Vitest

Vitest Setup

Detect the project type, generate the right Vitest configuration, and produce working test infrastructure. Not a reference card — this skill creates files.

Workflow

1. Detect — scan the project to determine type and existing setup
2. Configure — generate vitest.config.ts tailored to the environment
3. Scaffold — create test setup, utilities, and a sample test
4. Wire up — add package.json scripts and TypeScript config

Step 1: Detect Project Type

Read these files to determine the project:

package.json          → dependencies, scripts, type field
tsconfig.json         → paths, compiler options
wrangler.toml         → Cloudflare Workers project
vite.config.ts        → existing Vite setup (extend, don't replace)
vitest.config.ts      → already configured? just fill gaps
jest.config.*         → migration candidate
src/                  → source structure

Classify as one of:

| Type | Signals | Environment | |------|---------|-------------| | Cloudflare Workers | wrangler.toml, @cloudflare/workers-types, cloudflare vite plugin | node with Workers-specific setup | | React (Vite) | @vitejs/plugin-react, react-dom | jsdom or happy-dom | | React (SSR/TanStack Start) | @tanstack/start, vinxi | Split: node for server, jsdom for client | | Node/Hono API | hono, express, no react-dom | node | | Library | exports field, no framework deps | node |

If a vite.config.ts already exists, extend it rather than creating a separate vitest.config.ts — Vitest reads Vite config natively.

Step 2: Install Dependencies

Generate the install command based on detected type:

# Base (always)
pnpm add -D vitest

# React projects — add jsdom and Testing Library
pnpm add -D @vitest/coverage-v8 jsdom @testing-library/react @testing-library/jest-dom @testing-library/user-event

# Workers projects — add Cloudflare test utilities
pnpm add -D @vitest/coverage-v8 @cloudflare/vitest-pool-workers

# Node/Hono projects
pnpm add -D @vitest/coverage-v8

# If migrating from Jest, also remove:
pnpm remove jest ts-jest @types/jest jest-environment-jsdom babel-jest

Use the project's package manager (check for pnpm-lock.yaml, yarn.lock, bun.lockb, or package-lock.json).

Step 3: Generate vitest.config.ts

Cloudflare Workers

import { defineWorkersConfig } from "@cloudflare/vitest-pool-workers/config";

export default defineWorkersConfig({
  test: {
    globals: true,
    poolOptions: {
      workers: {
        wrangler: { configPath: "./wrangler.toml" },
      },
    },
  },
});

If the project uses the Cloudflare Vite plugin (@cloudflare/vite-plugin), integrate into the existing vite.config.ts instead:

/// 
import { defineConfig } from "vite";
import { cloudflare } from "@cloudflare/vite-plugin";

export default defineConfig({
  plugins: [cloudflare()],
  test: {
    globals: true,
  },
});

React (Vite)

/// 
import { defineConfig } from "vite";
import react from "@vitejs/plugin-react";

export default defineConfig({
  plugins: [react()],
  test: {
    globals: true,
    environment: "jsdom",
    setupFiles: ["./src/test/setup.ts"],
    css: true,
  },
});

If a vite.config.ts already exists, add the test block to it rather than creating a new file.

Node / Hono API

import { defineConfig } from "vitest/config";

export default defineConfig({
  test: {
    globals: true,
    environment: "node",
  },
});

With Coverage (add to any config)

  test: {
    // ... existing config
    coverage: {
      provider: "v8",
      reporter: ["text", "html", "lcov"],
      exclude: [
        "node_modules/",
        "**/*.config.*",
        "**/*.d.ts",
        "**/test/**",
      ],
    },
  },

Step 4: Generate Test Setup File

Create src/test/setup.ts (React projects only):

import "@testing-library/jest-dom/vitest";

That single import adds all the custom matchers (toBeInTheDocument, toHaveTextContent, etc.) and registers the Vitest expect.extend automatically.

Step 5: Add TypeScript Config

Add to tsconfig.json compilerOptions:

{
  "compilerOptions": {
    "types": ["vitest/globals"]
  }
}

For projects with multiple tsconfig files (e.g. tsconfig.app.json + tsconfig.node.json), add to the one that covers test files — usually the root tsconfig.json or create a tsconfig.test.json that extends it.

Step 6: Add Package.json Scripts

{
  "scripts": {
    "test": "vitest",
    "test:run": "vitest run",
    "test:coverage": "vitest run --coverage",
    "test:ui": "vitest --ui"
  }
}

Don't overwrite existing scripts — merge with what's there.

Step 7: Generate Sample Test

Write one test file that demonstrates the right patterns for this specific project. Place it next to real source code, not in a separate __tests__ directory.

For a Hono API route (e.g. src/routes/health.ts):

import { describe, it, expect } from "vitest";
import { app } from "../index";

describe("GET /health", () => {
  it("returns 200 with status ok", async () => {
    const res = await app.request("/health");
    expect(res.status).toBe(200);

    const body = await res.json();
    expect(body).toEqual({ status: "ok" });
  });
});

For a React component (e.g. src/components/Button.tsx):

import { describe, it, expect } from "vitest";
import { render, screen } from "@testing-library/react";
import userEvent from "@testing-library/user-event";
import { Button } from "./Button";

describe("Button", () => {
  it("renders with label", () => {
    render(Click me);
    expect(screen.getByRole("button", { name: /click me/i })).toBeInTheDocument();
  });

  it("calls onClick when clicked", async () => {
    const user = userEvent.setup();
    const handleClick = vi.fn();
    render(Click me);

    await user.click(screen.getByRole("button"));
    expect(handleClick).toHaveBeenCalledOnce();
  });
});

For a utility function (e.g. src/utils/format.ts):

import { describe, it, expect } from "vitest";
import { formatCurrency } from "./format";

describe("formatCurrency", () => {
  it("formats whole numbers", () => {
    expect(formatCurrency(1000)).toBe("$1,000.00");
  });

  it("formats decimals", () => {
    expect(formatCurrency(49.9)).toBe("$49.90");
  });

  it("handles zero", () => {
    expect(formatCurrency(0)).toBe("$0.00");
  });
});

Pick a real file from the project to test. Don't invent a fake module — the sample test should run immediately after setup.

Step 8: Verify

Run the tests to confirm everything works:

pnpm test:run

If it fails, diagnose and fix. Common issues:

| Error | Fix | |-------|-----| | Cannot find module 'vitest' | Check install completed, check node_modules/.vitest exists | | ReferenceError: describe is not defined | Add globals: true to config, or add types: ["vitest/globals"] to tsconfig | | document is not defined | Wrong environment — set environment: "jsdom" for React tests | | Cannot use import.meta | Ensure vitest.config uses .ts extension and project has "type": "module" or Vite handles transforms | | Workers bindings undefined | Use @cloudflare/vitest-pool-workers instead of plain vitest, check wrangler.toml path |

---

Mocking Reference

These patterns are for writing tests after setup is complete. Include them in the sample test or a src/test/examples.test.ts if the user asks for mocking examples.

Module mocking (vi.mock)

import { vi, describe, it, expect } from "vitest";
import { getUser } from "./api";

vi.mock("./api", () => ({
  getUser: vi.fn(),
}));

it("mocks a module function", async () => {
  vi.mocked(getUser).mockResolvedValue({ id: 1, name: "Test" });
  const user = await getUser(1);
  expect(user.name).toBe("Test");
  expect(getUser).toHaveBeenCalledWith(1);
});

Spy on methods (vi.spyOn)

it("spies on console.warn", () => {
  const spy = vi.spyOn(console, "warn").mockImplementation(() => {});
  doSomethingThatWarns();
  expect(spy).toHaveBeenCalledOnce();
  spy.mockRestore();
});

Fake timers

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

beforeEach(() => {
  vi.useFakeTimers();
  vi.setSystemTime(new Date("2026-01-15T10:00:00Z"));
});

afterEach(() => {
  vi.useRealTimers();
});

it("uses controlled time", () => {
  expect(new Date().toISOString()).toBe("2026-01-15T10:00:00.000Z");
});

Global stubs

it("stubs fetch", async () => {
  const mockFetch = vi.fn().mockResolvedValue({
    ok: true,
    json: () => Promise.resolve({ data: "test" }),
  });
  vi.stubGlobal("fetch", mockFetch);

  const res = await fetch("/api/data");
  expect(mockFetch).toHaveBeenCalledWith("/api/data");

  vi.unstubAllGlobals();
});

Snapshot testing

it("matches snapshot", () => {
  const result = generateConfig({ debug: true });
  expect(result).toMatchSnapshot();
});

it("matches inline snapshot", () => {
  expect({ status: "ok", count: 3 }).toMatchInlineSnapshot(`
    {
      "count": 3,
      "status": "ok",
    }
  `);
});

Parameterized tests

describe.each([
  { input: "hello", expected: "HELLO" },
  { input: "world", expected: "WORLD" },
  { input: "", expected: "" },
])("toUpperCase($input)", ({ input, expected }) => {
  it(`returns ${expected}`, () => {
    expect(input.toUpperCase()).toBe(expected);
  });
});

---

Jest Migration

When the detected project has Jest (jest.config.*, @types/jest, ts-jest in dependencies):

1. Generate the vitest.config.ts using the steps above
2. Update imports in existing test files:

// Before
import { jest } from "@jest/globals";
jest.mock("./api");
jest.fn();
jest.spyOn(obj, "method");

// After
import { vi } from "vitest";
vi.mock("./api");
vi.fn();
vi.spyOn(obj, "method");

1. Remove Jest packages:

pnpm remove jest ts-jest @types/jest jest-environment-jsdom babel-jest @jest/globals

1. Update tsconfig — replace "types": ["jest"] with "types": ["vitest/globals"]

1. Run tests and fix any remaining issues

Key replacements:

| Jest | Vitest | |------|--------| | jest.fn() | vi.fn() | | jest.mock() | vi.mock() | | jest.spyOn() | vi.spyOn() | | jest.useFakeTimers() | vi.useFakeTimers() | | jest.clearAllMocks() | vi.clearAllMocks() | | jest.requireActual() | vi.importActual() | | @jest/globals | vitest | | jest.config.js | vitest.config.ts |

---

Workspace Setup (Monorepos)

For monorepo projects with multiple packages:

// vitest.workspace.ts
import { defineWorkspace } from "vitest/config";

export default defineWorkspace([
  "packages/*/vitest.config.ts",
]);

Each package gets its own config. The workspace file just points to them.

---

What This Skill Produces

After running, the project should have:

• vitest.config.ts (or test block added to existing vite.config.ts)
• src/test/setup.ts (React projects)
• Updated tsconfig.json with vitest/globals type
• Updated package.json with test scripts
• At least one passing sample test against real source code
• Dependencies installed

The tests should pass on first run. If they don't, fix them before finishing.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: jezweb
• Source: jezweb/claude-skills
• License: MIT
• Homepage: https://jezweb.com.au

Install and usage instructions live in the source repository linked above.