vitest

Vitest is a next-generation testing framework powered by Vite. It provides a Jest-compatible API with native ESM, TypeScript, and JSX support out of the box. Vitest shares the same config, transformers, resolvers, and plugins with your Vite app.

Key Features:

• Vite-native: Uses Vite's transformation pipeline for fast HMR-like test updates
• Jest-compatible: Drop-in replacement for most Jest test suites
• Smart watch mode: Only reruns affected tests based on module graph
• Native ESM, TypeScript, JSX support without configuration
• Multi-threaded workers for parallel test execution
• Built-in coverage via V8 or Istanbul
• Snapshot testing, mocking, and spy utilities

The skill is based on Vitest 3.x, generated at 2026-01-28.

Core

| Topic | Description | Reference | | ------------- | --------------------------------------------------------------- | -------------------------------------------- | | Configuration | Vitest and Vite config integration, defineConfig usage | core-config | | CLI | Command line interface, commands and options | core-cli | | Test API | test/it function, modifiers like skip, only, concurrent | core-test-api | | Describe API | describe/suite for grouping tests and nested suites | core-describe | | Expect API | Assertions with toBe, toEqual, matchers and asymmetric matchers | core-expect | | Hooks | beforeEach, afterEach, beforeAll, afterAll, aroundEach | core-hooks |

Features

| Topic | Description | Reference | | ------------ | -------------------------------------------------------------- | ---------------------------------------------------------- | | Mocking | Mock functions, modules, timers, dates with vi utilities | features-mocking | | Snapshots | Snapshot testing with toMatchSnapshot and inline snapshots | features-snapshots | | Coverage | Code coverage with V8 or Istanbul providers | features-coverage | | Test Context | Test fixtures, context.expect, test.extend for custom fixtures | features-context | | Concurrency | Concurrent tests, parallel execution, sharding | features-concurrency | | Filtering | Filter tests by name, file patterns, tags | features-filtering |

Advanced

| Topic | Description | Reference | | ------------ | ------------------------------------------------------- | ------------------------------------------------------------ | | Vi Utilities | vi helper: mock, spyOn, fake timers, hoisted, waitFor | advanced-vi | | Environments | Test environments: node, jsdom, happy-dom, custom | advanced-environments | | Type Testing | Type-level testing with expectTypeOf and assertType | advanced-type-testing | | Projects | Multi-project workspaces, different configs per project | advanced-projects |