
-        "See https://github.com/sass/dart-sass#embedded-dart-sass for "
-        "details.",
-      );
-      // USAGE error from https://bit.ly/2poTt90
-      exitCode = 64;
-      return;
-  }
-
-  IsolateDispatcher(
-    StreamChannel.withGuarantees(
-      stdin,
-      stdout,
-      allowSinkErrors: false,
-    ).transform(lengthDelimited),
-  ).listen();
-}
+export 'vm/executable.dart' if (dart.library.js) 'js/executable.dart';

lib/src/embedded/js/concurrency.dart

@@ -0,0 +1,10 @@
+// Copyright 2025 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 'dart:js_interop';
+
+@JS('os.cpus')
+external JSArray _cpus();
+
+int get concurrencyLimit => _cpus().length;

lib/src/embedded/js/executable.dart

@@ -0,0 +1,27 @@
+// Copyright 2025 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:stream_channel/stream_channel.dart';
+
+import '../options.dart';
+import '../util/length_delimited_transformer.dart';
+import '../worker_dispatcher.dart';
+import '../worker_entrypoint.dart';
+import 'io.dart';
+import 'sync_receive_port.dart';
+import 'worker_threads.dart';
+
+void main(List<String> args) {
+  if (parseOptions(args)) {
+    if (isMainThread) {
+      WorkerDispatcher(StreamChannel.withGuarantees(stdin, stdout,
+                  allowSinkErrors: false)
+              .transform(lengthDelimited))
+          .listen();
+    } else {
+      var port = workerData! as MessagePort;
+      workerEntryPoint(JSSyncReceivePort(port), JSSendPort(port));
+    }
+  }
+}

lib/src/embedded/js/io.dart

@@ -0,0 +1,66 @@
+// Copyright 2025 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 'dart:async';
+import 'dart:js_interop';
+import 'dart:typed_data';
+
+@JS('process.exitCode')
+external int? get _exitCode;
+int get exitCode => _exitCode ?? 0;
+
+@JS('process.exitCode')
+external set exitCode(int code);
+
+@JS('process.exit')
+external void exit([int code]);
+
+@JS()
+extension type _ReadStream(JSObject _) implements JSObject {
+  external void destroy();
+  external void on(String type, JSFunction listener);
+}
+
+@JS('process.stdin')
+external _ReadStream get _stdin;
+
+@JS()
+extension type _WriteStream(JSObject _) implements JSObject {
+  external void write(JSUint8Array chunk);
+}
+
+@JS('process.stdout')
+external _WriteStream get _stdout;
+
+Stream<List<int>> get stdin {
+  var controller = StreamController<Uint8List>(
+      onCancel: () {
+        _stdin.destroy();
+      },
+      sync: true);
+  _stdin.on(
+      'data',
+      (JSUint8Array chunk) {
+        controller.sink.add(chunk.toDart);
+      }.toJS);
+  _stdin.on(
+      'end',
+      () {
+        controller.sink.close();
+      }.toJS);
+  _stdin.on(
+      'error',
+      (JSObject e) {
+        controller.sink.addError(e);
+      }.toJS);
+  return controller.stream;
+}
+
+StreamSink<List<int>> get stdout {
+  var controller = StreamController<Uint8List>(sync: true);
+  controller.stream.listen((buffer) {
+    _stdout.write(buffer.toJS);
+  });
+  return controller.sink;
+}

lib/src/embedded/js/isolate.dart

@@ -0,0 +1,17 @@
+// Copyright 2025 Google LLC. 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 'dart:isolate' show SendPort;
+export 'dart:isolate' show SendPort;
+
+import 'io.dart' as io;
+
+abstract class Isolate {
+  static Never exit([SendPort? finalMessagePort, Object? message]) {
+    if (message != null) {
+      finalMessagePort?.send(message);
+    }
+    io.exit(io.exitCode) as Never;
+  }
+}

lib/src/embedded/js/js.dart

@@ -0,0 +1,13 @@
+// Copyright 2025 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 'dart:js_interop';
+
+extension JSTypedArrayExtension on JSTypedArray {
+  external JSArrayBuffer get buffer;
+}
+
+extension JSArrayExtension<T extends JSAny?> on JSArray<T> {
+  external JSArray<T> slice([int start, int end]);
+}