jest-skill

name jest-skill description Generates Jest unit and integration tests in JavaScript or TypeScript. Covers mocking, snapshots, async testing, and React component testing. Use when user mentions "Jest", "describe/it/expect", "jest.mock", "toMatchSnapshot". Triggers on: "Jest", "expect().toBe()", "jest.mock", "snapshot test", "JS test", "React test". languages ["JavaScript","TypeScript"] category unit-testing license MIT metadata {"author":"TestMu AI","version":"1.0"} Jest Testing Skill Core Patterns Basic Test describe ( 'Calculator' , () => { let calc; beforeEach ( () => { calc = new Calculator (); }); test ( 'adds two numbers' , () => { expect (calc. add ( 2 , 3 )). toBe ( 5 ); }); test ( 'throws on division by zero' , () => { expect ( () => calc. divide ( 10 , 0 )). toThrow ( 'Division by zero' ); }); }); Matchers expect (value). toBe (exact); // === strict expect (value). toEqual (object); // deep equality expect (value). toBeTruthy (); expect (value). toBeNull (); expect (value). toBeGreaterThan ( 3 ); expect (value). toBeCloseTo ( 0.3 , 5 ); expect (str). toMatch ( /regex/ ); expect (arr). toContain (item); expect (arr). toHaveLength ( 3 ); expect (obj). toHaveProperty ( 'name' ); expect (obj). toMatchObject ({ name : 'Alice' }); expect ( () => fn ()). toThrow ( CustomError ); Mocking // Mock function const mockFn = jest. fn (); mockFn. mockReturnValue ( 42 ); mockFn. mockResolvedValue ({ data : 'test' }); expect (mockFn). toHaveBeenCalledWith ( 'arg1' ); expect (mockFn). toHaveBeenCalledTimes ( 1 ); // Mock module jest. mock ( './database' ); const db = require ( './database' ); db. getUser . mockResolvedValue ({ name : 'Alice' }); // Mock with implementation jest. mock ( './api' , () => ({ fetchUsers : jest. fn (). mockResolvedValue ([{ name : 'Alice' }]), })); // Spy const spy = jest. spyOn ( console , 'log' ). mockImplementation (); expect (spy). toHaveBeenCalledWith ( 'expected' ); spy. mockRestore (); // Fake timers jest. useFakeTimers (); jest. advanceTimersByTime ( 1000 ); jest. useRealTimers (); Async Testing test ( 'fetches users' , async () => { const users = await fetchUsers (); expect (users). toHaveLength ( 3 ); }); test ( 'resolves with data' , () => { return expect ( fetchData ()). resolves . toEqual ({ data : 'value' }); }); test ( 'rejects with error' , () => { return expect ( fetchBadData ()). rejects . toThrow ( 'not found' ); }); React Component Testing (Testing Library) import { render, screen, fireEvent, waitFor } from '@testing-library/react' ; import '@testing-library/jest-dom' ; import LoginForm from './LoginForm' ; test ( 'submits login form' , async () => { const onSubmit = jest. fn (); render ( < LoginForm onSubmit = {onSubmit} /> ); fireEvent. change (screen. getByLabelText ( 'Email' ), { target : { value : 'user@test.com' }, }); fireEvent. change (screen. getByLabelText ( 'Password' ), { target : { value : 'password123' }, }); fireEvent. click (screen. getByRole ( 'button' , { name : /login/i })); await waitFor ( () => { expect (onSubmit). toHaveBeenCalledWith ({ email : 'user@test.com' , password : 'password123' , }); }); }); Snapshot Testing test ( 'renders correctly' , () => { const tree = renderer. create ( < Button label = "Click" /> ). toJSON (); expect (tree). toMatchSnapshot (); }); // Update: jest --updateSnapshot Anti-Patterns Bad Good Why expect(x === y).toBe(true) expect(x).toBe(y) Better errors No await on async Always await Swallows failures Snapshot everything Snapshot UI, assert logic Snapshot fatigue Quick Reference Task Command Run all npx jest Watch npx jest --watch Coverage npx jest --coverage Update snapshots npx jest --updateSnapshot Run file npx jest tests/calc.test.js Single test test.only('name', () => {}) Deep Patterns For production-grade patterns, see reference/playbook.md : Section What's Inside §1 Production Config Node + React configs, path aliases, coverage thresholds §2 Mocking Deep Dive Module/partial/manual mocks, spies, timers, env vars §3 Async Patterns Promises, rejections, event emitters, streams §4 test.each Array, tagged template, describe.each for table-driven tests §5 Custom Matchers toBeWithinRange, toBeValidEmail, TypeScript declarations §6 React Testing Library userEvent, hooks, context providers §7 Snapshot Testing Component, inline, property matchers §8 API Service Testing Mocked axios, CRUD patterns, error handling §9 Global Setup Multi-project config, DB setup/teardown §10 CI/CD GitHub Actions with coverage gates §11 Debugging Table 10 common problems with fixes §12 Best Practices 15-item production checklist