[browser] fix marshaling reject(null)

src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JSImportTest.cs

@@ -127,6 +127,27 @@ public unsafe void OutOfRange()
             Assert.Contains("Overflow: value 9007199254740991 is out of -2147483648 2147483647 range", ex.Message);
         }
 
+        [Fact]
+        public async Task RejectString()
+        {
+            var ex = await Assert.ThrowsAsync<JSException>(() => JavaScriptTestHelper.Reject("noodles"));
+            Assert.Contains("noodles", ex.Message);
+        }
+
+        [Fact]
+        public async Task RejectException()
+        {
+            var expected = new Exception("noodles");
+            var actual = await Assert.ThrowsAsync<Exception>(() => JavaScriptTestHelper.Reject(expected));
+            Assert.Equal(expected, actual);
+        }
+
+        [Fact]
+        public async Task RejectNull()
+        {
+            var ex = await Assert.ThrowsAsync<JSException>(() => JavaScriptTestHelper.Reject(null));
+        }
+
         [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsNotWasmThreadingSupported))]
         public unsafe void OptimizedPaths()
         {

src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JavaScriptTestHelper.cs

@@ -37,6 +37,9 @@ public static void ConsoleWriteLine([JSMarshalAs<JSType.String>] string message)
         [JSImport("delay", "JavaScriptTestHelper")]
         public static partial Task Delay(int ms);
 
+        [JSImport("reject", "JavaScriptTestHelper")]
+        public static partial Task Reject([JSMarshalAs<JSType.Any>] object what);
+
         [JSImport("intentionallyMissingImport", "JavaScriptTestHelper")]
         public static partial void IntentionallyMissingImport();
 

src/libraries/System.Runtime.InteropServices.JavaScript/tests/System.Runtime.InteropServices.JavaScript.UnitTests/System/Runtime/InteropServices/JavaScript/JavaScriptTestHelper.mjs

@@ -448,3 +448,7 @@ export async function setup() {
 export function delay(ms) {
     return new Promise(resolve => globalThis.setTimeout(resolve, ms));
 }
+
+export function reject(what) {
+    return new Promise((_, reject) => globalThis.setTimeout(() => reject(what), 0));
+}

src/mono/browser/runtime/cancelable-promise.ts

@@ -111,9 +111,12 @@ export class PromiseHolder extends ManagedObject {
             mono_log_debug("This promise rejection can't be propagated to managed code, mono runtime already exited.");
             return;
         }
+        if (!reason) {
+            reason = new Error() as any;
+        }
         mono_assert(!this.isResolved, "reject could be called only once");
         mono_assert(!this.isDisposed, "resolve is already disposed.");
-        const isCancelation = reason && reason[promise_holder_symbol] === this;
+        const isCancelation = reason[promise_holder_symbol] === this;
         if (WasmEnableThreads && !isCancelation && !this.setIsResolving()) {
             // we know that cancelation is in flight
             // because we need to keep the GCHandle alive until until the cancelation arrives