testing-with-vitest

name	Testing with Vitest
description	Claude writes fast, reliable tests using Vitest for TypeScript/JavaScript projects. Use when writing unit tests, component tests, setting up mocks, snapshot testing, or configuring test coverage.

Testing with Vitest

Quick Start

// vitest.config.ts
import { defineConfig } from "vitest/config";

export default defineConfig({
  test: {
    globals: true,
    environment: "jsdom",
    setupFiles: ["./tests/setup.ts"],
    coverage: {
      provider: "v8",
      reporter: ["text", "html"],
      thresholds: { global: { branches: 80, functions: 80, lines: 80 } },
    },
  },
});

// tests/setup.ts
import { afterEach, vi } from "vitest";
import { cleanup } from "@testing-library/vue";
import "@testing-library/jest-dom/vitest";

afterEach(() => { vi.clearAllMocks(); cleanup(); });

Features

Feature	Description	Reference
Unit Testing	Fast isolated tests with describe/it/expect	Vitest API
Mocking	Module, function, and timer mocking with vi	Mocking Guide
Snapshot Testing	Component and data structure snapshots	Snapshot Testing
Component Testing	Vue/React component testing with Testing Library	Vue Test Utils
Coverage Reports	V8/Istanbul coverage with thresholds	Coverage
Parallel Execution	Multi-threaded test runner	Test Runner

Common Patterns

Unit Testing with Parametrization

import { describe, it, expect, test } from "vitest";

describe("String Utils", () => {
  test.each([
    ["hello", 3, "hel..."],
    ["hi", 10, "hi"],
    ["test", 4, "test"],
  ])('truncate("%s", %d) returns "%s"', (str, len, expected) => {
    expect(truncate(str, len)).toBe(expected);
  });
});

Mocking Modules and Services

import { describe, it, expect, vi, beforeEach, Mock } from "vitest";
import { UserService } from "@/services/user";
import { apiClient } from "@/lib/api";

vi.mock("@/lib/api", () => ({
  apiClient: { get: vi.fn(), post: vi.fn() },
}));

describe("UserService", () => {
  beforeEach(() => vi.clearAllMocks());

  it("fetches user from API", async () => {
    const mockUser = { id: "1", name: "John" };
    (apiClient.get as Mock).mockResolvedValue({ data: mockUser });

    const user = await new UserService().getUser("1");

    expect(apiClient.get).toHaveBeenCalledWith("/users/1");
    expect(user).toEqual(mockUser);
  });
});

Vue Component Testing

import { describe, it, expect, vi } from "vitest";
import { mount } from "@vue/test-utils";
import Button from "@/components/Button.vue";

describe("Button", () => {
  it("emits click event", async () => {
    const wrapper = mount(Button, { slots: { default: "Click" } });
    await wrapper.trigger("click");
    expect(wrapper.emitted("click")).toHaveLength(1);
  });

  it("is disabled when loading", () => {
    const wrapper = mount(Button, { props: { loading: true } });
    expect(wrapper.attributes("disabled")).toBeDefined();
  });
});

Testing Async Operations with Timers

import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";

describe("Debounce", () => {
  beforeEach(() => vi.useFakeTimers());
  afterEach(() => vi.useRealTimers());

  it("delays function execution", () => {
    const fn = vi.fn();
    const debouncedFn = debounce(fn, 100);

    debouncedFn();
    expect(fn).not.toHaveBeenCalled();

    vi.advanceTimersByTime(100);
    expect(fn).toHaveBeenCalledTimes(1);
  });
});

Best Practices

Do	Avoid
Use descriptive test names explaining behavior	Testing implementation details
Test behavior, not internal state	Sharing state between tests
Use test.each for multiple similar cases	Using arbitrary timeouts
Mock external dependencies	Over-mocking internal modules
Keep tests focused and isolated	Duplicating test coverage
Write tests alongside code	Ignoring flaky tests

References

• Vitest Documentation
• Vue Test Utils
• Testing Library
• Vitest Coverage