test(shared): add unit tests for createDeferredEventBuffer (#98670)

src/shared/deferred-event-buffer.test.ts

@@ -0,0 +1,61 @@
+// Deferred event buffer tests cover buffered event dispatch with flush/discard.
+import { describe, expect, it, vi } from "vitest";
+import { createDeferredEventBuffer } from "./deferred-event-buffer.js";
+
+describe("createDeferredEventBuffer", () => {
+  it("buffers events on push and delivers on flush", () => {
+    const sink = { push: vi.fn() };
+    const buffer = createDeferredEventBuffer(sink);
+    buffer.push("a");
+    buffer.push("b");
+    expect(sink.push).not.toHaveBeenCalled();
+    buffer.flush();
+    expect(sink.push).toHaveBeenCalledTimes(2);
+    expect(sink.push).toHaveBeenNthCalledWith(1, "a");
+    expect(sink.push).toHaveBeenNthCalledWith(2, "b");
+  });
+
+  it("discards buffered events without delivering to sink", () => {
+    const sink = { push: vi.fn() };
+    const buffer = createDeferredEventBuffer(sink);
+    buffer.push("a");
+    buffer.discard();
+    buffer.flush();
+    expect(sink.push).not.toHaveBeenCalled();
+  });
+
+  it("calls onBufferedEvent callback on each push", () => {
+    const onEvent = vi.fn();
+    const buffer = createDeferredEventBuffer({ push() {} }, onEvent);
+    buffer.push("a");
+    buffer.push("b");
+    expect(onEvent).toHaveBeenCalledTimes(2);
+  });
+
+  it("does not throw when onBufferedEvent is not provided", () => {
+    const buffer = createDeferredEventBuffer({ push() {} });
+    expect(() => buffer.push("a")).not.toThrow();
+  });
+
+  it("allows push after flush to start a new buffer", () => {
+    const sink = { push: vi.fn() };
+    const buffer = createDeferredEventBuffer(sink);
+    buffer.push("a");
+    buffer.flush();
+    expect(sink.push).toHaveBeenCalledTimes(1);
+    buffer.push("b");
+    buffer.flush();
+    expect(sink.push).toHaveBeenCalledTimes(2);
+  });
+
+  it("allows push after discard to start fresh", () => {
+    const sink = { push: vi.fn() };
+    const buffer = createDeferredEventBuffer(sink);
+    buffer.push("a");
+    buffer.discard();
+    buffer.push("b");
+    buffer.flush();
+    expect(sink.push).toHaveBeenCalledTimes(1);
+    expect(sink.push).toHaveBeenCalledWith("b");
+  });
+});