vitest

Create a vitest.config.ts file using defineConfig from 'vitest/config' and set test options like globals and environment. You can also add a test block directly to an existing vite.config.ts or merge both configs with mergeConfig.

Use vi.fn() for mock functions, vi.spyOn() for spying on object methods, and vi.mock() for module mocking, which is hoisted to the top of the file. Partial mocks use importOriginal to spread the actual module and override specific exports.

Yes, Vitest supports DOM testing through the jsdom or happy-dom environments, configured via the environment option or per-file with the @vitest-environment comment. It also offers browser mode with Playwright for real browser testing.

Use test.concurrent for parallel tests within a file and fileParallelism with pool options for parallel files. For CI, split runs with --shard=1/3 across machines and merge results using --merge-reports.

Mock state persists unless cleared. Call mockClear, mockReset, or mockRestore on individual mocks, or enable clearMocks, restoreMocks, and unstubGlobals in the test config to reset automatically between tests.

Yes, Vitest supports type testing through .test-d.ts files using expectTypeOf and assertType. Run them with vitest typecheck or vitest --typecheck to validate type-level assertions separately from runtime tests.