Jest JavaScript Testing Framework

Jest is a free and open-source JavaScript testing framework maintained by the OpenJS Foundation, originally created by Christoph Nakazawa at Facebook (Meta) in 2014 to test React applications. Jest has since become one of the most popular testing frameworks for JavaScript and TypeScript, known for its zero-configuration setup, fast performance, and excellent developer experience. Key features: zero configuration: Jest works out of the box for most JavaScript and TypeScript projects without any configuration. It automatically finds test files (*.test.js, *.spec.js, __tests__/), transpiles Babel/TypeScript, and runs tests. For Create React App and Next.js, Jest is pre-configured. Snapshot testing: capture the rendered output of React components (or any serializable value) and compare it against future renders. If the output changes, the test fails, highlighting the difference. Developers can update snapshots with --updateSnapshot. Ideal for UI regression testing. Mocking: comprehensive mocking capabilities. jest.fn() creates mock functions, jest.mock() replaces entire modules with mocks, jest.spyOn() wraps existing methods to track calls. Manual mocks in __mocks__/ directories. Timer mocks (jest.useFakeTimers) for testing time-dependent code. Assertions: built-in expect with matchers including toBe, toEqual, toBeTruthy, toBeNull, toContain, toHaveLength, toMatch, toThrow, toHaveBeenCalledWith, and custom matchers via expect.extend. Code coverage: built-in coverage reports (HTML, text, lcov) showing line, branch, function, and statement coverage via Istanbul/nyc integration. Parallel test execution: Jest runs tests in parallel across worker processes for speed, with intelligent test sequencing. Watch mode: automatically re-runs affected tests when files change. Module isolation: each test file runs in its own sandbox, preventing state leakage between tests. Setup and teardown: beforeEach, afterEach, beforeAll, afterAll. JavaScript/TypeScript. MIT.