fix(vitest-pool-workers): responses from fetchMock should have immutable headers (#8254)

Author: edmundhung

.changeset/vast-trains-joke.md

@@ -0,0 +1,5 @@
+---
+"@cloudflare/vitest-pool-workers": patch
+---
+
+fix: responses from `fetchMock` should have immutable headers

fixtures/vitest-pool-workers-examples/misc/test/fetch-mock.test.ts

@@ -61,6 +61,19 @@ it("intercepts URLs with query parameters with repeated keys", async () => {
 	expect(await response3.text()).toBe("baz");
 });
 
+it("throws if you try to mutate the headers", async () => {
+	fetchMock
+		.get("https://example.com")
+		.intercept({ path: "/" })
+		.reply(200, "body");
+
+	let response = await fetch("https://example.com");
+
+	expect(() => response.headers.set("foo", "bar")).toThrowError();
+	expect(() => response.headers.append("foo", "baz")).toThrowError();
+	expect(() => response.headers.delete("foo")).toThrowError();
+});
+
 describe("AbortSignal", () => {
 	let abortSignalTimeoutMock: MockInstance;
 

packages/vitest-pool-workers/src/worker/fetch-mock.ts

@@ -170,7 +170,15 @@ globalThis.fetch = async (input, init) => {
 					statusText: responseStatusText,
 					headers: responseHeaders,
 				});
+				const throwImmutableHeadersError = () => {
+					throw new TypeError("Can't modify immutable headers");
+				};
 				Object.defineProperty(response, "url", { value: url.href });
+				Object.defineProperties(response.headers, {
+					set: { value: throwImmutableHeadersError },
+					append: { value: throwImmutableHeadersError },
+					delete: { value: throwImmutableHeadersError },
+				});
 				responseResolve(response);
 			} else {
 				responseResolve(maybeResponse);