Generate Jest unit and integration tests for JavaScript

A skill for generating Jest unit and integration tests in JavaScript or TypeScript, covering mocking, snapshots, async testing, and React component testing. Core patterns cover basic describe/beforeEach/test structure with common matchers (toBe for strict equality, toEqual for deep equality, toBeTruthy, toBeGreaterThan, toBeCloseTo, toMatch, toContain, toHaveLength, toHaveProperty, toMatchObject, toThrow). Mocking covers jest.fn() with mockReturnValue/mockResolvedValue plus toHaveBeenCalledWith/toHaveBeenCalledTimes assertions, module mocking via jest.mock() (both auto-mocked and with an explicit factory implementation), jest.spyOn for spying on existing methods (with mockRestore to undo it), and fake timers via jest.useFakeTimers/advanceTimersByTime/useRealTimers. Async testing covers async/await tests, and the resolves/rejects matcher forms for asserting a promise's resolution or rejection directly. React component testing uses Testing Library (render, screen, fireEvent, waitFor) with a worked login-form example simulating field changes and a button click, then waiting for the onSubmit callback to be called with the expected payload. Snapshot testing uses toMatchSnapshot with jest --updateSnapshot to refresh snapshots. A documented anti-pattern table contrasts bad vs. good: expect(x === y).toBe(true) versus expect(x).toBe(y) for better failure messages, missing await on async assertions (silently swallows failures), and snapshotting everything versus snapshotting UI while asserting logic directly (avoiding snapshot fatigue). A quick-reference table covers running all tests, watch mode, coverage, updating snapshots, running a single file, and test.only for a single test. A deeper reference/playbook.md is pointed to for production-grade patterns across twelve sections: production config (Node/React configs, path aliases, coverage thresholds), a mocking deep dive (module/partial/manual mocks, spies, timers, env vars), async patterns (promises, rejections, event emitters, streams), table-driven testing with test.each/describe.each, custom matchers with TypeScript declarations, deeper React Testing Library patterns (userEvent, hooks, context providers), deeper snapshot testing (component/inline/property matchers), API service testing (mocked axios, CRUD, error handling), global setup for multi-project configs and DB setup/teardown, CI/CD with GitHub Actions and coverage gates, a debugging table of ten common problems with fixes, and a fifteen-item production best-practices checklist.

Use it when the user mentions Jest, describe/it/expect, jest.mock, or toMatchSnapshot, or needs unit/integration tests written for JavaScript or TypeScript code including React components.

Given code to test, it produces Jest test files using the appropriate matchers, mocks, async patterns, or React Testing Library queries for the code under test, following the documented anti-pattern avoidance and quick-reference commands.

npx jest --coverage

Uses Jest as the test runner and matcher library, @testing-library/react and @testing-library/jest-dom for React component testing, and jest.mock/jest.spyOn/fake timers for isolating dependencies.

Developers writing Jest tests - unit tests, mocked integration tests, async code, or React components - who want correct matcher choice, proper mocking patterns, and awareness of common anti-patterns like missing awaits or over-snapshotting.