javascript-testing-patterns

JavaScript Testing Patterns

Comprehensive guide for implementing robust testing strategies in JavaScript/TypeScript applications using modern testing frameworks and best practices.

When to Use This Skill

Setting up test infrastructure for new projects
Writing unit tests for functions and classes
Creating integration tests for APIs and services
Implementing end-to-end tests for user flows
Mocking external dependencies and APIs
Testing React, Vue, or other frontend components
Implementing test-driven development (TDD)
Setting up continuous testing in CI/CD pipelines

Testing Frameworks

Jest - Full-Featured Testing Framework

Setup:

// jest.config.ts import type { Config } from 'jest';

const config: Config = { preset: 'ts-jest', testEnvironment: 'node', roots: ['<rootDir>/src'], testMatch: ['/tests//.ts', '**/?(.)+(spec|test).ts'], collectCoverageFrom: [ 'src//*.ts', '!src//.d.ts', '!src/**/.interface.ts', ], coverageThreshold: { global: { branches: 80, functions: 80, lines: 80, statements: 80, }, }, setupFilesAfterEnv: ['<rootDir>/src/test/setup.ts'], };

export default config;

Vitest - Fast, Vite-Native Testing

Setup:

// vitest.config.ts import { defineConfig } from 'vitest/config';

export default defineConfig({ test: { globals: true, environment: 'node', coverage: { provider: 'v8', reporter: ['text', 'json', 'html'], exclude: ['/*.d.ts', '/*.config.ts', '/dist/'], }, setupFiles: ['./src/test/setup.ts'], }, });

Unit Testing Patterns

Pattern 1: Testing Pure Functions

// utils/calculator.ts export function add(a: number, b: number): number { return a + b; }

export function divide(a: number, b: number): number { if (b === 0) { throw new Error('Division by zero'); } return a / b; }

// utils/calculator.test.ts import { describe, it, expect } from 'vitest'; import { add, divide } from './calculator';

describe('Calculator', () => { describe('add', () => { it('should add two positive numbers', () => { expect(add(2, 3)).toBe(5); });

it('should add negative numbers', () => {
  expect(add(-2, -3)).toBe(-5);
});

it('should handle zero', () => {
  expect(add(0, 5)).toBe(5);
  expect(add(5, 0)).toBe(5);
});

});

describe('divide', () => { it('should divide two numbers', () => { expect(divide(10, 2)).toBe(5); });

it('should handle decimal results', () => {
  expect(divide(5, 2)).toBe(2.5);
});

it('should throw error when dividing by zero', () => {
  expect(() => divide(10, 0)).toThrow('Division by zero');
});

}); });

Pattern 2: Testing Classes

// services/user.service.ts export class UserService { private users: Map<string, User> = new Map();

create(user: User): User { if (this.users.has(user.id)) { throw new Error('User already exists'); } this.users.set(user.id, user); return user; }

findById(id: string): User | undefined { return this.users.get(id); }

update(id: string, updates: Partial<User>): User { const user = this.users.get(id); if (!user) { throw new Error('User not found'); } const updated = { ...user, ...updates }; this.users.set(id, updated); return updated; }

delete(id: string): boolean { return this.users.delete(id); } }

// services/user.service.test.ts import { describe, it, expect, beforeEach } from 'vitest'; import { UserService } from './user.service';

describe('UserService', () => { let service: UserService;

beforeEach(() => { service = new UserService(); });

describe('create', () => { it('should create a new user', () => { const user = { id: '1', name: 'John', email: 'john@example.com' }; const created = service.create(user);

  expect(created).toEqual(user);
  expect(service.findById('1')).toEqual(user);
});

it('should throw error if user already exists', () => {
  const user = { id: '1', name: 'John', email: 'john@example.com' };
  service.create(user);

  expect(() => service.create(user)).toThrow('User already exists');
});

});

describe('update', () => { it('should update existing user', () => { const user = { id: '1', name: 'John', email: 'john@example.com' }; service.create(user);

  const updated = service.update('1', { name: 'Jane' });

  expect(updated.name).toBe('Jane');
  expect(updated.email).toBe('john@example.com');
});

it('should throw error if user not found', () => {
  expect(() => service.update('999', { name: 'Jane' }))
    .toThrow('User not found');
});

}); });

Pattern 3: Testing Async Functions

// services/api.service.ts export class ApiService { async fetchUser(id: string): Promise<User> { const response = await fetch(https://api.example.com/users/${id}); if (!response.ok) { throw new Error('User not found'); } return response.json(); }

async createUser(user: CreateUserDTO): Promise<User> { const response = await fetch('https://api.example.com/users', { method: 'POST', headers: { 'Content-Type': 'application/json' }, body: JSON.stringify(user), }); return response.json(); } }

// services/api.service.test.ts import { describe, it, expect, vi, beforeEach } from 'vitest'; import { ApiService } from './api.service';

// Mock fetch globally global.fetch = vi.fn();

describe('ApiService', () => { let service: ApiService;

beforeEach(() => { service = new ApiService(); vi.clearAllMocks(); });

describe('fetchUser', () => { it('should fetch user successfully', async () => { const mockUser = { id: '1', name: 'John', email: 'john@example.com' };

  (fetch as any).mockResolvedValueOnce({
    ok: true,
    json: async () => mockUser,
  });

  const user = await service.fetchUser('1');

  expect(user).toEqual(mockUser);
  expect(fetch).toHaveBeenCalledWith('https://api.example.com/users/1');
});

it('should throw error if user not found', async () => {
  (fetch as any).mockResolvedValueOnce({
    ok: false,
  });

  await expect(service.fetchUser('999')).rejects.toThrow('User not found');
});

});

describe('createUser', () => { it('should create user successfully', async () => { const newUser = { name: 'John', email: 'john@example.com' }; const createdUser = { id: '1', ...newUser };

  (fetch as any).mockResolvedValueOnce({
    ok: true,
    json: async () => createdUser,
  });

  const user = await service.createUser(newUser);

  expect(user).toEqual(createdUser);
  expect(fetch).toHaveBeenCalledWith(
    'https://api.example.com/users',
    expect.objectContaining({
      method: 'POST',
      body: JSON.stringify(newUser),
    })
  );
});

}); });

Mocking Patterns

Pattern 1: Mocking Modules

// services/email.service.ts import nodemailer from 'nodemailer';

export class EmailService { private transporter = nodemailer.createTransport({ host: process.env.SMTP_HOST, port: 587, auth: { user: process.env.SMTP_USER, pass: process.env.SMTP_PASS, }, });

async sendEmail(to: string, subject: string, html: string) { await this.transporter.sendMail({ from: process.env.EMAIL_FROM, to, subject, html, }); } }

// services/email.service.test.ts import { describe, it, expect, vi, beforeEach } from 'vitest'; import { EmailService } from './email.service';

vi.mock('nodemailer', () => ({ default: { createTransport: vi.fn(() => ({ sendMail: vi.fn().mockResolvedValue({ messageId: '123' }), })), }, }));

describe('EmailService', () => { let service: EmailService;

beforeEach(() => { service = new EmailService(); });

it('should send email successfully', async () => { await service.sendEmail( 'test@example.com', 'Test Subject', '<p>Test Body</p>' );

expect(service['transporter'].sendMail).toHaveBeenCalledWith(
  expect.objectContaining({
    to: 'test@example.com',
    subject: 'Test Subject',
  })
);

}); });

Pattern 2: Dependency Injection for Testing

// services/user.service.ts export interface IUserRepository { findById(id: string): Promise<User | null>; create(user: User): Promise<User>; }

export class UserService { constructor(private userRepository: IUserRepository) {}

async getUser(id: string): Promise<User> { const user = await this.userRepository.findById(id); if (!user) { throw new Error('User not found'); } return user; }

async createUser(userData: CreateUserDTO): Promise<User> { // Business logic here const user = { id: generateId(), ...userData }; return this.userRepository.create(user); } }

// services/user.service.test.ts import { describe, it, expect, vi, beforeEach } from 'vitest'; import { UserService, IUserRepository } from './user.service';

describe('UserService', () => { let service: UserService; let mockRepository: IUserRepository;

beforeEach(() => { mockRepository = { findById: vi.fn(), create: vi.fn(), }; service = new UserService(mockRepository); });

describe('getUser', () => { it('should return user if found', async () => { const mockUser = { id: '1', name: 'John', email: 'john@example.com' }; vi.mocked(mockRepository.findById).mockResolvedValue(mockUser);

  const user = await service.getUser('1');

  expect(user).toEqual(mockUser);
  expect(mockRepository.findById).toHaveBeenCalledWith('1');
});

it('should throw error if user not found', async () => {
  vi.mocked(mockRepository.findById).mockResolvedValue(null);

  await expect(service.getUser('999')).rejects.toThrow('User not found');
});

});

describe('createUser', () => { it('should create user successfully', async () => { const userData = { name: 'John', email: 'john@example.com' }; const createdUser = { id: '1', ...userData };

  vi.mocked(mockRepository.create).mockResolvedValue(createdUser);

  const user = await service.createUser(userData);

  expect(user).toEqual(createdUser);
  expect(mockRepository.create).toHaveBeenCalled();
});

}); });

Pattern 3: Spying on Functions

// utils/logger.ts export const logger = { info: (message: string) => console.log(INFO: ${message}), error: (message: string) => console.error(ERROR: ${message}), };

// services/order.service.ts import { logger } from '../utils/logger';

export class OrderService { async processOrder(orderId: string): Promise<void> { logger.info(Processing order ${orderId}); // Process order logic logger.info(Order ${orderId} processed successfully); } }

// services/order.service.test.ts import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest'; import { OrderService } from './order.service'; import { logger } from '../utils/logger';

describe('OrderService', () => { let service: OrderService; let loggerSpy: any;

beforeEach(() => { service = new OrderService(); loggerSpy = vi.spyOn(logger, 'info'); });

afterEach(() => { loggerSpy.mockRestore(); });

it('should log order processing', async () => { await service.processOrder('123');

expect(loggerSpy).toHaveBeenCalledWith('Processing order 123');
expect(loggerSpy).toHaveBeenCalledWith('Order 123 processed successfully');
expect(loggerSpy).toHaveBeenCalledTimes(2);

}); });

Integration Testing

Pattern 1: API Integration Tests

// tests/integration/user.api.test.ts import request from 'supertest'; import { app } from '../../src/app'; import { pool } from '../../src/config/database';

describe('User API Integration Tests', () => { beforeAll(async () => { // Setup test database await pool.query('CREATE TABLE IF NOT EXISTS users (...)'); });

afterAll(async () => { // Cleanup await pool.query('DROP TABLE IF EXISTS users'); await pool.end(); });

beforeEach(async () => { // Clear data before each test await pool.query('TRUNCATE TABLE users CASCADE'); });

describe('POST /api/users', () => { it('should create a new user', async () => { const userData = { name: 'John Doe', email: 'john@example.com', password: 'password123', };

  const response = await request(app)
    .post('/api/users')
    .send(userData)
    .expect(201);

  expect(response.body).toMatchObject({
    name: userData.name,
    email: userData.email,
  });
  expect(response.body).toHaveProperty('id');
  expect(response.body).not.toHaveProperty('password');
});

it('should return 400 if email is invalid', async () => {
  const userData = {
    name: 'John Doe',
    email: 'invalid-email',
    password: 'password123',
  };

  const response = await request(app)
    .post('/api/users')
    .send(userData)
    .expect(400);

  expect(response.body).toHaveProperty('error');
});

it('should return 409 if email already exists', async () => {
  const userData = {
    name: 'John Doe',
    email: 'john@example.com',
    password: 'password123',
  };

  await request(app).post('/api/users').send(userData);

  const response = await request(app)
    .post('/api/users')
    .send(userData)
    .expect(409);

  expect(response.body.error).toContain('already exists');
});

});

describe('GET /api/users/:id', () => { it('should get user by id', async () => { const createResponse = await request(app) .post('/api/users') .send({ name: 'John Doe', email: 'john@example.com', password: 'password123', });

  const userId = createResponse.body.id;

  const response = await request(app)
    .get(`/api/users/${userId}`)
    .expect(200);

  expect(response.body).toMatchObject({
    id: userId,
    name: 'John Doe',
    email: 'john@example.com',
  });
});

it('should return 404 if user not found', async () => {
  await request(app)
    .get('/api/users/999')
    .expect(404);
});

});

describe('Authentication', () => { it('should require authentication for protected routes', async () => { await request(app) .get('/api/users/me') .expect(401); });

it('should allow access with valid token', async () => {
  // Create user and login
  await request(app)
    .post('/api/users')
    .send({
      name: 'John Doe',
      email: 'john@example.com',
      password: 'password123',
    });

  const loginResponse = await request(app)
    .post('/api/auth/login')
    .send({
      email: 'john@example.com',
      password: 'password123',
    });

  const token = loginResponse.body.token;

  const response = await request(app)
    .get('/api/users/me')
    .set('Authorization', `Bearer ${token}`)
    .expect(200);

  expect(response.body.email).toBe('john@example.com');
});

}); });

Pattern 2: Database Integration Tests

// tests/integration/user.repository.test.ts import { describe, it, expect, beforeAll, afterAll, beforeEach } from 'vitest'; import { Pool } from 'pg'; import { UserRepository } from '../../src/repositories/user.repository';

describe('UserRepository Integration Tests', () => { let pool: Pool; let repository: UserRepository;

beforeAll(async () => { pool = new Pool({ host: 'localhost', port: 5432, database: 'test_db', user: 'test_user', password: 'test_password', });

repository = new UserRepository(pool);

// Create tables
await pool.query(`
  CREATE TABLE IF NOT EXISTS users (
    id SERIAL PRIMARY KEY,
    name VARCHAR(255) NOT NULL,
    email VARCHAR(255) UNIQUE NOT NULL,
    password VARCHAR(255) NOT NULL,
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
  )
`);

});

afterAll(async () => { await pool.query('DROP TABLE IF EXISTS users'); await pool.end(); });

beforeEach(async () => { await pool.query('TRUNCATE TABLE users CASCADE'); });

it('should create a user', async () => { const user = await repository.create({ name: 'John Doe', email: 'john@example.com', password: 'hashed_password', });

expect(user).toHaveProperty('id');
expect(user.name).toBe('John Doe');
expect(user.email).toBe('john@example.com');

});

it('should find user by email', async () => { await repository.create({ name: 'John Doe', email: 'john@example.com', password: 'hashed_password', });

const user = await repository.findByEmail('john@example.com');

expect(user).toBeTruthy();
expect(user?.name).toBe('John Doe');

});

it('should return null if user not found', async () => { const user = await repository.findByEmail('nonexistent@example.com'); expect(user).toBeNull(); }); });

Frontend Testing with Testing Library

Pattern 1: React Component Testing

// components/UserForm.tsx import { useState } from 'react';

interface Props { onSubmit: (user: { name: string; email: string }) => void; }

export function UserForm({ onSubmit }: Props) { const [name, setName] = useState(''); const [email, setEmail] = useState('');

const handleSubmit = (e: React.FormEvent) => { e.preventDefault(); onSubmit({ name, email }); };

return ( <form onSubmit={handleSubmit}> <input type="text" placeholder="Name" value={name} onChange={(e) => setName(e.target.value)} data-testid="name-input" /> <input type="email" placeholder="Email" value={email} onChange={(e) => setEmail(e.target.value)} data-testid="email-input" /> <button type="submit">Submit</button> </form> ); }

// components/UserForm.test.tsx import { render, screen, fireEvent } from '@testing-library/react'; import { describe, it, expect, vi } from 'vitest'; import { UserForm } from './UserForm';

describe('UserForm', () => { it('should render form inputs', () => { render(<UserForm onSubmit={vi.fn()} />);

expect(screen.getByPlaceholderText('Name')).toBeInTheDocument();
expect(screen.getByPlaceholderText('Email')).toBeInTheDocument();
expect(screen.getByRole('button', { name: 'Submit' })).toBeInTheDocument();

});

it('should update input values', () => { render(<UserForm onSubmit={vi.fn()} />);

const nameInput = screen.getByTestId('name-input') as HTMLInputElement;
const emailInput = screen.getByTestId('email-input') as HTMLInputElement;

fireEvent.change(nameInput, { target: { value: 'John Doe' } });
fireEvent.change(emailInput, { target: { value: 'john@example.com' } });

expect(nameInput.value).toBe('John Doe');
expect(emailInput.value).toBe('john@example.com');

});

it('should call onSubmit with form data', () => { const onSubmit = vi.fn(); render(<UserForm onSubmit={onSubmit} />);

fireEvent.change(screen.getByTestId('name-input'), {
  target: { value: 'John Doe' },
});
fireEvent.change(screen.getByTestId('email-input'), {
  target: { value: 'john@example.com' },
});
fireEvent.click(screen.getByRole('button', { name: 'Submit' }));

expect(onSubmit).toHaveBeenCalledWith({
  name: 'John Doe',
  email: 'john@example.com',
});

}); });

Pattern 2: Testing Hooks

// hooks/useCounter.ts import { useState, useCallback } from 'react';

export function useCounter(initialValue = 0) { const [count, setCount] = useState(initialValue);

const increment = useCallback(() => setCount((c) => c + 1), []); const decrement = useCallback(() => setCount((c) => c - 1), []); const reset = useCallback(() => setCount(initialValue), [initialValue]);

return { count, increment, decrement, reset }; }

// hooks/useCounter.test.ts import { renderHook, act } from '@testing-library/react'; import { describe, it, expect } from 'vitest'; import { useCounter } from './useCounter';

describe('useCounter', () => { it('should initialize with default value', () => { const { result } = renderHook(() => useCounter()); expect(result.current.count).toBe(0); });

it('should initialize with custom value', () => { const { result } = renderHook(() => useCounter(10)); expect(result.current.count).toBe(10); });

it('should increment count', () => { const { result } = renderHook(() => useCounter());

act(() => {
  result.current.increment();
});

expect(result.current.count).toBe(1);

});

it('should decrement count', () => { const { result } = renderHook(() => useCounter(5));

act(() => {
  result.current.decrement();
});

expect(result.current.count).toBe(4);

});

it('should reset to initial value', () => { const { result } = renderHook(() => useCounter(10));

act(() => {
  result.current.increment();
  result.current.increment();
});

expect(result.current.count).toBe(12);

act(() => {
  result.current.reset();
});

expect(result.current.count).toBe(10);

}); });

Test Fixtures and Factories

// tests/fixtures/user.fixture.ts import { faker } from '@faker-js/faker';

export function createUserFixture(overrides?: Partial<User>): User { return { id: faker.string.uuid(), name: faker.person.fullName(), email: faker.internet.email(), createdAt: faker.date.past(), ...overrides, }; }

export function createUsersFixture(count: number): User[] { return Array.from({ length: count }, () => createUserFixture()); }

// Usage in tests import { createUserFixture, createUsersFixture } from '../fixtures/user.fixture';

describe('UserService', () => { it('should process user', () => { const user = createUserFixture({ name: 'John Doe' }); // Use user in test });

it('should handle multiple users', () => { const users = createUsersFixture(10); // Use users in test }); });

Snapshot Testing

// components/UserCard.test.tsx import { render } from '@testing-library/react'; import { describe, it, expect } from 'vitest'; import { UserCard } from './UserCard';

describe('UserCard', () => { it('should match snapshot', () => { const user = { id: '1', name: 'John Doe', email: 'john@example.com', avatar: 'https://example.com/avatar.jpg', };

const { container } = render(&#x3C;UserCard user={user} />);

expect(container.firstChild).toMatchSnapshot();

});

it('should match snapshot with loading state', () => { const { container } = render(<UserCard loading />); expect(container.firstChild).toMatchSnapshot(); }); });

Coverage Reports

// package.json { "scripts": { "test": "vitest", "test:coverage": "vitest --coverage", "test:ui": "vitest --ui" } }

Best Practices

Follow AAA Pattern: Arrange, Act, Assert
One assertion per test: Or logically related assertions
Descriptive test names: Should describe what is being tested
Use beforeEach/afterEach: For setup and teardown
Mock external dependencies: Keep tests isolated
Test edge cases: Not just happy paths
Avoid implementation details: Test behavior, not implementation
Use test factories: For consistent test data
Keep tests fast: Mock slow operations
Write tests first (TDD): When possible
Maintain test coverage: Aim for 80%+ coverage
Use TypeScript: For type-safe tests
Test error handling: Not just success cases
Use data-testid sparingly: Prefer semantic queries
Clean up after tests: Prevent test pollution

Common Patterns

Test Organization

describe('UserService', () => { describe('createUser', () => { it('should create user successfully', () => {}); it('should throw error if email exists', () => {}); it('should hash password', () => {}); });

describe('updateUser', () => { it('should update user', () => {}); it('should throw error if not found', () => {}); }); });

Testing Promises

// Using async/await it('should fetch user', async () => { const user = await service.fetchUser('1'); expect(user).toBeDefined(); });

// Testing rejections it('should throw error', async () => { await expect(service.fetchUser('invalid')).rejects.toThrow('Not found'); });

Testing Timers

import { vi } from 'vitest';

it('should call function after delay', () => { vi.useFakeTimers();

const callback = vi.fn(); setTimeout(callback, 1000);

expect(callback).not.toHaveBeenCalled();

vi.advanceTimersByTime(1000);

expect(callback).toHaveBeenCalled();

vi.useRealTimers(); });

Resources

Jest Documentation: https://jestjs.io/
Vitest Documentation: https://vitest.dev/
Testing Library: https://testing-library.com/
Kent C. Dodds Testing Blog: https://kentcdodds.com/blog/

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