Vitest Testing Skill

Core Patterns

Basic Test

import { describe, it, expect, vi, beforeEach } from 'vitest';
import { Calculator } from './calculator';

describe('Calculator', () => {
  let calc: Calculator;
  beforeEach(() => { calc = new Calculator(); });

  it('adds two numbers', () => {
    expect(calc.add(2, 3)).toBe(5);
  });

  it('throws on divide by zero', () => {
    expect(() => calc.divide(10, 0)).toThrow();
  });
});

Mocking (vi instead of jest)

import { vi } from 'vitest';

// Mock module
vi.mock('./database', () => ({
  getUser: vi.fn().mockResolvedValue({ name: 'Alice' }),
  saveUser: vi.fn().mockResolvedValue(true),
}));

// Mock function
const mockFn = vi.fn();
mockFn.mockReturnValue(42);
mockFn.mockResolvedValue({ data: 'test' });

// Spy
const spy = vi.spyOn(console, 'log').mockImplementation(() => {});
expect(spy).toHaveBeenCalledWith('message');
spy.mockRestore();

// Timers
vi.useFakeTimers();
vi.advanceTimersByTime(1000);
vi.runAllTimers();
vi.useRealTimers();

In-Source Testing

// src/math.ts — tests alongside code!
export function add(a: number, b: number) { return a + b; }
export function multiply(a: number, b: number) { return a * b; }

if (import.meta.vitest) {
  const { it, expect } = import.meta.vitest;
  it('adds', () => { expect(add(2, 3)).toBe(5); });
  it('multiplies', () => { expect(multiply(3, 4)).toBe(12); });
}

Snapshot Testing

it('serializes user', () => {
  expect(serializeUser(user)).toMatchSnapshot();
});

it('inline snapshot', () => {
  expect(serializeUser(user)).toMatchInlineSnapshot(`
    { "name": "Alice", "email": "alice@test.com" }
  `);
});

React Component Testing

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

describe('Button', () => {
  it('renders with label', () => {
    render(<Button label="Click me" />);
    expect(screen.getByText('Click me')).toBeDefined();
  });
});

Anti-Patterns

| Bad | Good | Why |
|-----|------|-----|
| jest.fn() | vi.fn() | Vitest uses vi |
| jest.mock() | vi.mock() | Different namespace |
| No type safety | TypeScript + strict | Vitest is TS-first |

Quick Reference

| Task | Command |
|------|---------|
| Run once | npx vitest run |
| Watch | npx vitest (default) |
| UI | npx vitest --ui |
| Coverage | npx vitest --coverage |
| Specific file | npx vitest run src/math.test.ts |
| Filter | npx vitest run -t "adds" |

vitest.config.ts

import { defineConfig } from 'vitest/config';
export default defineConfig({
  test: {
    globals: true,
    environment: 'jsdom',
    coverage: { provider: 'v8', reporter: ['text', 'html'] },
    include: ['src/**/*.{test,spec}.{js,ts,tsx}'],
    includeSource: ['src/**/*.{js,ts}'],
  },
});

Deep Patterns → reference/playbook.md

| § | Section | Lines |
|---|---------|-------|
| 1 | Production Configuration | Config, workspace, setup file |
| 2 | Mocking Patterns | vi.mock, spies, timers, fetch |
| 3 | React Testing Library | Components, hooks, providers |
| 4 | Snapshot & Inline Snapshots | File, inline, serializers |
| 5 | Table-Driven Tests | test.each, describe.each |
| 6 | In-Source Testing | Co-located tests in source |
| 7 | API / Integration Testing | Server tests with fetch |
| 8 | CI/CD Integration | GitHub Actions, scripts |
| 9 | Debugging Quick-Reference | 10 common problems |
| 10 | Best Practices Checklist | 13 items |