Vitest Skill

> Generates Vitest tests in JavaScript/TypeScript with Vite-native speed. Jest-compatible API with ESM support and HMR. Use when user mentions "Vitest", "Vite test", "vitest config".

5k tokens

context cost

the whole folder, loaded on every use

files

instructions only

copies elsewhere

how many repositories repackaged it

343

stars on the repo

on the repository, not the skill itself

Install

one command, takes just this skill from the repository

npx skills add https://github.com/LambdaTest/agent-skills --skill vitest-skill

The instruction itself

11 sections, as written by the author

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": "[email protected]" }
  `);
});

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 |

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19k tokens

How to use it

Copy the folder

Take lambdatest/vitest-skill from the repository into ~/.claude/skills for personal use, or into .claude/skills inside a project.

Check the name does not clash

The agent identifies a skill by the name field in its header. Two skills with the same name cannot sit side by side — one of them will be ignored.

Install what it needs

The instructions reference npx. Without those the skill loads but fails at the first command.