Use when writing unit/integration tests for Vite projects - configure vitest.config.ts, write test suites with describe/it, create mock implementations with vi.fn and vi.mock, set up code coverage thresholds, and run tests in parallel

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 5.0.1, generated at 2026 09 25. Core Topic Description Reference Configuration Vitest and Vite config integration, defineConfig usage [core config](references/core config.md) CLI Command line interface, commands and options [core cli](references/core cli.md) Test API test/it function, modifiers like skip, only, concurrent [core test api](references/core test api.md) Describe API describe/suite for grouping tests and nested suites [core describe](references/core describe.md) Expect API Assertions with toBe, toEqual, matchers and asymmetric matchers [core expect](references/core expect.md) Hooks beforeEach, afterEach, beforeAll, afterAll, aroundEach [core hooks](references/core hooks.md) Features Topic Description Reference Mocking Mock functions, modules, timers, dates with vi utilities [features mocking](references/features mocking.md) Snapshots Snapshot testing with toMatchSnapshot and inline snapshots [features snapshots](references/features snapshots.md) Coverage Code coverage with V8 or Istanbul providers [features coverage](references/features coverage.md) Test Context Test fixtures, context.expect, test.extend for custom fixtures [features context](references/features context.md) Concurrency Concurrent tests, parallel execution, sharding [features concurrency](references/features concurrency.md) Filtering Filter tests by name, file patterns, tags [features filtering](references/features filtering.md) Test Tags Label tests with tags to filter runs and apply shared options [features test tags](references/features test tags.md) Reporters Built in reporters, default selection, CI/output config [features reporters](references/features reporters.md) Benchmarking Write benchmarks with the bench fixture (Tinybench) [features benchmarking](references/features benchmarking.md) Advanced Topic Description Reference Vi Utilities vi helper: mock, spyOn, fake timers, hoisted, waitFor [advanced vi](references/advanced vi.md) Environments Test environments: node, jsdom, happy dom, custom [advanced environments](references/advanced environments.md) Type Testing Type level testing with expectTypeOf and assertType [advanced type testing](references/advanced type testing.md) Projects Multi project workspaces, different configs per project [advanced projects](references/advanced projects.md)