test: Make sure response is still writable before setting headers (#12478)

packages/lib/server/defaultResponder.test.ts

@@ -0,0 +1,22 @@
+import type { NextApiRequest, NextApiResponse } from "next";
+import { describe, it, expect, vi } from "vitest";
+
+import { defaultResponder } from "./defaultResponder";
+
+describe("defaultResponder", () => {
+  it("should call res.json when response is still writable and result is not null", async () => {
+    const f = vi.fn().mockResolvedValue({});
+    const req = {} as NextApiRequest;
+    const res = { json: vi.fn(), writableEnded: false } as unknown as NextApiResponse;
+    await defaultResponder(f)(req, res);
+    expect(res.json).toHaveBeenCalled();
+  });
+
+  it("should not call res.json when response is not writable", async () => {
+    const f = vi.fn().mockResolvedValue({});
+    const req = {} as NextApiRequest;
+    const res = { json: vi.fn(), writableEnded: true } as unknown as NextApiResponse;
+    await defaultResponder(f)(req, res);
+    expect(res.json).not.toHaveBeenCalled();
+  });
+});