diff --git a/LibTest/js_interop/JSAnyUtilityExtension/instanceOfString_A01_t01.dart b/LibTest/js_interop/JSAnyUtilityExtension/instanceOfString_A01_t01.dart
new file mode 100644
index 0000000000..19c55bfd31
--- /dev/null
+++ b/LibTest/js_interop/JSAnyUtilityExtension/instanceOfString_A01_t01.dart
@@ -0,0 +1,43 @@
+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/// @assertion bool instanceOfString( String constructorName )
+///
+/// Whether this [JSAny?] is an `instanceof` the constructor that is defined by
+/// `constructorName`, which is looked up in the `globalContext`.
+///
+/// If `constructorName` contains '.'s, the name is split into several parts in
+/// order to get the constructor. For example, `library1.JSClass` would involve
+/// fetching `library1` off of the `globalContext`, and then fetching `JSClass`
+/// off of `library1` to get the constructor.
+///
+/// If `constructorName` is empty or any of the parts or the constructor don't
+/// exist, returns `false`.
+///
+/// @description Checks that `instanceOfString()` returns `true` if this
+/// `JSAny?` is is an `instanceof` the constructor that is defined by
+/// `constructorName`.
+/// @author sgrekhov22@gmail.com
+
+import 'dart:js_interop';
+import 'dart:js_interop_unsafe';
+import '../../../Utils/expect.dart';
+import '../js_utils.dart';
+
+main() {
+  eval(r'''
+    function A(id, name) {
+      this.id = id;
+      this.name = name;
+    }
+    function B() {}
+    
+    globalThis.objA = new A(42, "A form JS");
+    globalThis.objB = new B();
+  ''');
+  Expect.isTrue(globalContext["objA"].instanceOfString("A"));
+  Expect.isTrue(globalContext["objB"].instanceOfString("B"));
+  Expect.isFalse(globalContext["objA"].instanceOfString("B"));
+  Expect.isFalse(globalContext["objB"].instanceOfString("A"));
+}
diff --git a/LibTest/js_interop/JSAnyUtilityExtension/instanceOfString_A01_t02.dart b/LibTest/js_interop/JSAnyUtilityExtension/instanceOfString_A01_t02.dart
new file mode 100644
index 0000000000..51d28b176f
--- /dev/null
+++ b/LibTest/js_interop/JSAnyUtilityExtension/instanceOfString_A01_t02.dart
@@ -0,0 +1,49 @@
+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/// @assertion bool instanceOfString( String constructorName )
+///
+/// Whether this [JSAny?] is an `instanceof` the constructor that is defined by
+/// `constructorName`, which is looked up in the `globalContext`.
+///
+/// If `constructorName` contains '.'s, the name is split into several parts in
+/// order to get the constructor. For example, `library1.JSClass` would involve
+/// fetching `library1` off of the `globalContext`, and then fetching `JSClass`
+/// off of `library1` to get the constructor.
+///
+/// If `constructorName` is empty or any of the parts or the constructor don't
+/// exist, returns `false`.
+///
+/// @description Checks that `instanceOfString()` returns `true` if this
+/// `JSAny?` is is an `instanceof` the constructor that is defined by
+/// `constructorName`.
+/// @author sgrekhov22@gmail.com
+
+import 'dart:js_interop';
+import 'dart:js_interop_unsafe';
+import '../../../Utils/expect.dart';
+import '../js_utils.dart';
+
+main() {
+  eval(r'''
+    class A {
+      constructor(id, name) {
+        this.id = id;
+        this.name = name;
+      }
+    }
+    class B {
+      constructor() {}
+    }
+    
+    globalThis.fnB = B;
+    globalThis.fnA = A;
+    globalThis.objA = new A(42, "A form JS");
+    globalThis.objB = new B();
+  ''');
+  Expect.isTrue(globalContext["objA"].instanceOfString("fnA"));
+  Expect.isTrue(globalContext["objB"].instanceOfString("fnB"));
+  Expect.isFalse(globalContext["objA"].instanceOfString("B"));
+  Expect.isFalse(globalContext["objB"].instanceOfString("A"));
+}
diff --git a/LibTest/js_interop/JSAnyUtilityExtension/instanceOfString_A02_t01.dart b/LibTest/js_interop/JSAnyUtilityExtension/instanceOfString_A02_t01.dart
new file mode 100644
index 0000000000..0854f8ee68
--- /dev/null
+++ b/LibTest/js_interop/JSAnyUtilityExtension/instanceOfString_A02_t01.dart
@@ -0,0 +1,44 @@
+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/// @assertion bool instanceOfString( String constructorName )
+///
+/// Whether this [JSAny?] is an `instanceof` the constructor that is defined by
+/// `constructorName`, which is looked up in the `globalContext`.
+///
+/// If `constructorName` contains '.'s, the name is split into several parts in
+/// order to get the constructor. For example, `library1.JSClass` would involve
+/// fetching `library1` off of the `globalContext`, and then fetching `JSClass`
+/// off of `library1` to get the constructor.
+///
+/// If `constructorName` is empty or any of the parts or the constructor don't
+/// exist, returns `false`.
+///
+/// @description Checks that `instanceOfString()` returns `false` if
+/// `constructorName` cannot be found in the `globalContext`.
+/// @author sgrekhov22@gmail.com
+
+import 'dart:js_interop';
+import 'dart:js_interop_unsafe';
+import '../../../Utils/expect.dart';
+import '../js_utils.dart';
+
+main() {
+  eval(r'''
+    class A {
+      constructor(id, name) {
+        this.id = id;
+        this.name = name;
+      }
+    }
+    class B {
+      constructor() {}
+    }
+    
+    globalThis.objA = new A(42, "A form JS");
+    globalThis.objB = new B();
+  ''');
+  Expect.isFalse(globalContext["objA"].instanceOfString("A"));
+  Expect.isFalse(globalContext["objB"].instanceOfString("B"));
+}
diff --git a/LibTest/js_interop/JSAnyUtilityExtension/instanceOfString_A03_t01.dart b/LibTest/js_interop/JSAnyUtilityExtension/instanceOfString_A03_t01.dart
new file mode 100644
index 0000000000..088f7824f0
--- /dev/null
+++ b/LibTest/js_interop/JSAnyUtilityExtension/instanceOfString_A03_t01.dart
@@ -0,0 +1,43 @@
+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/// @assertion bool instanceOfString( String constructorName )
+///
+/// Whether this [JSAny?] is an `instanceof` the constructor that is defined by
+/// `constructorName`, which is looked up in the `globalContext`.
+///
+/// If `constructorName` contains '.'s, the name is split into several parts in
+/// order to get the constructor. For example, `library1.JSClass` would involve
+/// fetching `library1` off of the `globalContext`, and then fetching `JSClass`
+/// off of `library1` to get the constructor.
+///
+/// If `constructorName` is empty or any of the parts or the constructor don't
+/// exist, returns `false`.
+///
+/// @description Checks that if `constructorName` then `instanceOfString`
+/// returns `false`.
+/// @author sgrekhov22@gmail.com
+
+import 'dart:js_interop';
+import 'dart:js_interop_unsafe';
+import '../../../Utils/expect.dart';
+import '../js_utils.dart';
+
+main() {
+  eval(r'''
+    class A {
+      constructor(id, name) {
+        this.id = id;
+        this.name = name;
+      }
+    }
+    class B {
+      constructor() {}
+    }
+    globalThis.objA = new A(42, "A form JS");
+    globalThis.objB = new B();
+  ''');
+  Expect.isFalse(globalContext["objA"].instanceOfString(""));
+  Expect.isFalse(globalContext["objB"].instanceOfString(""));
+}
diff --git a/LibTest/js_interop/JSAnyUtilityExtension/instanceOfString_A03_t02.dart b/LibTest/js_interop/JSAnyUtilityExtension/instanceOfString_A03_t02.dart
new file mode 100644
index 0000000000..e1fb7c9036
--- /dev/null
+++ b/LibTest/js_interop/JSAnyUtilityExtension/instanceOfString_A03_t02.dart
@@ -0,0 +1,43 @@
+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/// @assertion bool instanceOfString( String constructorName )
+///
+/// Whether this [JSAny?] is an `instanceof` the constructor that is defined by
+/// `constructorName`, which is looked up in the `globalContext`.
+///
+/// If `constructorName` contains '.'s, the name is split into several parts in
+/// order to get the constructor. For example, `library1.JSClass` would involve
+/// fetching `library1` off of the `globalContext`, and then fetching `JSClass`
+/// off of `library1` to get the constructor.
+///
+/// If `constructorName` is empty or any of the parts or the constructor don't
+/// exist, returns `false`.
+///
+/// @description Checks that if any of the parts or the constructor don't exist,
+/// `instanceOfString` returns `false`
+/// @author sgrekhov22@gmail.com
+
+import 'dart:js_interop';
+import 'dart:js_interop_unsafe';
+import '../../../Utils/expect.dart';
+import '../js_utils.dart';
+
+main() {
+  eval(r'''
+    class A {
+      constructor(id, name) {
+        this.id = id;
+        this.name = name;
+      }
+    }
+    class B {
+      constructor() {}
+    }
+    globalThis.objA = new A(42, "A form JS");
+    globalThis.objB = new B();
+  ''');
+  Expect.isFalse(globalContext["objA"].instanceOfString("lib1.A"));
+  Expect.isFalse(globalContext["objB"].instanceOfString("lib2.B"));
+}
diff --git a/LibTest/js_interop/JSAnyUtilityExtension/instanceOfString_A04_t01.dart b/LibTest/js_interop/JSAnyUtilityExtension/instanceOfString_A04_t01.dart
new file mode 100644
index 0000000000..80071ac571
--- /dev/null
+++ b/LibTest/js_interop/JSAnyUtilityExtension/instanceOfString_A04_t01.dart
@@ -0,0 +1,58 @@
+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/// @assertion bool instanceOfString( String constructorName )
+///
+/// Whether this [JSAny?] is an `instanceof` the constructor that is defined by
+/// `constructorName`, which is looked up in the `globalContext`.
+///
+/// If `constructorName` contains '.'s, the name is split into several parts in
+/// order to get the constructor. For example, `library1.JSClass` would involve
+/// fetching `library1` off of the `globalContext`, and then fetching `JSClass`
+/// off of `library1` to get the constructor.
+///
+/// If `constructorName` is empty or any of the parts or the constructor don't
+/// exist, returns `false`.
+///
+/// @description Checks that `instanceOfString()` works as expected if this
+/// `JSAny?` is is an `instanceof` the constructor that is defined by
+/// `constructorName`. Test the case when `constructorName` contains '.'.
+/// @author sgrekhov22@gmail.com
+
+// OtherResources=module.js
+import 'dart:async';
+import 'dart:js_interop';
+import 'dart:js_interop_unsafe';
+import '../../../Utils/expect.dart';
+import '../js_utils.dart';
+
+final completer = Completer<String>();
+
+void complete(String value) {
+  completer.complete(value);
+}
+
+main() {
+  globalContext["complete"] = complete.toJS;
+  eval(r'''
+    var lib1;
+    (async () => { 
+      // This is path to the module on tryjobs. May not work locally.
+      lib1 = await import('/root_dart/tests/co19/src/LibTest/js_interop/JSAnyUtilityExtension/module.js');
+      globalThis.objA = new lib1.A(42, "A form JS");
+      globalThis.objB = new lib1.B();
+      console.log('initialized');
+    })().then(function(v) {
+      globalThis.complete("");
+    });
+  ''');
+  asyncStart();
+  completer.future.then((_) {
+    Expect.isTrue(globalContext["objA"].instanceOfString("lib1.A"));
+    Expect.isTrue(globalContext["objB"].instanceOfString("lib1.B"));
+    Expect.isFalse(globalContext["objA"].instanceOfString("A"));
+    Expect.isFalse(globalContext["objB"].instanceOfString("B"));
+    asyncEnd();
+  });
+}
diff --git a/LibTest/js_interop/JSAnyUtilityExtension/instanceof_A01_t01.dart b/LibTest/js_interop/JSAnyUtilityExtension/instanceof_A01_t01.dart
new file mode 100644
index 0000000000..f7c41db6ba
--- /dev/null
+++ b/LibTest/js_interop/JSAnyUtilityExtension/instanceof_A01_t01.dart
@@ -0,0 +1,43 @@
+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/// @assertion bool instanceof( JSFunction constructor )
+///
+/// Whether this `JSAny?` is an instance of `constructor`.
+///
+/// @description Checks that `instanceof()` returns `true` if this `JSAny?` is
+/// an instance of `constructor` and `false` otherwise.
+/// @author sgrekhov22@gmail.com
+
+import 'dart:js_interop';
+import 'dart:js_interop_unsafe';
+import '../../../Utils/expect.dart';
+import '../js_utils.dart';
+
+main() {
+  eval(r'''
+    function A(id, name) {
+      this.id = id;
+      this.name = name;
+    }
+    function B() {}
+    
+    globalThis.fnB = B;
+    globalThis.fnA = A;
+    globalThis.objA = new A(42, "A form JS");
+    globalThis.objB = new B();
+  ''');
+  Expect.isTrue(
+    globalContext["objA"].instanceof(globalContext["fnA"] as JSFunction),
+  );
+  Expect.isTrue(
+    globalContext["objB"].instanceof(globalContext["fnB"] as JSFunction),
+  );
+  Expect.isFalse(
+    globalContext["objA"].instanceof(globalContext["fnB"] as JSFunction),
+  );
+  Expect.isFalse(
+    globalContext["objF"].instanceof(globalContext["fnA"] as JSFunction),
+  );
+}
diff --git a/LibTest/js_interop/JSAnyUtilityExtension/instanceof_A01_t02.dart b/LibTest/js_interop/JSAnyUtilityExtension/instanceof_A01_t02.dart
new file mode 100644
index 0000000000..e5e357e2c7
--- /dev/null
+++ b/LibTest/js_interop/JSAnyUtilityExtension/instanceof_A01_t02.dart
@@ -0,0 +1,47 @@
+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/// @assertion bool instanceof( JSFunction constructor )
+///
+/// Whether this `JSAny?` is an instance of `constructor`.
+///
+/// @description Checks that `instanceof()` returns `true` if this `JSAny?` is
+/// an instance of `constructor` and `false` otherwise.
+/// @author sgrekhov22@gmail.com
+
+import 'dart:js_interop';
+import 'dart:js_interop_unsafe';
+import '../../../Utils/expect.dart';
+import '../js_utils.dart';
+
+main() {
+  eval(r'''
+    class A {
+      constructor(id, name) {
+        this.id = id;
+        this.name = name;
+      }
+    }
+    class B {
+      constructor() {}
+    }
+    
+    globalThis.fnB = B;
+    globalThis.fnA = A;
+    globalThis.objA = new A(42, "A form JS");
+    globalThis.objB = new B();
+  ''');
+  Expect.isTrue(
+    globalContext["objA"].instanceof(globalContext["fnA"] as JSFunction),
+  );
+  Expect.isTrue(
+    globalContext["objB"].instanceof(globalContext["fnB"] as JSFunction),
+  );
+  Expect.isFalse(
+    globalContext["objA"].instanceof(globalContext["fnB"] as JSFunction),
+  );
+  Expect.isFalse(
+    globalContext["objF"].instanceof(globalContext["fnA"] as JSFunction),
+  );
+}
diff --git a/LibTest/js_interop/JSAnyUtilityExtension/module.js b/LibTest/js_interop/JSAnyUtilityExtension/module.js
new file mode 100644
index 0000000000..8488df28e2
--- /dev/null
+++ b/LibTest/js_interop/JSAnyUtilityExtension/module.js
@@ -0,0 +1,6 @@
+export function A(id, name) {
+  this.id = id;
+  this.name = name;
+}
+
+export function B() {}