Vitest Migration & Config

This skill makes the agent migrate a Jest test suite to Vitest correctly and configure Vitest from scratch. Vitest is mostly Jest-compatible, but the differences bite: the vi namespace replaces jest, vi.mock hoisting needs vi.hoisted, ESM is first-class, and Vitest v4 removed workspace files in favor of inline projects. The agent should produce a config that runs fast, types cleanly, and reports coverage.

Use this skill when migrating from Jest, setting up vitest.config.ts, splitting unit vs browser tests, or resolving v4 upgrade breakage.

Core Principles

1. `vi` replaces `jest`. jest.fn -> vi.fn, jest.mock -> vi.mock, jest.spyOn -> vi.spyOn, jest.useFakeTimers -> vi.useFakeTimers. The assertion API (expect, matchers) is unchanged.
2. Mocks must reset explicitly. Set test.clearMocks/restoreMocks in config or call vi.clearAllMocks() in beforeEach — Vitest does not reset by default.
3. `vi.mock` is hoisted; use `vi.hoisted` for shared values. Variables the factory needs must be created via vi.hoisted(() => ...), since the factory runs before imports.
4. Decide on globals up front. Either enable globals: true (Jest-like, no imports) or import { describe, it, expect, vi } from vitest. Pick one and add types: ['vitest/globals'] if using globals.
5. v4 uses `projects`, not `workspace`. The standalone vitest.workspace.ts file is removed; declare multiple environments via the inline projects array.
6. Browser mode replaces jsdom for real-DOM tests. Use browser.instances (v4) to run component tests in a real browser engine via Playwright.

Workflow / Patterns

Pattern 1 — Mechanical Jest -> Vitest swap

Most test bodies migrate with a namespace rename. With globals: true, even the imports can stay as-is.

// Before (Jest):
//   const fn = jest.fn();
//   jest.spyOn(obj, 'method');
//   jest.useFakeTimers();

// After (Vitest) — explicit imports (recommended, ESM-safe):
import { describe, it, expect, vi, beforeEach } from 'vitest';

describe('cart', () => {
  beforeEach(() => vi.clearAllMocks());

  it('adds an item', () => {
    const onChange = vi.fn();
    const cart = createCart(onChange);
    cart.add({ sku: 'A1', qty: 2 });

    expect(cart.total).toBe(2);
    expect(onChange).toHaveBeenCalledWith(expect.objectContaining({ sku: 'A1' }));
  });
});

Pattern 2 — vi.mock with vi.hoisted (the hoisting fix)

In Jest you prefix variables with mock. In Vitest, wrap them in vi.hoisted so they exist when the hoisted factory runs.

import { vi, test, expect, beforeEach } from 'vitest';
import { getUser } from './user-service';
import axios from 'axios';

// Create the spy in a hoisted block so the factory below can reference it.
const { mockGet } = vi.hoisted(() => ({ mockGet: vi.fn() }));

vi.mock('axios', () => ({
  default: { get: mockGet }, // note: ESM default export shape
}));

beforeEach(() => vi.clearAllMocks());

test('resolves user from API', async () => {
  mockGet.mockResolvedValue({ data: { id: 1, name: 'Ada' } });

  const user = await getUser(1);

  expect(user).toEqual({ id: 1, name: 'Ada' });
  expect(axios.get).toHaveBeenCalledWith('/api/users/1');
});

Pattern 3 — Partial mock with importActual

The Vitest equivalent of jest.requireActual. The factory is async.

vi.mock('./config', async (importOriginal) => {
  const actual = await importOriginal<typeof import('./config')>();
  return {
    ...actual,
    isProduction: vi.fn(() => false), // override one export, keep the rest real
  };
});

Pattern 4 — A complete vitest.config.ts

Covers globals, environment, setup files, and coverage. This replaces jest.config.js.

import { defineConfig } from 'vitest/config';

export default defineConfig({
  test: {
    globals: true, // Jest-like; otherwise import from 'vitest'
    environment: 'node', // 'jsdom' for component DOM tests
    setupFiles: ['./test/setup.ts'],
    clearMocks: true, // reset vi.fn() call data before each test
    restoreMocks: true, // restore spies to original impl
    coverage: {
      provider: 'v8', // fast, no instrumentation step
      reporter: ['text', 'html', 'lcov'],
      include: ['src/**/*.ts'],
      exclude: ['src/**/*.d.ts', 'src/**/index.ts'],
      thresholds: { lines: 80, functions: 80, branches: 75 },
    },
  },
});

If using globals: true, add to tsconfig.json:

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

Pattern 5 — Multiple environments via projects (v4)

The v4 replacement for vitest.workspace.ts. Run Node unit tests and jsdom component tests in one command, each with its own config.

import { defineConfig } from 'vitest/config';

export default defineConfig({
  test: {
    coverage: { provider: 'v8' },
    projects: [
      {
        // Fast pure-logic tests in Node.
        test: {
          name: 'unit',
          environment: 'node',
          include: ['src/**/*.unit.test.ts'],
        },
      },
      {
        // DOM/component tests in jsdom.
        test: {
          name: 'dom',
          environment: 'jsdom',
          setupFiles: ['./test/dom-setup.ts'],
          include: ['src/**/*.dom.test.ts'],
        },
      },
    ],
  },
});

Pattern 6 — Browser mode (real engine, v4 instances)

For component tests that need a real browser. v4 replaced the singular browser.name with a browser.instances array.

import { defineConfig } from 'vitest/config';

export default defineConfig({
  test: {
    browser: {
      enabled: true,
      provider: 'playwright',
      headless: true,
      instances: [
        { browser: 'chromium' },
        { browser: 'firefox' },
      ],
    },
  },
});

// component.browser.test.ts — runs in a real browser
import { render } from 'vitest-browser-react';
import { expect, test } from 'vitest';
import { Counter } from './Counter';

test('increments in a real browser', async () => {
  const screen = render(<Counter />);
  await screen.getByRole('button', { name: 'Increment' }).click();
  await expect.element(screen.getByText('Count: 1')).toBeVisible();
});

Pattern 7 — package.json scripts and the runner

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

Best Practices

1. Choose globals vs imports once and apply it suite-wide. If globals: true, add types: ['vitest/globals'] so TypeScript knows expect/vi.
2. Set `clearMocks` and `restoreMocks` in config — Vitest does not auto-reset, so omitting them recreates Jest's "leaky mock" bug.
3. Use `vi.hoisted` for any value a `vi.mock` factory references. This is the single most common migration failure.
4. Mock ESM default exports as `{ default: ... }` in the factory — the shape differs from Jest's CJS interop.
5. Use the `v8` coverage provider for speed; reserve istanbul only if you need its specific report nuances.
6. Split slow DOM/browser tests into a separate `project` so the fast Node unit tests give quick feedback.
7. On v4 upgrade, replace `vitest.workspace.ts` with inline `projects` and convert browser.name to browser.instances.

Anti-Patterns

1. Search-replacing `jest` -> `vi` and assuming you're done. Mock hoisting and ESM default shapes still need vi.hoisted and { default }.
2. Forgetting mocks reset because Vitest, unlike a default Jest setup, won't clear them for you.
3. Referencing an outer variable in `vi.mock` without `vi.hoisted`. It is undefined when the hoisted factory executes.
4. Keeping `vitest.workspace.ts` on v4. It is removed; the suite silently ignores it or errors. Use projects.
5. Enabling `globals: true` but not adding `vitest/globals` types, producing a flood of "cannot find name 'expect'" TS errors.
6. Running everything in jsdom when most tests are pure logic — jsdom is slower and unnecessary. Default to node.
7. Mixing `@jest/globals` imports into Vitest files — import from vitest instead.

When to Trigger This Skill

• "Migrate my Jest tests to Vitest"
• "Set up vitest.config.ts" / "configure Vitest coverage"
• "vi.mock isn't working / variable is undefined in the factory"
• "How do I mock a default ESM export in Vitest?"
• "Run component tests in a real browser with Vitest"
• "Vitest v4 broke my workspace / browser.name config"
• "Split unit and DOM tests into projects"
• "Jest requireActual equivalent in Vitest"