diff --git a/ngdart/lib/src/testability/js_api.dart b/ngdart/lib/src/testability/js_api.dart
index 258b7b8c1..ea3bf7faa 100644
--- a/ngdart/lib/src/testability/js_api.dart
+++ b/ngdart/lib/src/testability/js_api.dart
@@ -8,9 +8,7 @@ import 'package:web/web.dart';
 /// A JavaScript interface for interacting with AngularDart's `Testability` API.
 ///
 /// This interfaces with a running AngularDart application.
-@JS()
-@anonymous
-abstract class JsTestability {
+extension type JsTestability._(JSObject _) implements JSObject {
   external factory JsTestability({
     required JSFunction isStable,
     required JSFunction whenStable,
@@ -22,7 +20,7 @@ abstract class JsTestability {
   /// framework. By default, this is determined by no known asynchronous tasks
   /// (microtasks, or timers) being present but not yet executed within the
   /// framework context.
-  bool isStable();
+  external bool isStable();
 
   /// Invokes the provided [callback] when the application [isStable].
   ///
@@ -30,23 +28,21 @@ abstract class JsTestability {
   /// invoked, [callback] is invoked with a value of `false` for `didWork`,
   /// indicating that no asynchronous work was awaited before execution.
   /// Otherwise a value of `true` is passed.
-  void whenStable(JSFunction callback);
+  external void whenStable(JSFunction callback);
 }
 
 /// A JavaScript interface for interacting with AngularDart's `TestabilityRegistry` API.
 ///
 /// A global registry of `Testability` instances given an app root element.
-@JS()
-@anonymous
-abstract class JsTestabilityRegistry {
+extension type JsTestabilityRegistry._(JSObject _) implements JSObject {
   external factory JsTestabilityRegistry({
     required JSFunction getAngularTestability,
     required JSFunction getAllAngularTestabilities,
   });
 
   /// Returns the registered testability instance for [appRoot], or `null`.
-  JsTestability? getAngularTestability(Element appRoot);
+  external JsTestability? getAngularTestability(Element appRoot);
 
   /// Returns all testability instances registered.
-  List<JsTestability> getAllAngularTestabilities();
+  external JSArray<JsTestability> getAllAngularTestabilities();
 }
diff --git a/ngdart/lib/src/testability/js_impl.dart b/ngdart/lib/src/testability/js_impl.dart
index 27a6834f3..f49dac9b1 100644
--- a/ngdart/lib/src/testability/js_impl.dart
+++ b/ngdart/lib/src/testability/js_impl.dart
@@ -1,7 +1,7 @@
 part of 'testability.dart';
 
 @JS('ngTestabilityRegistries')
-external List<JsTestabilityRegistry>? _ngJsTestabilityRegistries;
+external JSArray<JsTestabilityRegistry>? _ngJsTestabilityRegistries;
 
 @JS('getAngularTestability')
 external set _jsGetAngularTestability(JSFunction function);
@@ -10,7 +10,7 @@ external set _jsGetAngularTestability(JSFunction function);
 external set _jsGetAllAngularTestabilities(JSFunction function);
 
 @JS('frameworkStabilizers')
-external List<Object?>? _jsFrameworkStabilizers;
+external JSArray<JSFunction>? _jsFrameworkStabilizers;
 
 class _JSTestabilityProxy implements _TestabilityProxy {
   const _JSTestabilityProxy();
@@ -19,23 +19,25 @@ class _JSTestabilityProxy implements _TestabilityProxy {
   void addToWindow(TestabilityRegistry registry) {
     var registries = _ngJsTestabilityRegistries;
     if (registries == null) {
-      registries = <JsTestabilityRegistry>[];
+      registries = JSArray();
       _ngJsTestabilityRegistries = registries;
       _jsGetAngularTestability = _getAngularTestability.toJS;
       _jsGetAllAngularTestabilities = _getAllAngularTestabilities.toJS;
-      (_jsFrameworkStabilizers ??= <Object?>[]).add(_whenAllStable.toJS);
+      ((_jsFrameworkStabilizers ??= JSArray()) as List<JSFunction>).add(
+          ((JSFunction callback) => _whenAllStable(callback as void Function()))
+              .toJS);
     }
     registries.add(registry.asJsApi());
   }
 
   /// For every registered [TestabilityRegistry], tries `getAngularTestability`.
   static JsTestability? _getAngularTestability(Element element) {
-    final registry = _ngJsTestabilityRegistries;
-    if (registry == null) {
+    final registries = _ngJsTestabilityRegistries;
+    if (registries == null) {
       return null;
     }
-    for (var i = 0; i < registry.length; i++) {
-      final result = registry[i].getAngularTestability(element);
+    for (final registry in (registries as List<JsTestabilityRegistry>)) {
+      final result = registry.getAngularTestability(element);
       if (result != null) {
         return result;
       }
@@ -44,22 +46,23 @@ class _JSTestabilityProxy implements _TestabilityProxy {
   }
 
   /// For every registered [TestabilityRegistry], returns the JS API for it.
-  static List<JsTestability> _getAllAngularTestabilities() {
-    final registry = _ngJsTestabilityRegistries;
-    if (registry == null) {
-      return <JsTestability>[];
+  static JSArray<JsTestability> _getAllAngularTestabilities() {
+    final registries = _ngJsTestabilityRegistries;
+    if (registries == null) {
+      return JSArray();
     }
     final result = <JsTestability>[];
-    for (var i = 0; i < registry.length; i++) {
-      final testabilities = registry[i].getAllAngularTestabilities();
+    for (final registry in (registries as List<JsTestabilityRegistry>)) {
+      final testabilities =
+          registry.getAllAngularTestabilities() as List<JsTestability>;
       result.addAll(testabilities);
     }
-    return result;
+    return result.toJS;
   }
 
   /// For every testability, calls [callback] when they _all_ report stable.
   static void _whenAllStable(void Function() callback) {
-    final testabilities = _getAllAngularTestabilities();
+    final testabilities = _getAllAngularTestabilities() as List<JsTestability>;
 
     var pendingStable = testabilities.length;
 
@@ -80,7 +83,8 @@ extension on Testability {
   JsTestability asJsApi() {
     return JsTestability(
       isStable: (() => isStable).toJS,
-      whenStable: whenStable.toJS,
+      whenStable: ((JSFunction callback) =>
+          whenStable(callback as void Function())).toJS,
     );
   }
 }
@@ -92,10 +96,11 @@ extension on TestabilityRegistry {
       return dartTestability?.asJsApi();
     }
 
-    List<JsTestability> getAllAngularTestabilities() {
+    JSArray<JsTestability> getAllAngularTestabilities() {
       return allTestabilities
           .map((testability) => testability.asJsApi())
-          .toList();
+          .toList()
+          .toJS;
     }
 
     return JsTestabilityRegistry(
diff --git a/ngdart/lib/src/testability/testability.dart b/ngdart/lib/src/testability/testability.dart
index 9f10c0786..d5d1caf9e 100644
--- a/ngdart/lib/src/testability/testability.dart
+++ b/ngdart/lib/src/testability/testability.dart
@@ -34,6 +34,8 @@ const testabilityProvider = [
 /// must be injected too.
 // TODO(b/168535057): Add `dispose` function (to unsubscribe, remove elements).
 @sealed
+// @JS()
+// @staticInterop
 class Testability {
   final NgZone _ngZone;
 
@@ -46,6 +48,7 @@ class Testability {
   /// This is commonly referred to as _stable_, that-is that the DOM
   /// representation of the app is synchronized with the Dart data and template
   /// models, and no more changes are (currently) epected.
+  // @JSExport('whenStable')
   void whenStable(void Function() callback) {
     _storeCallback(callback);
     _runCallbacksIfStable();
@@ -69,6 +72,7 @@ class Testability {
   /// Whether the framework is no longer anticipating change detection.
   ///
   /// See [whenStable] for details.
+  // @JSExport('isStable')
   bool get isStable => !_ngZone.isRunning && !_ngZone.hasPendingMacrotasks;
 
   void _runCallbacksIfStable() {