Vitest Mocking

Test Double Types

Type	Purpose	Changes Implementation?
Spy	Record calls, verify interactions	No (uses real code)
Mock	Replace module entirely	Yes
Stub	Return canned responses	Yes
Fake	Working but simplified impl	Yes

Decision Guide

Use spy when: Verifying a function was called correctly while keeping real behavior

const fetchSpy = vi.spyOn(globalThis, "fetch");
await doSomething();
expect(fetchSpy).toHaveBeenCalledWith("https://api.example.com");

Use mock when: Replacing external dependencies (network, database, third-party modules)

vi.mock("./api-client");
vi.mocked(apiClient.fetch).mockResolvedValue({ data: "test" });

Use stubGlobal when: Replacing browser/Node globals

vi.stubGlobal("fetch", vi.fn().mockResolvedValue(mockResponse));

Essential Patterns

Mock Module (hoisted)

import { someFunc } from "./module";
vi.mock("./module"); // hoisted to top

it("test", () => {
  vi.mocked(someFunc).mockReturnValue("mocked");
});

Mock with Implementation

vi.mock("./module", () => ({
  someFunc: vi.fn().mockReturnValue("mocked"),
}));

Spy with Fake Return

const spy = vi.spyOn(obj, "method").mockReturnValue("fake");

Mock Global Fetch

globalThis.fetch = vi.fn().mockResolvedValue({
  ok: true,
  json: async () => ({ data: "test" }),
} as Response);

Fake Timers

beforeAll(() => vi.useFakeTimers());
afterAll(() => vi.useRealTimers());

it("test", async () => {
  // advance time
  await vi.advanceTimersByTimeAsync(5000);
});

Clean Up

beforeEach(() => vi.clearAllMocks()); // reset call counts
afterEach(() => vi.restoreAllMocks()); // restore originals

Reference

For complete examples including default imports, named imports, classes, partial mocks, snapshot testing, composables, and verification patterns, see references/cheat-sheet.md.