# Vitest — Next Generation Testing Framework Powered by Vite

> Vitest is a blazing-fast unit testing framework powered by Vite, with native ESM, TypeScript, and JSX support. Jest-compatible API, instant HMR for tests, and in-source testing make it the go-to test runner for Vite projects.

## Install

Save in your project root:

## Quick Use

```bash
npm i -D vitest
```

```json
// package.json
{
  "scripts": { "test": "vitest" }
}
```

```ts
// sum.test.ts
import { expect, test } from "vitest";
import { sum } from "./sum";

test("adds 1 + 2 to equal 3", () => {
  expect(sum(1, 2)).toBe(3);
});
```

Run:
```bash
npm test          # watch mode
npm test -- --run # single run
npm test -- --ui  # browser UI
```

## Intro

Vitest is a next-generation testing framework powered by Vite. Jest-compatible API with massive performance gains thanks to Vite ESM-native dev server and Rollup bundling. Default choice for Vite projects (Vue, Svelte, Solid, React+Vite).

- **Repo**: https://github.com/vitest-dev/vitest
- **Stars**: 16K+
- **Language**: TypeScript
- **License**: MIT

## What Vitest Does

- **Unit tests** — Jest-compatible `describe/test/expect`
- **Watch mode** — instant re-run via Vite HMR
- **TypeScript** — native, no ts-jest
- **ESM** — native ES modules
- **Mocks** — `vi.fn()`, `vi.mock()` matching Jest API
- **Snapshots** — inline and file-based
- **Coverage** — v8 or istanbul providers
- **Browser mode** — run tests in real browser via Playwright
- **In-source testing** — tests next to code with `if (import.meta.vitest)`

## Architecture

Uses the same `vite.config.ts` as the app. Test files transformed by Vite plugins (same as dev). Workers spin up for parallel test execution. Vite dep pre-bundling caches third-party libs for fast startup.

## Self-Hosting

Dev tool. Install as devDependency. CI example:
```yaml
- run: npm ci
- run: npx vitest run --coverage
```

## Key Features

- Jest-compatible API (easy migration)
- Native TS/JSX/ESM
- Vite HMR-powered watch
- UI mode (browser dashboard)
- Browser mode (real browser tests)
- In-source testing
- Workspace support (monorepos)
- Benchmark mode (bench/describe)

## Comparison

| Runner | Speed | TS | ESM | Vite |
|---|---|---|---|---|
| Vitest | Fast | Native | Native | Integrated |
| Jest | Slow | ts-jest | Experimental | No |
| Mocha | Medium | ts-node | Yes | No |
| Bun test | Fastest | Native | Native | No |

## 常见问题 FAQ

**Q: 从 Jest 迁移难吗？**
A: API 几乎一致。大部分项目只需改 import `jest` → `vi`，替换 `jest.config` 为 `vite.config` 的 `test` 字段。

**Q: 不用 Vite 能用 Vitest 吗？**
A: 可以。Vitest 只需 vite.config 的配置字段，不需要 Vite 构建应用。但 Vite 项目收益最大。

**Q: 和 Playwright 冲突吗？**
A: 不。Vitest 做单元/组件测试，Playwright 做 E2E。可共存。

## 来源与致谢 Sources

- Docs: https://vitest.dev
- GitHub: https://github.com/vitest-dev/vitest
- License: MIT

---
Source: https://tokrepo.com/en/workflows/267275ed-355f-11f1-9bc6-00163e2b0d79
Author: AI Open Source