From 1e374512a1385a8721888c1fc87ec67b275ae7a8 Mon Sep 17 00:00:00 2001
From: Marc Zwart <marc@briskey.nl>
Date: Tue, 20 Jan 2026 00:57:03 +0100
Subject: [PATCH 1/3] add failing test for completed task of long marshalling

---
 .../JavaScript/JSExportTest.cs                | 26 +++++++++++++++++++
 .../JavaScript/JavaScriptTestHelper.cs        | 12 ++++++++-
 2 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JSExportTest.cs b/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JSExportTest.cs
index 7e6364265b335b..4eae0ab9cb10c8 100644
--- a/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JSExportTest.cs
+++ b/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JSExportTest.cs
@@ -390,6 +390,32 @@ public async Task JsExportTaskOfInt(int value)
             //GC.Collect();
         }
 
+        [Theory]
+        [MemberData(nameof(MarshalBigInt64Cases))]
+        public async Task JsExportTaskOfLong(long value)
+        {
+            TaskCompletionSource<long> tcs = new TaskCompletionSource<long>();
+            var res = JavaScriptTestHelper.invoke1_TaskOfLong(tcs.Task, nameof(JavaScriptTestHelper.AwaitTaskOfInt64));
+            tcs.SetResult(value); // incompleted task marshalls promise and resolves on completion
+            await Task.Yield();
+            var rr = await res;
+            await Task.Yield();
+            Assert.Equal(value, rr);
+        }
+
+        [Theory]
+        [MemberData(nameof(MarshalBigInt64Cases))]
+        public async Task JsExportCompletedTaskOfLong(long value)
+        {
+            TaskCompletionSource<long> tcs = new TaskCompletionSource<long>();
+            tcs.SetResult(value); // completed task marshalls value immediately
+            var res = JavaScriptTestHelper.invoke1_TaskOfLong(tcs.Task, nameof(JavaScriptTestHelper.AwaitTaskOfInt64));
+            await Task.Yield();
+            var rr = await res;
+            await Task.Yield();
+            Assert.Equal(value, rr);
+        }
+
         [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsNotWasmThreadingSupported))]
         public void JsExportCallback_FunctionIntInt()
         {
diff --git a/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JavaScriptTestHelper.cs b/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JavaScriptTestHelper.cs
index 26b81d5bf52273..c064839d530677 100644
--- a/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JavaScriptTestHelper.cs
+++ b/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JavaScriptTestHelper.cs
@@ -441,7 +441,9 @@ public static Exception EchoException([JSMarshalAs<JSType.Error>] Exception arg1
         [JSImport("invoke1", "JavaScriptTestHelper")]
         [return: JSMarshalAs<JSType.Promise<JSType.Number>>]
         internal static partial Task<int> invoke1_TaskOfInt([JSMarshalAs<JSType.Promise<JSType.Number>>] Task<int> value, [JSMarshalAs<JSType.String>] string name);
-    
+        [JSImport("invoke1", "JavaScriptTestHelper")]
+        [return: JSMarshalAs<JSType.Promise<JSType.BigInt>>]
+        internal static partial Task<long> invoke1_TaskOfLong([JSMarshalAs<JSType.Promise<JSType.BigInt>>] Task<long> value, [JSMarshalAs<JSType.String>] string name);
         [JSImport("returnResolvedPromise", "JavaScriptTestHelper")]
         internal static partial Task ReturnResolvedPromise();
 
@@ -462,6 +464,14 @@ public static async Task<object> AwaitTaskOfObject([JSMarshalAs<JSType.Promise<J
             return res;
         }
 
+        [JSExport]
+        [return: JSMarshalAs<JSType.Promise<JSType.BigInt>>]
+        public static async Task<long> AwaitTaskOfInt64([JSMarshalAs<JSType.Promise<JSType.BigInt>>] Task<long> arg1)
+        {
+            var res = await arg1;
+            return res;
+        }
+
         #endregion
 
         #region Action + Func

From 52e64bc5e0d17fd03fcf83d635abc49fe61bfe7c Mon Sep 17 00:00:00 2001
From: Marc Zwart <marc@briskey.nl>
Date: Tue, 20 Jan 2026 00:58:22 +0100
Subject: [PATCH 2/3] use marshaller provided by codegen in completed task path

---
 .../JavaScript/Marshaling/JSMarshalerArgument.Task.cs           | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/Marshaling/JSMarshalerArgument.Task.cs b/src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/Marshaling/JSMarshalerArgument.Task.cs
index 38a8e20487571c..910a6f472e9c00 100644
--- a/src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/Marshaling/JSMarshalerArgument.Task.cs
+++ b/src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/Marshaling/JSMarshalerArgument.Task.cs
@@ -357,7 +357,7 @@ public void ToJS<T>(Task<T>? value, ArgumentToJSCallback<T> marshaler)
                 else
                 {
                     T result = task.Result;
-                    ToJS(result);
+                    marshaler(ref this, result);
                     slot.ElementType = slot.Type;
                     slot.Type = MarshalerType.TaskResolved;
                     return;

From 3d1966dc587fee8bf327307c5fc5d41561d64667 Mon Sep 17 00:00:00 2001
From: Pavel Savara <pavelsavara@microsoft.com>
Date: Tue, 20 Jan 2026 12:47:21 +0100
Subject: [PATCH 3/3] nit

---
 .../System/Runtime/InteropServices/JavaScript/JSExportTest.cs   | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JSExportTest.cs b/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JSExportTest.cs
index 4eae0ab9cb10c8..f6e7e99abd310b 100644
--- a/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JSExportTest.cs
+++ b/src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JSExportTest.cs
@@ -396,7 +396,7 @@ public async Task JsExportTaskOfLong(long value)
         {
             TaskCompletionSource<long> tcs = new TaskCompletionSource<long>();
             var res = JavaScriptTestHelper.invoke1_TaskOfLong(tcs.Task, nameof(JavaScriptTestHelper.AwaitTaskOfInt64));
-            tcs.SetResult(value); // incompleted task marshalls promise and resolves on completion
+            tcs.SetResult(value); // unresolved task marshalls promise and resolves on completion
             await Task.Yield();
             var rr = await res;
             await Task.Yield();