Jest

Jest is a JavaScript testing framework built by Meta (formerly Facebook) that runs unit and integration tests for JavaScript/TypeScript code in Node.js and browser environments. It's the zero-config default: npm test works without any setup, comes with an assertion library (expect), mocking utilities (jest.mock, jest.fn), and coverage tools built-in. Jest's parallel test execution and intelligent caching make it blazingly fast, 500 tests in 3-5 seconds is standard. In 2026, Jest dominates React, Vue, and full-stack Node.js projects. It's the de-facto standard for frontend engineers and recommended by every major framework (Next.js, Create React App, Remix). Alternatives like Vitest are rising for Vite projects, but Jest's ecosystem maturity and "works out of the box" philosophy remain unmatched for large codebases with legacy tests.