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