framework-testing-javascript

# Automated Testing in JavaScript & TypeScript: Jest & Mocha This skill establishes the engineering guidelines and standards for developing and automating tests in the JavaScript and TypeScript ecosystem, spanning batteries-included solutions such as **Jest** and modular compositions with **Mocha + Chai + Sinon.js**. --- ## 🧭 1. Framework Comparison and Selection Guidelines | Characteristic | Jest | Mocha + Chai + Sinon.js | | :--- | :--- | :--- | | **Architecture** | "All-in-one" framework (Runner + Assertions + Mocks) | Decoupled runner with a plugin ecosystem | | **Mocks & Spies** | Native (`jest.fn()`, `jest.spyOn()`, `jest.mock()`) | Via Sinon.js (`sinon.stub()`, `sinon.spy()`) | | **Assertions** | Rich native matchers (`expect(val).toBe()`) | Chai library (`expect`, `should`, `assert`) | | **Snapshot Testing** | Native support (`toMatchSnapshot()`) | Requires additional plugins | | **Typical Environment** | React, Vue, NestJS, TypeScript, Next.js | Legacy Node.js microservices, Express, pure ESM | --- ## ⚡ 2. Jest: Native Test and Mock Patterns ### 2.1 Unit Tests and Native Matchers ```typescript import { calculateOrderTotal } from './calculator'; describe('calculateOrderTotal', () => { it('should calculate the total with discount and shipping correctly', () => { const items = [{ price: 100, quantity: 2 }, { price: 50, quantity: 1 }]; const discount = 20; const shipping = 15; const result = calculateOrderTotal(items, discount, shipping); expect(result).toBe(245); // (200 + 50) - 20 + 15 }); it('should throw an error for negative quantity', () => { const invalidItems = [{ price: 100, quantity: -1 }]; expect(() => calculateOrderTotal(invalidItems, 0, 0)).toThrow('Invalid quantity'); }); }); ``` ### 2.2 Module and Asynchronous Function Mocks ```typescript import { UserService } from './user.service'; import { UserRepository } from './user.repository'; jest.mock('./user.repository'); describe('UserService', () => { let userService: UserService; let mockRepo: jest.Mocked<UserRepository>; beforeEach(() => { jest.clearAllMocks(); mockRepo = new UserRepository() as jest.Mocked<UserRepository>; userService = new UserService(mockRepo); }); it('should return the user when found in the repository', async () => { mockRepo.findById.mockResolvedValue({ id: 'u1', name: 'Alice' }); const user = await userService.getUser('u1'); expect(mockRepo.findById).toHaveBeenCalledWith('u1'); expect(user.name).toBe('Alice'); }); }); ``` --- ## ☕ 3. Mocha + Chai + Sinon.js: Modular Architecture ### 3.1 BDD Testing with Chai and a Sinon.js Sandbox ```javascript const { expect } = require('chai'); const sinon = require('sinon'); const { PaymentService } = require('../src/payment.service'); const { PaymentGateway } = require('../src/payment.gateway'); describe('PaymentService (Mocha + Chai + Sinon)', () => { let sandbox; let paymentService; let gatewayMock; beforeEach(() => { sandbox = sinon.createSandbox(); gatewayMock = new PaymentGateway(); paymentService = new PaymentService(gatewayMock); }); afterEach(() => { sandbox.restore(); // Restores all stubs and spies }); it('should process the payment and record the transaction', async () => { const stub = sandbox.stub(gatewayMock, 'charge').resolves({ status: 'SUCCESS', id: 'tx-999' }); const result = await paymentService.processPayment({ amount: 100 }); expect(stub.calledOnce).to.be.true; expect(result.status).to.equal('SUCCESS'); expect(result.id).to.equal('tx-999'); }); }); ``` --- ## 🧪 4. Fake Timers and Asynchronous Tests ### 4.1 Jest Fake Timers ```typescript jest.useFakeTimers(); it('should execute the callback after a 300ms debounce', () => { const callback = jest.fn(); const debounced = debounce(callback, 300); debounced(); expect(callback).not.toHaveBeenCalled(); jest.advanceTimersByTime(300); expect(callback).toHaveBeenCalledTimes(1); }); ```