# Chavangorakh1999 Sde Skills Unit Testing > Unit Testing with Jest - Skill: `tomevault-io/chavangorakh1999-sde-skills-unit-testing` (Agent Skill, multi-file: 2 files) - Install (CLI): `npx skillmds@latest add tomevault-io/chavangorakh1999-sde-skills-unit-testing` - Raw SKILL.md: https://api.skillmd.com/api/skills/tomevault-io/chavangorakh1999-sde-skills-unit-testing/raw - Safety review: pending (external: skill-scanner PASS, skillspector PASS) - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - Author: tomevault-io (https://skillmd.com/u/tomevault-io) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/tomevault-io/chavangorakh1999-sde-skills-unit-testing --- ## Unit Testing with Jest ### Context Unit to test or testing problem: **$ARGUMENTS** --- ### Test Anatomy (AAA Pattern) ```javascript // Arrange → Act → Assert describe('userService.calculateTier', () => { it('returns gold tier for spend >= 1000', () => { // Arrange const user = { totalSpend: 1500, joinDate: new Date('2023-01-01') }; // Act const tier = userService.calculateTier(user); // Assert expect(tier).toBe('gold'); }); it('returns silver tier for spend between 500 and 999', () => { const user = { totalSpend: 750, joinDate: new Date('2023-01-01') }; expect(userService.calculateTier(user)).toBe('silver'); }); it('returns bronze tier for spend < 500', () => { const user = { totalSpend: 100, joinDate: new Date() }; expect(userService.calculateTier(user)).toBe('bronze'); }); }); ``` --- ### Describe Block Organization ```javascript // Group by function, then by scenario describe('CartService', () => { describe('addItem', () => { it('adds new item to empty cart'); it('increments quantity when item already in cart'); it('throws when quantity exceeds stock'); }); describe('removeItem', () => { it('removes item from cart'); it('throws NotFoundError when item not in cart'); }); describe('calculateTotal', () => { it('sums all items'); it('applies discount code correctly'); it('returns 0 for empty cart'); }); }); ``` --- ### Mocking Modules ```javascript // jest.mock — replace entire module jest.mock('../repositories/userRepository.js'); import { userRepository } from '../repositories/userRepository.js'; // After jest.mock(), all exported functions become jest.fn() // userRepository.findById is now a jest.fn() describe('UserService.getById', () => { beforeEach(() => { jest.clearAllMocks(); // reset call counts and implementations between tests }); it('returns mapped user when found', async () => { const mockUser = { _id: '123', email: 'alice@example.com', role: 'user' }; userRepository.findById.mockResolvedValue(mockUser); const result = await userService.getById('123'); expect(userRepository.findById).toHaveBeenCalledWith('123'); expect(result).toEqual({ id: '123', email: 'alice@example.com', role: 'user' }); }); it('throws NotFoundError when repository returns null', async () => { userRepository.findById.mockResolvedValue(null); await expect(userService.getById('123')) .rejects.toMatchObject({ code: 'NOT_FOUND' }); }); it('propagates unexpected repository errors', async () => { userRepository.findById.mockRejectedValue(new Error('connection lost')); await expect(userService.getById('123')) .rejects.toThrow('connection lost'); }); }); ``` --- ### Spying (Partial Mocks) ```javascript // jest.spyOn — mock one method, keep rest real import { emailService } from '../services/emailService.js'; describe('UserService.register', () => { it('sends welcome email after successful registration', async () => { // Spy on just sendWelcome, don't mock the whole module const spy = jest.spyOn(emailService, 'sendWelcome') .mockResolvedValue({ messageId: 'abc' }); await userService.register({ email: 'alice@example.com', password: 'Test1234!' }); expect(spy).toHaveBeenCalledWith( expect.objectContaining({ email: 'alice@example.com' }) ); spy.mockRestore(); // restore original implementation }); }); ``` --- ### Async Testing ```javascript // Always use async/await for async tests it('handles concurrent requests correctly', async () => { // Run assertions after all promises resolve const [result1, result2] = await Promise.all([ userService.getById('1'), userService.getById('2'), ]); expect(result1.id).toBe('1'); expect(result2.id).toBe('2'); }); // Testing rejection — two patterns: it('throws on invalid input', async () => { // Pattern 1: rejects.toThrow await expect(userService.getById('')).rejects.toThrow('ID is required'); // Pattern 2: try/catch with assertions try { await userService.getById(''); fail('Expected error was not thrown'); } catch (err) { expect(err.message).toBe('ID is required'); } }); // Fake timers for setTimeout/setInterval/Date it('expires cache after TTL', async () => { jest.useFakeTimers(); const cache = new Cache({ ttl: 60_000 }); cache.set('key', 'value'); jest.advanceTimersByTime(61_000); expect(cache.get('key')).toBeNull(); jest.useRealTimers(); }); ``` --- ### Parameterized Tests ```javascript // test.each — avoid duplicating test structure describe('validateEmail', () => { test.each([ ['alice@example.com', true], ['alice+tag@example.co.uk', true], ['not-an-email', false], ['@nodomain.com', false], ['', false], ])('validateEmail(%s) returns %s', (email, expected) => { expect(validateEmail(email)).toBe(expected); }); }); // Object form for readability test.each([ { spend: 1500, expected: 'gold' }, { spend: 750, expected: 'silver' }, { spend: 100, expected: 'bronze' }, ])('tier calculation: spend=$spend → $expected', ({ spend, expected }) => { expect(userService.calculateTier({ totalSpend: spend })).toBe(expected); }); ``` --- ### Coverage Configuration ```javascript // jest.config.js export default { collectCoverageFrom: [ 'src/**/*.js', '!src/**/*.test.js', '!src/config/**', // don't cover config files '!src/migrations/**', // don't cover migrations ], coverageThresholds: { global: { lines: 80, branches: 75, functions: 85, statements: 80, }, // Higher bar for critical modules: './src/services/': { lines: 90, }, }, coverageReporters: ['text', 'html', 'lcov'], // lcov for CI integration }; ``` --- ### Test Quality Anti-Patterns ```javascript // ANTI-PATTERN: testing implementation, not behavior it('sets this._cache[key] in the internal map', () => { cache.set('key', 'val'); expect(cache._cache.has('key')).toBe(true); // testing internals }); // Better: it('returns value after setting', () => { cache.set('key', 'val'); expect(cache.get('key')).toBe('val'); }); // ANTI-PATTERN: meaningless assertions expect(result).toBeDefined(); // truthy check adds no value expect(result).not.toBeNull(); // same // Better: assert the actual shape expect(result).toEqual({ id: '123', email: 'alice@example.com' }); // ANTI-PATTERN: test titles that just repeat the assertion it('returns true'); // useless // Better: it('returns true when user has verified email and active subscription'); ``` --- > Source: [chavangorakh1999/sde-skills](https://github.com/chavangorakh1999/sde-skills) — distributed by [TomeVault](https://tomevault.io).