vitest

Use vi.mock with a factory function, which is hoisted to the top of the file before imports. For partial mocks, call importOriginal inside the factory and spread the actual module, overriding only specific exports with vi.fn.

Enable coverage in vitest.config.ts under test.coverage with provider 'v8' or 'istanbul', then run vitest run --coverage. Install @vitest/coverage-v8 or @vitest/coverage-istanbul and set thresholds for lines, functions, branches, and statements.

jsdom provides full browser API simulation including localStorage and complete DOM behavior, while happy-dom runs faster with fewer APIs. Set the environment in config or per file with the // @vitest-environment comment.

Yes, test files run in parallel across worker threads by default. Within a file, use test.concurrent or describe.concurrent for parallel tests, and use the context's expect for correct assertion tracking.

vi.mock calls are hoisted above imports, so regular variables are undefined at that point. Wrap shared values in vi.hoisted so they are initialized before the mock factory executes.

Filter by file path as a CLI argument, by test name with -t or --testNamePattern, or by tags with --tags. Use test.only to focus a single test during development, and vitest related to run tests importing changed files.