/jest-unit

name: jest-unit
description: Unit testing skill using Jest for TypeScript and JavaScript, covering mocking, spies, snapshots, coverage, async testing, and custom matchers.
license: MIT
metadata:
  author: thetestingacademy
  version: 1.0.0
  source: https://qaskills.sh/skills/thetestingacademy/jest-unit

Jest Unit Testing Skill

You are an expert software engineer specializing in unit testing with Jest. When the user asks you to write, review, or debug Jest unit tests, follow these detailed instructions.

Core Principles

1. **Test behavior, not implementation** -- Tests should verify what code does, not how it does it. 2. **One assertion focus per test** -- Each test should verify a single logical concept. 3. **Arrange-Act-Assert** -- Structure every test into setup, execution, and verification. 4. **Fast and isolated** -- Unit tests must run in milliseconds and have no external dependencies. 5. **Descriptive names** -- Test names should read as specifications of the code's behavior.

Project Structure

src/
  services/
    user.service.ts
    user.service.test.ts
    order.service.ts
    order.service.test.ts
  utils/
    validators.ts
    validators.test.ts
    formatters.ts
    formatters.test.ts
  models/
    user.model.ts
  __mocks__/
    axios.ts
    database.ts
  __tests__/
    integration/
      user-order.test.ts
jest.config.ts

Configuration

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

const config: Config = {
  preset: 'ts-jest',
  testEnvironment: 'node',
  roots: ['<rootDir>/src'],
  testMatch: ['**/*.test.ts', '**/*.spec.ts'],
  collectCoverageFrom: [
    'src/**/*.ts',
    '!src/**/*.d.ts',
    '!src/**/*.test.ts',
    '!src/**/index.ts',
  ],
  coverageThresholds: {
    global: {
      branches: 80,
      functions: 80,
      lines: 80,
      statements: 80,
    },
  },
  coverageReporters: ['text', 'lcov', 'json-summary'],
  moduleNameMapper: {
    '^@/(.*)$': '<rootDir>/src/$1',
  },
  setupFilesAfterSetup: ['<rootDir>/jest.setup.ts'],
  clearMocks: true,
  restoreMocks: true,
};

export default config;

Writing Tests

Basic Test Structure

// validators.ts
export function isValidEmail(email: string): boolean {
  const regex = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
  return regex.test(email);
}

export function isStrongPassword(password: string): boolean {
  return (
    password.length >= 8 &&
    /[A-Z]/.test(password) &&
    /[a-z]/.test(password) &&
    /[0-9]/.test(password) &&
    /[!@#$%^&*]/.test(password)
  );
}

// validators.test.ts
import { isValidEmail, isStrongPassword } from './validators';

describe('isValidEmail', () => {
  it('should return true for valid email addresses', () => {
    expect(isValidEmail('user@example.com')).toBe(true);
    expect(isValidEmail('first.last@domain.co.uk')).toBe(true);
    expect(isValidEmail('user+tag@example.com')).toBe(true);
  });

  it('should return false for invalid email addresses', () => {
    expect(isValidEmail('')).toBe(false);
    expect(isValidEmail('not-an-email')).toBe(false);
    expect(isValidEmail('@missing-local.com')).toBe(false);
    expect(isValidEmail('missing-at.com')).toBe(false);
    expect(isValidEmail('spaces here@bad.com')).toBe(false);
  });
});

describe('isStrongPassword', () => {
  it('should accept a strong password', () => {
    expect(isStrongPassword('SecurePass1!')).toBe(true);
  });

  it('should reject passwords shorter than 8 characters', () => {
    expect(isStrongPassword('Ab1!')).toBe(false);
  });

  it('should reject passwords without uppercase letters', () => {
    expect(isStrongPassword('lowercase1!')).toBe(false);
  });

  it('should reject passwords without lowercase letters', () => {
    expect(isStrongPassword('UPPERCASE1!')).toBe(false);
  });

  it('should reject passwords without numbers', () => {
    expect(isStrongPassword('NoNumbers!')).toBe(false);
  });

  it('should reject passwords without special characters', () => {
    expect(isStrongPassword('NoSpecial1')).toBe(false);
  });
});

Testing Classes and Services

// user.service.ts
import { UserRepository } from './user.repository';
import { EmailService } from './email.service';

export class UserService {
  constructor(
    private userRepo: UserRepository,
    private emailService: EmailService
  ) {}

  async createUser(email: string, name: string): Promise<User> {
    const existing = await this.userRepo.findByEmail(email);
    if (existing) {
      throw new Error('User already exists');
    }

    const user = await this.userRepo.create({ email, name });
    await this.emailService.sendWelcomeEmail(user.email, user.name);
    return user;
  }

  async getUser(id: string): Promise<User | null> {
    return this.userRepo.findById(id);
  }

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

// user.service.test.ts
import { UserService } from './user.service';
import { UserRepository } from './user.repository';
import { EmailService } from './email.service';

// Mock the dependencies
jest.mock('./user.repository');
jest.mock('./email.service');

describe('UserService', () => {
  let userService: UserService;
  let mockUserRepo: jest.Mocked<UserRepository>;
  let mockEmailService: jest.Mocked<EmailService>;

  beforeEach(() => {
    mockUserRepo = new UserRepository() as jest.Mocked<UserRepository>;
    mockEmailService = new EmailService() as jest.Mocked<EmailService>;
    userService = new UserService(mockUserRepo, mockEmailService);
  });

  describe('createUser', () => {
    it('should create a user and send welcome email', async () => {
      const newUser = { id: '1', email: 'new@example.com', name: 'New User' };
      mockUserRepo.findByEmail.mockResolvedValue(null);
      mockUserRepo.create.mockResolvedValue(newUser);