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:

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:

/// <reference types="vitest/config" />
import { defineConfig } from "vite";
import { cloudflare } from "@cloudflare/vite-plugin";

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

React (Vite)

/// <reference types="vitest/config" />
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(<Button>Click me</Button>);
    expect(screen.getByRole("button", { name: /click me/i })).toBeInTheDocument();
  });

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

    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:

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