Sets up and repairs JavaScript / TypeScript unit tests - a suite that cannot resolve the project's path aliases (`@lib/...` declared in jsconfig or tsconfig), a regression that shipped because the tests asserted too little, or a module dependency that needs mocking, spying, or fake timers. Jest and Vitest as co-primary frameworks: install, config (`jest.config.js` / `vite.config.ts` test block), mocking (`jest.fn`/`jest.mock`/`jest.spyOn`, `vi.fn`/`vi.mock`/`vi.spyOn`, `__mocks__/`, fake timers), coverage (Istanbul/babel vs v8 providers, `coverageThreshold` gating), watch mode, and CI (`jest --ci`, `vitest run`, JUnit XML). Use when authoring, configuring, or repairing unit tests in a JS/TS project.