return early if falling back to setTimeout for MessageChannel

.changeset/olive-owls-search.md

@@ -0,0 +1,5 @@
+---
+'@remote-dom/core': patch
+---
+
+return early if falling back to setTimeout for MessageChannel

packages/core/source/elements/connection.ts

@@ -58,6 +58,7 @@ function createDefaultBatchFunction() {
       setTimeout(() => {
         queue();
       }, 0);
+      return;
     }
 
     // `MessageChannel` trick that forces the code to run on the next task.

packages/core/source/elements/tests/connection.test.ts

@@ -29,6 +29,30 @@ describe('BatchingRemoteConnection', () => {
     expect(connection.mutate).toHaveBeenCalledWith([7, 8, 9]);
   });
 
+  it('batches mutations with setTimeout when there is no MessageChannel', async () => {
+    vi.useFakeTimers();
+    vi.spyOn(globalThis, 'MessageChannel', 'get').mockReturnValue(
+      undefined as any,
+    );
+    const setTimeoutSpy = vi.spyOn(globalThis, 'setTimeout');
+    const connection = createRemoteConnectionSpy();
+    const batchingConnection = new BatchingRemoteConnection(connection);
+
+    batchingConnection.mutate([1, 2, 3]);
+    batchingConnection.mutate([4, 5, 6]);
+
+    expect(connection.mutate).not.toHaveBeenCalled();
+
+    vi.runAllTimers();
+
+    expect(setTimeoutSpy).toHaveBeenCalledTimes(1);
+    expect(connection.mutate).toHaveBeenCalledTimes(1);
+    expect(connection.mutate).toHaveBeenCalledWith([1, 2, 3, 4, 5, 6]);
+
+    vi.restoreAllMocks();
+    vi.useRealTimers();
+  });
+
   it('flushes mutations', async () => {
     const connection = createRemoteConnectionSpy();
     const batchingConnection = new BatchingRemoteConnection(connection);