How to use
- Copy the installation command and review the source package before installing it in your own agent environment.
- Check the source document for required tools, credentials and supported hosts.
- Describe your input and desired deliverable; review the output against the source material.
npx skills add https://github.com/lambdatest/agent-skills --skill vitest-skillBefore you start
- Installation does not include a model subscription, API allowance or permission to access your files.
- Repository stars and installation counts describe activity, not an independent quality or security test.
Original skill document
Rendered from the source repository. Headings, examples, code, tables, links, and referenced images are preserved.
Vitest Testing Skill
Core Patterns
Basic Test
typescript
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)
typescript
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
typescript
// 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
typescript
it('serializes user', () => {
expect(serializeUser(user)).toMatchSnapshot();
});
it('inline snapshot', () => {
expect(serializeUser(user)).toMatchInlineSnapshot(`
{ "name": "Alice", "email": "alice@test.com" }
`);
});React Component Testing
typescript
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
typescript
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 |
