js-unit-tests

Overview

The two mainstream JS/TS unit frameworks share one Jest-shaped API and split by build tool:

• Jest (jestjs.io/docs/getting-started) - Meta-built, batteries-included: expect matchers, snapshot testing, mocking (jest.fn / jest.mock / jest.spyOn), and Istanbul coverage in one package. Home turf: React (CRA / older Next.js), React Native, Node services.
• Vitest (vitest.dev/guide) - Vite-native: "Vitest reads your vite.config.* by default, so your existing Vite plugins and configuration work out-of-the-box." Jest-compatible API (expect, vi.fn, vi.mock), native ESM, in-source testing, browser mode.