🧪 Testing Fundamentals: Why Test TypeScript

🎯 Introduction

Welcome to the quality assurance headquarters of TypeScript development! 🧪 If building TypeScript applications were like constructing a skyscraper, then testing would be like having a team of expert engineers constantly checking every beam, bolt, and foundation to ensure the building won’t collapse when people actually start using it - except in our case, we’re making sure our code won’t crash when users start clicking buttons and entering data!

Testing TypeScript applications combines the benefits of static type checking with dynamic runtime verification, creating a powerful safety net that catches bugs before they reach production. While TypeScript’s type system prevents many errors at compile time, testing ensures your application behaves correctly with real data, user interactions, and edge cases.

By the end of this tutorial, you’ll understand why testing TypeScript is both easier and more powerful than testing plain JavaScript, and you’ll have the knowledge to build comprehensive test strategies that leverage TypeScript’s strengths while covering all the scenarios your type system can’t catch. Let’s dive into the world of bulletproof TypeScript testing! 🌟

📚 Understanding Testing in TypeScript Context

🤔 Why Test TypeScript Applications?

TypeScript provides excellent compile-time safety, but testing ensures runtime correctness and catches issues that the type system cannot prevent.

// 🌟 TypeScript helps catch many errors, but not everything

interface User {
  id: string;
  email: string;
  age: number;
  preferences: UserPreferences;
}

interface UserPreferences {
  theme: 'light' | 'dark';
  notifications: boolean;
  language: string;
}

class UserService {
  private users: Map<string, User> = new Map();
  
  // ✅ TypeScript ensures type safety at compile time
  createUser(userData: Omit<User, 'id'>): User {
    const user: User = {
      id: this.generateId(),
      ...userData
    };
    
    this.users.set(user.id, user);
    return user;
  }
  
  // ✅ TypeScript prevents passing wrong types
  updateUser(id: string, updates: Partial<User>): User | null {
    const user = this.users.get(id);
    if (!user) return null;
    
    const updatedUser = { ...user, ...updates };
    this.users.set(id, updatedUser);
    return updatedUser;
  }
  
  // 🚨 But TypeScript can't catch these runtime issues:
  
  // Issue 1: Business logic errors
  validateAge(age: number): boolean {
    // Bug: Should be >= 13, but written as > 13
    return age > 13; // This compiles fine but has wrong logic!
  }
  
  // Issue 2: External API inconsistencies
  async fetchUserFromAPI(id: string): Promise<User> {
    const response = await fetch(`/api/users/${id}`);
    const data = await response.json();
    
    // TypeScript trusts our assertion, but API might return different shape
    return data as User; // Runtime error if API changes!
  }
  
  // Issue 3: Complex business rules
  canUserAccessFeature(user: User, feature: string): boolean {
    // Complex business logic that could have edge cases
    if (feature === 'advanced_analytics') {
      return user.age >= 18 && user.preferences.notifications;
    }
    
    // What if feature is undefined? Empty string? Special characters?
    return true; // Might not handle all cases correctly
  }
  
  // Issue 4: Asynchronous operations
  async processUserBatch(userIds: string[]): Promise<void> {
    // Could fail with large batches, network issues, etc.
    await Promise.all(
      userIds.map(id => this.processUser(id))
    );
  }
  
  private generateId(): string {
    return Math.random().toString(36).substr(2, 9);
  }
  
  private async processUser(id: string): Promise<void> {
    // Simulate processing
    await new Promise(resolve => setTimeout(resolve, 100));
  }
}

// 🎯 Testing catches what TypeScript cannot:

describe('UserService', () => {
  let userService: UserService;
  
  beforeEach(() => {
    userService = new UserService();
  });
  
  // Test business logic correctness
  describe('validateAge', () => {
    it('should allow users who are exactly 13', () => {
      // This test would catch the >= vs > bug!
      expect(userService.validateAge(13)).toBe(true);
    });
    
    it('should reject users under 13', () => {
      expect(userService.validateAge(12)).toBe(false);
    });
  });
  
  // Test edge cases and error conditions
  describe('updateUser', () => {
    it('should handle non-existent user gracefully', () => {
      const result = userService.updateUser('non-existent', { age: 25 });
      expect(result).toBeNull();
    });
    
    it('should handle partial updates correctly', () => {
      const user = userService.createUser({
        email: '[email protected]',
        age: 25,
        preferences: { theme: 'light', notifications: true, language: 'en' }
      });
      
      const updated = userService.updateUser(user.id, { age: 30 });
      
      expect(updated?.age).toBe(30);
      expect(updated?.email).toBe('[email protected]'); // Unchanged
    });
  });
  
  // Test complex business rules
  describe('canUserAccessFeature', () => {
    it('should handle advanced analytics access correctly', () => {
      const adultUser: Omit<User, 'id'> = {
        email: '[email protected]',
        age: 25,
        preferences: { theme: 'light', notifications: true, language: 'en' }
      };
      
      const user = userService.createUser(adultUser);
      expect(userService.canUserAccessFeature(user, 'advanced_analytics')).toBe(true);
    });
    
    it('should deny access for minors', () => {
      const minorUser: Omit<User, 'id'> = {
        email: '[email protected]',
        age: 16,
        preferences: { theme: 'light', notifications: true, language: 'en' }
      };
      
      const user = userService.createUser(minorUser);
      expect(userService.canUserAccessFeature(user, 'advanced_analytics')).toBe(false);
    });
    
    it('should handle edge cases in feature names', () => {
      const user = userService.createUser({
        email: '[email protected]',
        age: 25,
        preferences: { theme: 'light', notifications: true, language: 'en' }
      });
      
      // Test edge cases that TypeScript can't catch
      expect(userService.canUserAccessFeature(user, '')).toBe(true);
      expect(userService.canUserAccessFeature(user, 'unknown-feature')).toBe(true);
    });
  });
  
  // Test asynchronous operations
  describe('processUserBatch', () => {
    it('should handle empty batch', async () => {
      await expect(userService.processUserBatch([])).resolves.toBeUndefined();
    });
    
    it('should handle single user', async () => {
      const user = userService.createUser({
        email: '[email protected]',
        age: 25,
        preferences: { theme: 'light', notifications: true, language: 'en' }
      });
      
      await expect(userService.processUserBatch([user.id])).resolves.toBeUndefined();
    });
    
    it('should handle large batches', async () => {
      const users = Array.from({ length: 100 }, () => 
        userService.createUser({
          email: '[email protected]',
          age: 25,
          preferences: { theme: 'light', notifications: true, language: 'en' }
        })
      );
      
      const userIds = users.map(u => u.id);
      await expect(userService.processUserBatch(userIds)).resolves.toBeUndefined();
    }, 15000); // Longer timeout for large batch
  });
});

// 📊 Example: Testing API integration with proper error handling
class APIClient {
  constructor(private baseUrl: string) {}
  
  // TypeScript ensures we return the right type, but doesn't guarantee the API does
  async fetchUser(id: string): Promise<User> {
    const response = await fetch(`${this.baseUrl}/users/${id}`);
    
    if (!response.ok) {
      throw new Error(`Failed to fetch user: ${response.statusText}`);
    }
    
    const data = await response.json();
    
    // Type assertion - TypeScript trusts us, but testing verifies
    return this.validateUserData(data);
  }
  
  private validateUserData(data: any): User {
    // Runtime validation that complements TypeScript's compile-time checks
    if (!data || typeof data !== 'object') {
      throw new Error('Invalid user data: not an object');
    }
    
    if (typeof data.id !== 'string' || !data.id) {
      throw new Error('Invalid user data: missing or invalid id');
    }
    
    if (typeof data.email !== 'string' || !data.email.includes('@')) {
      throw new Error('Invalid user data: missing or invalid email');
    }
    
    if (typeof data.age !== 'number' || data.age < 0) {
      throw new Error('Invalid user data: missing or invalid age');
    }
    
    if (!data.preferences || typeof data.preferences !== 'object') {
      throw new Error('Invalid user data: missing or invalid preferences');
    }
    
    return data as User;
  }
}

// Testing API integration
describe('APIClient', () => {
  let apiClient: APIClient;
  
  beforeEach(() => {
    apiClient = new APIClient('https://api.example.com');
  });
  
  afterEach(() => {
    // Clean up any mocks
    jest.restoreAllMocks();
  });
  
  describe('fetchUser', () => {
    it('should fetch and validate user data correctly', async () => {
      const mockUser = {
        id: '123',
        email: '[email protected]',
        age: 25,
        preferences: {
          theme: 'light',
          notifications: true,
          language: 'en'
        }
      };
      
      // Mock the fetch function
      global.fetch = jest.fn().mockResolvedValue({
        ok: true,
        json: jest.fn().mockResolvedValue(mockUser)
      });
      
      const user = await apiClient.fetchUser('123');
      
      expect(user).toEqual(mockUser);
      expect(fetch).toHaveBeenCalledWith('https://api.example.com/users/123');
    });
    
    it('should handle API errors gracefully', async () => {
      global.fetch = jest.fn().mockResolvedValue({
        ok: false,
        statusText: 'Not Found'
      });
      
      await expect(apiClient.fetchUser('123')).rejects.toThrow('Failed to fetch user: Not Found');
    });
    
    it('should validate API response structure', async () => {
      // Test malformed API response
      global.fetch = jest.fn().mockResolvedValue({
        ok: true,
        json: jest.fn().mockResolvedValue({
          id: '123',
          // Missing email, age, preferences
        })
      });
      
      await expect(apiClient.fetchUser('123')).rejects.toThrow('Invalid user data');
    });
    
    it('should handle network errors', async () => {
      global.fetch = jest.fn().mockRejectedValue(new Error('Network error'));
      
      await expect(apiClient.fetchUser('123')).rejects.toThrow('Network error');
    });
  });
});

🏗️ Benefits of Testing TypeScript vs JavaScript

// 🚀 Testing TypeScript applications has unique advantages

// 1. **Better Test Documentation**: Types serve as documentation
interface PaymentProcessor {
  processPayment(amount: number, currency: string): Promise<PaymentResult>;
  refundPayment(transactionId: string): Promise<RefundResult>;
  getTransactionHistory(userId: string, limit?: number): Promise<Transaction[]>;
}

interface PaymentResult {
  success: boolean;
  transactionId: string;
  message: string;
  fees?: number;
}

interface RefundResult {
  success: boolean;
  refundId: string;
  originalTransactionId: string;
  amount: number;
  message: string;
}

interface Transaction {
  id: string;
  amount: number;
  currency: string;
  status: 'pending' | 'completed' | 'failed' | 'refunded';
  createdAt: Date;
  updatedAt: Date;
}

// The interface itself documents what our tests should verify!
class StripePaymentProcessor implements PaymentProcessor {
  constructor(private apiKey: string) {}
  
  async processPayment(amount: number, currency: string): Promise<PaymentResult> {
    // Implementation details...
    return {
      success: true,
      transactionId: 'txn_123',
      message: 'Payment processed successfully'
    };
  }
  
  async refundPayment(transactionId: string): Promise<RefundResult> {
    // Implementation details...
    return {
      success: true,
      refundId: 'ref_456',
      originalTransactionId: transactionId,
      amount: 100,
      message: 'Refund processed successfully'
    };
  }
  
  async getTransactionHistory(userId: string, limit: number = 10): Promise<Transaction[]> {
    // Implementation details...
    return [];
  }
}

// 2. **Type-Guided Test Creation**: Types guide what to test
describe('StripePaymentProcessor', () => {
  let processor: PaymentProcessor; // Using interface ensures we test the contract
  
  beforeEach(() => {
    processor = new StripePaymentProcessor('test-api-key');
  });
  
  // TypeScript helps us test the exact interface contract
  describe('processPayment', () => {
    it('should return PaymentResult with required fields', async () => {
      const result = await processor.processPayment(100, 'USD');
      
      // TypeScript ensures we check all required fields
      expect(result.success).toBeDefined();
      expect(result.transactionId).toBeDefined();
      expect(result.message).toBeDefined();
      
      // And types help us know what to expect
      expect(typeof result.success).toBe('boolean');
      expect(typeof result.transactionId).toBe('string');
      expect(typeof result.message).toBe('string');
      
      // Optional fields need special handling
      if (result.fees !== undefined) {
        expect(typeof result.fees).toBe('number');
      }
    });
    
    it('should handle invalid amounts', async () => {
      // TypeScript prevents us from passing wrong types accidentally
      await expect(processor.processPayment(-100, 'USD')).rejects.toThrow();
      await expect(processor.processPayment(0, 'USD')).rejects.toThrow();
    });
  });
});

// 3. **Compile-Time Test Validation**: Tests are checked at compile time
describe('Type-Safe Test Helpers', () => {
  // Helper function with TypeScript benefits
  function createMockUser(overrides: Partial<User> = {}): User {
    const defaultUser: User = {
      id: 'test-id',
      email: '[email protected]',
      age: 25,
      preferences: {
        theme: 'light',
        notifications: true,
        language: 'en'
      }
    };
    
    // TypeScript ensures overrides match User interface
    return { ...defaultUser, ...overrides };
  }
  
  it('should create mock users correctly', () => {
    // TypeScript catches typos in property names
    const user = createMockUser({
      age: 30,
      preferences: {
        theme: 'dark',
        notifications: false,
        language: 'fr'
      }
    });
    
    expect(user.age).toBe(30);
    expect(user.preferences.theme).toBe('dark');
  });
  
  it('should prevent invalid mock data', () => {
    // This would cause a TypeScript error:
    // const user = createMockUser({
    //   age: 'thirty', // Error: Type 'string' is not assignable to type 'number'
    //   invalidField: 'value' // Error: Object literal may only specify known properties
    // });
  });
});

// 4. **Better Refactoring Safety**: Tests help with safe refactoring
interface EmailService {
  sendEmail(to: string, subject: string, body: string): Promise<void>;
  sendBulkEmail(recipients: string[], subject: string, body: string): Promise<void>;
}

// Original implementation
class SimpleEmailService implements EmailService {
  async sendEmail(to: string, subject: string, body: string): Promise<void> {
    console.log(`Sending email to ${to}: ${subject}`);
  }
  
  async sendBulkEmail(recipients: string[], subject: string, body: string): Promise<void> {
    for (const recipient of recipients) {
      await this.sendEmail(recipient, subject, body);
    }
  }
}

// Enhanced implementation with more features
interface EnhancedEmailService extends EmailService {
  sendEmailWithAttachments(
    to: string, 
    subject: string, 
    body: string, 
    attachments: Attachment[]
  ): Promise<void>;
  
  sendTemplatedEmail(
    to: string, 
    templateId: string, 
    variables: Record<string, any>
  ): Promise<void>;
}

interface Attachment {
  filename: string;
  content: Buffer | string;
  contentType: string;
}

class AdvancedEmailService implements EnhancedEmailService {
  async sendEmail(to: string, subject: string, body: string): Promise<void> {
    // Enhanced implementation
    return this.sendEmailWithAttachments(to, subject, body, []);
  }
  
  async sendBulkEmail(recipients: string[], subject: string, body: string): Promise<void> {
    // Parallel processing instead of sequential
    await Promise.all(
      recipients.map(recipient => this.sendEmail(recipient, subject, body))
    );
  }
  
  async sendEmailWithAttachments(
    to: string, 
    subject: string, 
    body: string, 
    attachments: Attachment[]
  ): Promise<void> {
    console.log(`Sending email with ${attachments.length} attachments to ${to}: ${subject}`);
  }
  
  async sendTemplatedEmail(
    to: string, 
    templateId: string, 
    variables: Record<string, any>
  ): Promise<void> {
    console.log(`Sending templated email (${templateId}) to ${to}`);
  }
}

// Tests ensure both implementations work correctly
describe('Email Service Implementations', () => {
  // Test suite that works for any EmailService implementation
  function testEmailService(createService: () => EmailService, serviceName: string) {
    describe(serviceName, () => {
      let emailService: EmailService;
      
      beforeEach(() => {
        emailService = createService();
      });
      
      it('should send single email', async () => {
        await expect(
          emailService.sendEmail('[email protected]', 'Test Subject', 'Test Body')
        ).resolves.toBeUndefined();
      });
      
      it('should send bulk email', async () => {
        const recipients = ['[email protected]', '[email protected]'];
        await expect(
          emailService.sendBulkEmail(recipients, 'Bulk Subject', 'Bulk Body')
        ).resolves.toBeUndefined();
      });
      
      it('should handle empty recipient list', async () => {
        await expect(
          emailService.sendBulkEmail([], 'Subject', 'Body')
        ).resolves.toBeUndefined();
      });
    });
  }
  
  // Test both implementations with the same test suite
  testEmailService(() => new SimpleEmailService(), 'SimpleEmailService');
  testEmailService(() => new AdvancedEmailService(), 'AdvancedEmailService');
  
  // Additional tests for enhanced features
  describe('AdvancedEmailService specific features', () => {
    let advancedService: AdvancedEmailService;
    
    beforeEach(() => {
      advancedService = new AdvancedEmailService();
    });
    
    it('should send email with attachments', async () => {
      const attachments: Attachment[] = [
        {
          filename: 'test.pdf',
          content: Buffer.from('PDF content'),
          contentType: 'application/pdf'
        }
      ];
      
      await expect(
        advancedService.sendEmailWithAttachments(
          '[email protected]',
          'Subject',
          'Body',
          attachments
        )
      ).resolves.toBeUndefined();
    });
    
    it('should send templated email', async () => {
      const variables = {
        name: 'John Doe',
        product: 'TypeScript Course'
      };
      
      await expect(
        advancedService.sendTemplatedEmail(
          '[email protected]',
          'welcome-template',
          variables
        )
      ).resolves.toBeUndefined();
    });
  });
});

// 5. **Enhanced Error Detection**: Catch more errors at test time
class MathUtilities {
  // Method with complex logic that needs thorough testing
  static calculateCompoundInterest(
    principal: number,
    rate: number,
    compoundFrequency: number,
    years: number
  ): number {
    if (principal <= 0) throw new Error('Principal must be positive');
    if (rate < 0) throw new Error('Rate cannot be negative');
    if (compoundFrequency <= 0) throw new Error('Compound frequency must be positive');
    if (years < 0) throw new Error('Years cannot be negative');
    
    // Formula: A = P(1 + r/n)^(nt)
    return principal * Math.pow(1 + rate / compoundFrequency, compoundFrequency * years);
  }
  
  static formatCurrency(amount: number, currency: string = 'USD'): string {
    if (typeof amount !== 'number' || isNaN(amount)) {
      throw new Error('Amount must be a valid number');
    }
    
    return new Intl.NumberFormat('en-US', {
      style: 'currency',
      currency
    }).format(amount);
  }
  
  // Method with edge cases that TypeScript can't catch
  static calculatePercentage(part: number, total: number): number {
    if (total === 0) {
      throw new Error('Cannot calculate percentage when total is zero');
    }
    
    return (part / total) * 100;
  }
}

describe('MathUtilities', () => {
  describe('calculateCompoundInterest', () => {
    it('should calculate compound interest correctly', () => {
      // Test known calculation
      const result = MathUtilities.calculateCompoundInterest(1000, 0.05, 4, 10);
      expect(result).toBeCloseTo(1643.62, 2);
    });
    
    it('should handle edge cases', () => {
      // Zero interest rate
      expect(MathUtilities.calculateCompoundInterest(1000, 0, 4, 10)).toBe(1000);
      
      // Zero years
      expect(MathUtilities.calculateCompoundInterest(1000, 0.05, 4, 0)).toBe(1000);
    });
    
    it('should validate input parameters', () => {
      expect(() => MathUtilities.calculateCompoundInterest(-1000, 0.05, 4, 10))
        .toThrow('Principal must be positive');
      
      expect(() => MathUtilities.calculateCompoundInterest(1000, -0.05, 4, 10))
        .toThrow('Rate cannot be negative');
      
      expect(() => MathUtilities.calculateCompoundInterest(1000, 0.05, 0, 10))
        .toThrow('Compound frequency must be positive');
      
      expect(() => MathUtilities.calculateCompoundInterest(1000, 0.05, 4, -10))
        .toThrow('Years cannot be negative');
    });
  });
  
  describe('formatCurrency', () => {
    it('should format currency correctly', () => {
      expect(MathUtilities.formatCurrency(1234.56)).toBe('$1,234.56');
      expect(MathUtilities.formatCurrency(1234.56, 'EUR')).toBe('€1,234.56');
    });
    
    it('should handle edge cases', () => {
      expect(MathUtilities.formatCurrency(0)).toBe('$0.00');
      expect(MathUtilities.formatCurrency(-1234.56)).toBe('-$1,234.56');
    });
    
    it('should validate input', () => {
      expect(() => MathUtilities.formatCurrency(NaN)).toThrow('Amount must be a valid number');
      expect(() => MathUtilities.formatCurrency(Infinity)).toThrow('Amount must be a valid number');
    });
  });
  
  describe('calculatePercentage', () => {
    it('should calculate percentages correctly', () => {
      expect(MathUtilities.calculatePercentage(25, 100)).toBe(25);
      expect(MathUtilities.calculatePercentage(1, 3)).toBeCloseTo(33.33, 2);
    });
    
    it('should handle edge cases', () => {
      expect(MathUtilities.calculatePercentage(0, 100)).toBe(0);
      expect(MathUtilities.calculatePercentage(100, 100)).toBe(100);
      expect(MathUtilities.calculatePercentage(150, 100)).toBe(150);
    });
    
    it('should handle division by zero', () => {
      expect(() => MathUtilities.calculatePercentage(25, 0))
        .toThrow('Cannot calculate percentage when total is zero');
    });
  });
});

🛠️ Building a Testing Strategy Framework

Let’s create a comprehensive framework for developing testing strategies in TypeScript projects:

// 🏗️ Testing Strategy Framework for TypeScript Projects

namespace TestingStrategy {
  
  // 📋 Core interfaces for testing strategy
  export interface TestStrategy {
    project: ProjectInfo;
    coverage: CoverageTargets;
    testTypes: TestTypeConfig[];
    tools: TestingTools;
    environments: TestEnvironment[];
    pipeline: TestPipeline;
    metrics: QualityMetrics;
  }
  
  export interface ProjectInfo {
    name: string;
    type: ProjectType;
    size: ProjectSize;
    complexity: ComplexityLevel;
    domain: ApplicationDomain;
    team: TeamInfo;
    timeline: ProjectTimeline;
  }
  
  export type ProjectType = 
    | 'web-application' 
    | 'api-service' 
    | 'library' 
    | 'cli-tool' 
    | 'mobile-app' 
    | 'desktop-app';
  
  export type ProjectSize = 'small' | 'medium' | 'large' | 'enterprise';
  export type ComplexityLevel = 'low' | 'medium' | 'high' | 'very-high';
  export type ApplicationDomain = 
    | 'e-commerce' 
    | 'finance' 
    | 'healthcare' 
    | 'education' 
    | 'entertainment' 
    | 'productivity' 
    | 'infrastructure';
  
  export interface TeamInfo {
    size: number;
    experience: ExperienceLevel;
    testingExperience: ExperienceLevel;
    distributed: boolean;
  }
  
  export type ExperienceLevel = 'junior' | 'mid' | 'senior' | 'expert';
  
  export interface ProjectTimeline {
    duration: number; // months
    phase: 'planning' | 'development' | 'testing' | 'deployment' | 'maintenance';
    deadlines: ProjectDeadline[];
  }
  
  export interface ProjectDeadline {
    name: string;
    date: Date;
    critical: boolean;
  }
  
  export interface CoverageTargets {
    unit: CoverageTarget;
    integration: CoverageTarget;
    e2e: CoverageTarget;
    overall: CoverageTarget;
    critical: CoverageTarget;
  }
  
  export interface CoverageTarget {
    lines: number; // percentage
    branches: number; // percentage
    functions: number; // percentage
    statements: number; // percentage
  }
  
  export interface TestTypeConfig {
    type: TestType;
    priority: Priority;
    coverage: number; // percentage of codebase
    automation: AutomationLevel;
    tools: string[];
    framework: string;
    patterns: TestPattern[];
  }
  
  export type TestType = 
    | 'unit' 
    | 'integration' 
    | 'contract' 
    | 'component' 
    | 'e2e' 
    | 'performance' 
    | 'security' 
    | 'accessibility' 
    | 'visual' 
    | 'smoke';
  
  export type Priority = 'low' | 'medium' | 'high' | 'critical';
  export type AutomationLevel = 'manual' | 'semi-automated' | 'fully-automated';
  
  export interface TestPattern {
    name: string;
    description: string;
    applicability: string[];
    implementation: string;
  }
  
  export interface TestingTools {
    testRunner: string;
    assertionLibrary: string;
    mockingFramework: string;
    coverageReporter: string;
    e2eFramework?: string;
    visualTesting?: string;
    performanceTesting?: string;
    additionalTools: string[];
  }
  
  export interface TestEnvironment {
    name: string;
    type: EnvironmentType;
    configuration: EnvironmentConfig;
    testTypes: TestType[];
  }
  
  export type EnvironmentType = 'local' | 'ci' | 'staging' | 'production-like' | 'cloud';
  
  export interface EnvironmentConfig {
    node: string;
    typescript: string;
    dependencies: Record<string, string>;
    environment: Record<string, string>;
    databases?: DatabaseConfig[];
    services?: ServiceConfig[];
  }
  
  export interface DatabaseConfig {
    type: string;
    version: string;
    testData: boolean;
  }
  
  export interface ServiceConfig {
    name: string;
    type: 'mock' | 'sandbox' | 'real';
    endpoint: string;
  }
  
  export interface TestPipeline {
    stages: PipelineStage[];
    triggers: PipelineTrigger[];
    notifications: NotificationConfig[];
    artifacts: ArtifactConfig[];
  }
  
  export interface PipelineStage {
    name: string;
    testTypes: TestType[];
    parallelism: number;
    timeout: number; // minutes
    retries: number;
    conditions: StageCondition[];
  }
  
  export interface StageCondition {
    type: 'coverage' | 'performance' | 'security' | 'custom';
    threshold: number;
    action: 'warn' | 'fail' | 'block';
  }
  
  export interface PipelineTrigger {
    event: 'push' | 'pull-request' | 'schedule' | 'manual';
    branches?: string[];
    schedule?: string;
    conditions?: string[];
  }
  
  export interface NotificationConfig {
    type: 'slack' | 'email' | 'webhook';
    target: string;
    events: ('success' | 'failure' | 'coverage-drop' | 'performance-regression')[];
  }
  
  export interface ArtifactConfig {
    type: 'coverage-report' | 'test-results' | 'performance-report' | 'screenshots';
    retention: number; // days
    public: boolean;
  }
  
  export interface QualityMetrics {
    targets: QualityTarget[];
    monitoring: MetricMonitoring[];
    reporting: ReportingConfig;
  }
  
  export interface QualityTarget {
    metric: QualityMetric;
    target: number;
    threshold: number;
    trend: 'improving' | 'stable' | 'declining';
  }
  
  export type QualityMetric = 
    | 'test-coverage' 
    | 'test-performance' 
    | 'bug-detection-rate' 
    | 'false-positive-rate' 
    | 'test-maintenance-effort' 
    | 'flaky-test-rate';
  
  export interface MetricMonitoring {
    metric: QualityMetric;
    frequency: 'daily' | 'weekly' | 'monthly';
    alerting: AlertConfig[];
  }
  
  export interface AlertConfig {
    condition: string;
    severity: 'info' | 'warning' | 'critical';
    recipients: string[];
  }
  
  export interface ReportingConfig {
    frequency: 'daily' | 'weekly' | 'monthly';
    format: 'dashboard' | 'email' | 'pdf';
    recipients: string[];
    metrics: QualityMetric[];
  }
  
  // 🔧 Strategy Builder
  export class TestStrategyBuilder {
    private strategy: Partial<TestStrategy> = {};
    
    // 📝 Build strategy based on project characteristics
    buildStrategy(projectInfo: ProjectInfo): TestStrategy {
      console.log(`📝 Building test strategy for: ${projectInfo.name}`);
      
      this.strategy.project = projectInfo;
      this.strategy.coverage = this.determineCoverageTargets(projectInfo);
      this.strategy.testTypes = this.selectTestTypes(projectInfo);
      this.strategy.tools = this.recommendTools(projectInfo);
      this.strategy.environments = this.designEnvironments(projectInfo);
      this.strategy.pipeline = this.createPipeline(projectInfo);
      this.strategy.metrics = this.defineMetrics(projectInfo);
      
      return this.strategy as TestStrategy;
    }
    
    // 🎯 Determine coverage targets based on project characteristics
    private determineCoverageTargets(project: ProjectInfo): CoverageTargets {
      const baseTargets = {
        lines: 80,
        branches: 75,
        functions: 85,
        statements: 80
      };
      
      // Adjust based on domain criticality
      const domainMultipliers: Record<ApplicationDomain, number> = {
        'finance': 1.2,
        'healthcare': 1.25,
        'e-commerce': 1.1,
        'education': 1.0,
        'entertainment': 0.9,
        'productivity': 1.0,
        'infrastructure': 1.15
      };
      
      const multiplier = domainMultipliers[project.domain];
      
      const adjustTargets = (target: CoverageTarget): CoverageTarget => ({
        lines: Math.min(95, Math.round(target.lines * multiplier)),
        branches: Math.min(95, Math.round(target.branches * multiplier)),
        functions: Math.min(95, Math.round(target.functions * multiplier)),
        statements: Math.min(95, Math.round(target.statements * multiplier))
      });
      
      return {
        unit: adjustTargets(baseTargets),
        integration: adjustTargets({
          lines: baseTargets.lines - 10,
          branches: baseTargets.branches - 10,
          functions: baseTargets.functions - 10,
          statements: baseTargets.statements - 10
        }),
        e2e: adjustTargets({
          lines: baseTargets.lines - 20,
          branches: baseTargets.branches - 20,
          functions: baseTargets.functions - 20,
          statements: baseTargets.statements - 20
        }),
        overall: adjustTargets(baseTargets),
        critical: adjustTargets({
          lines: 95,
          branches: 90,
          functions: 100,
          statements: 95
        })
      };
    }
    
    // 🔍 Select appropriate test types
    private selectTestTypes(project: ProjectInfo): TestTypeConfig[] {
      const testTypes: TestTypeConfig[] = [];
      
      // Unit tests - always included
      testTypes.push({
        type: 'unit',
        priority: 'critical',
        coverage: 80,
        automation: 'fully-automated',
        tools: ['jest', '@types/jest'],
        framework: 'jest',
        patterns: [
          {
            name: 'AAA Pattern',
            description: 'Arrange, Act, Assert',
            applicability: ['all'],
            implementation: 'Organize tests with clear setup, execution, and verification phases'
          }
        ]
      });
      
      // Integration tests
      testTypes.push({
        type: 'integration',
        priority: 'high',
        coverage: 60,
        automation: 'fully-automated',
        tools: ['jest', 'supertest'],
        framework: 'jest',
        patterns: [
          {
            name: 'Test Containers',
            description: 'Use real dependencies in isolated containers',
            applicability: ['api-service', 'web-application'],
            implementation: 'Spin up database/service containers for integration tests'
          }
        ]
      });
      
      // E2E tests for user-facing applications
      if (['web-application', 'mobile-app', 'desktop-app'].includes(project.type)) {
        testTypes.push({
          type: 'e2e',
          priority: 'high',
          coverage: 30,
          automation: 'fully-automated',
          tools: ['playwright', '@playwright/test'],
          framework: 'playwright',
          patterns: [
            {
              name: 'Page Object Model',
              description: 'Encapsulate page interactions in objects',
              applicability: ['web-application'],
              implementation: 'Create page classes that expose high-level actions'
            }
          ]
        });
      }
      
      // Performance tests for high-load applications
      if (project.size === 'large' || project.size === 'enterprise') {
        testTypes.push({
          type: 'performance',
          priority: 'medium',
          coverage: 20,
          automation: 'semi-automated',
          tools: ['k6', 'clinic'],
          framework: 'k6',
          patterns: [
            {
              name: 'Load Testing Pyramid',
              description: 'Test at multiple load levels',
              applicability: ['api-service', 'web-application'],
              implementation: 'Gradually increase load to find breaking points'
            }
          ]
        });
      }
      
      // Security tests for sensitive domains
      if (['finance', 'healthcare'].includes(project.domain)) {
        testTypes.push({
          type: 'security',
          priority: 'high',
          coverage: 40,
          automation: 'semi-automated',
          tools: ['owasp-zap', 'snyk'],
          framework: 'custom',
          patterns: [
            {
              name: 'Security Scanning',
              description: 'Automated vulnerability scanning',
              applicability: ['web-application', 'api-service'],
              implementation: 'Integrate security scanners in CI pipeline'
            }
          ]
        });
      }
      
      return testTypes;
    }
    
    // 🔧 Recommend testing tools
    private recommendTools(project: ProjectInfo): TestingTools {
      const baseTools: TestingTools = {
        testRunner: 'jest',
        assertionLibrary: 'jest',
        mockingFramework: 'jest',
        coverageReporter: 'jest',
        additionalTools: ['@types/jest', 'ts-jest']
      };
      
      // Add tools based on project type
      if (['web-application', 'mobile-app'].includes(project.type)) {
        baseTools.e2eFramework = 'playwright';
        baseTools.visualTesting = 'percy';
        baseTools.additionalTools.push('@playwright/test', 'percy-playwright');
      }
      
      if (project.size === 'large' || project.size === 'enterprise') {
        baseTools.performanceTesting = 'k6';
        baseTools.additionalTools.push('k6', 'clinic');
      }
      
      return baseTools;
    }
    
    // 🌍 Design test environments
    private designEnvironments(project: ProjectInfo): TestEnvironment[] {
      const environments: TestEnvironment[] = [
        {
          name: 'local',
          type: 'local',
          configuration: {
            node: '18.x',
            typescript: '5.x',
            dependencies: {},
            environment: {
              NODE_ENV: 'test'
            }
          },
          testTypes: ['unit', 'integration']
        },
        {
          name: 'ci',
          type: 'ci',
          configuration: {
            node: '18.x',
            typescript: '5.x',
            dependencies: {},
            environment: {
              NODE_ENV: 'test',
              CI: 'true'
            }
          },
          testTypes: ['unit', 'integration', 'e2e']
        }
      ];
      
      if (project.size === 'large' || project.size === 'enterprise') {
        environments.push({
          name: 'staging',
          type: 'staging',
          configuration: {
            node: '18.x',
            typescript: '5.x',
            dependencies: {},
            environment: {
              NODE_ENV: 'staging'
            },
            databases: [
              {
                type: 'postgresql',
                version: '14',
                testData: true
              }
            ],
            services: [
              {
                name: 'payment-service',
                type: 'sandbox',
                endpoint: 'https://sandbox.payment.com'
              }
            ]
          },
          testTypes: ['e2e', 'performance', 'security']
        });
      }
      
      return environments;
    }
    
    // ⚙️ Create test pipeline
    private createPipeline(project: ProjectInfo): TestPipeline {
      const stages: PipelineStage[] = [
        {
          name: 'unit-tests',
          testTypes: ['unit'],
          parallelism: 4,
          timeout: 10,
          retries: 2,
          conditions: [
            {
              type: 'coverage',
              threshold: 80,
              action: 'fail'
            }
          ]
        },
        {
          name: 'integration-tests',
          testTypes: ['integration'],
          parallelism: 2,
          timeout: 20,
          retries: 3,
          conditions: [
            {
              type: 'coverage',
              threshold: 60,
              action: 'warn'
            }
          ]
        }
      ];
      
      if (['web-application', 'mobile-app'].includes(project.type)) {
        stages.push({
          name: 'e2e-tests',
          testTypes: ['e2e'],
          parallelism: 1,
          timeout: 60,
          retries: 2,
          conditions: [
            {
              type: 'performance',
              threshold: 5000, // 5 seconds max
              action: 'warn'
            }
          ]
        });
      }
      
      return {
        stages,
        triggers: [
          {
            event: 'push',
            branches: ['main', 'develop']
          },
          {
            event: 'pull-request'
          },
          {
            event: 'schedule',
            schedule: '0 2 * * *' // Daily at 2 AM
          }
        ],
        notifications: [
          {
            type: 'slack',
            target: '#dev-team',
            events: ['failure', 'coverage-drop']
          }
        ],
        artifacts: [
          {
            type: 'coverage-report',
            retention: 30,
            public: true
          },
          {
            type: 'test-results',
            retention: 7,
            public: false
          }
        ]
      };
    }
    
    // 📊 Define quality metrics
    private defineMetrics(project: ProjectInfo): QualityMetrics {
      const targets: QualityTarget[] = [
        {
          metric: 'test-coverage',
          target: 80,
          threshold: 75,
          trend: 'improving'
        },
        {
          metric: 'bug-detection-rate',
          target: 90,
          threshold: 85,
          trend: 'stable'
        },
        {
          metric: 'flaky-test-rate',
          target: 2,
          threshold: 5,
          trend: 'declining'
        }
      ];
      
      return {
        targets,
        monitoring: [
          {
            metric: 'test-coverage',
            frequency: 'daily',
            alerting: [
              {
                condition: 'coverage < 75%',
                severity: 'warning',
                recipients: ['[email protected]']
              }
            ]
          }
        ],
        reporting: {
          frequency: 'weekly',
          format: 'dashboard',
          recipients: ['[email protected]'],
          metrics: ['test-coverage', 'bug-detection-rate', 'flaky-test-rate']
        }
      };
    }
  }
  
  // 📊 Strategy Analyzer
  export class StrategyAnalyzer {
    // 🔍 Analyze strategy effectiveness
    analyzeStrategy(strategy: TestStrategy): StrategyAnalysis {
      console.log(`🔍 Analyzing test strategy for: ${strategy.project.name}`);
      
      const strengths = this.identifyStrengths(strategy);
      const weaknesses = this.identifyWeaknesses(strategy);
      const recommendations = this.generateRecommendations(strategy, weaknesses);
      const riskAssessment = this.assessRisks(strategy);
      
      return {
        strategy,
        strengths,
        weaknesses,
        recommendations,
        riskAssessment,
        score: this.calculateScore(strategy),
        analysisDate: new Date()
      };
    }
    
    private identifyStrengths(strategy: TestStrategy): string[] {
      const strengths: string[] = [];
      
      if (strategy.coverage.unit.lines >= 80) {
        strengths.push('High unit test coverage target');
      }
      
      if (strategy.testTypes.some(t => t.type === 'e2e' && t.automation === 'fully-automated')) {
        strengths.push('Automated end-to-end testing');
      }
      
      if (strategy.pipeline.stages.length >= 3) {
        strengths.push('Comprehensive test pipeline');
      }
      
      return strengths;
    }
    
    private identifyWeaknesses(strategy: TestStrategy): string[] {
      const weaknesses: string[] = [];
      
      if (strategy.coverage.integration.lines < 60) {
        weaknesses.push('Low integration test coverage target');
      }
      
      if (!strategy.testTypes.some(t => t.type === 'performance')) {
        weaknesses.push('Missing performance testing');
      }
      
      if (strategy.pipeline.notifications.length === 0) {
        weaknesses.push('No pipeline notifications configured');
      }
      
      return weaknesses;
    }
    
    private generateRecommendations(strategy: TestStrategy, weaknesses: string[]): string[] {
      const recommendations: string[] = [];
      
      weaknesses.forEach(weakness => {
        switch (weakness) {
          case 'Low integration test coverage target':
            recommendations.push('Increase integration test coverage to at least 60%');
            break;
          case 'Missing performance testing':
            recommendations.push('Add performance testing for critical user journeys');
            break;
          case 'No pipeline notifications configured':
            recommendations.push('Configure Slack/email notifications for test failures');
            break;
        }
      });
      
      return recommendations;
    }
    
    private assessRisks(strategy: TestStrategy): RiskAssessment {
      return {
        testMaintenance: 'medium',
        falsePositives: 'low',
        coverage: 'low',
        performance: 'medium',
        overall: 'medium'
      };
    }
    
    private calculateScore(strategy: TestStrategy): number {
      let score = 0;
      
      // Coverage score (40% weight)
      const avgCoverage = (
        strategy.coverage.unit.lines +
        strategy.coverage.integration.lines +
        strategy.coverage.overall.lines
      ) / 3;
      score += (avgCoverage / 100) * 40;
      
      // Test type diversity (30% weight)
      const testTypeScore = Math.min(strategy.testTypes.length / 6, 1) * 30;
      score += testTypeScore;
      
      // Automation level (20% weight)
      const automatedTests = strategy.testTypes.filter(t => t.automation === 'fully-automated').length;
      const automationScore = (automatedTests / strategy.testTypes.length) * 20;
      score += automationScore;
      
      // Pipeline maturity (10% weight)
      const pipelineScore = Math.min(strategy.pipeline.stages.length / 4, 1) * 10;
      score += pipelineScore;
      
      return Math.round(score);
    }
  }
  
  // 📊 Supporting interfaces
  interface StrategyAnalysis {
    strategy: TestStrategy;
    strengths: string[];
    weaknesses: string[];
    recommendations: string[];
    riskAssessment: RiskAssessment;
    score: number;
    analysisDate: Date;
  }
  
  interface RiskAssessment {
    testMaintenance: 'low' | 'medium' | 'high';
    falsePositives: 'low' | 'medium' | 'high';
    coverage: 'low' | 'medium' | 'high';
    performance: 'low' | 'medium' | 'high';
    overall: 'low' | 'medium' | 'high';
  }
}

// 🚀 Usage examples with the testing strategy framework
const projectInfo: TestingStrategy.ProjectInfo = {
  name: 'E-commerce Platform',
  type: 'web-application',
  size: 'large',
  complexity: 'high',
  domain: 'e-commerce',
  team: {
    size: 8,
    experience: 'mid',
    testingExperience: 'mid',
    distributed: true
  },
  timeline: {
    duration: 12,
    phase: 'development',
    deadlines: [
      {
        name: 'MVP Release',
        date: new Date('2024-06-01'),
        critical: true
      }
    ]
  }
};

const strategyBuilder = new TestingStrategy.TestStrategyBuilder();
const strategy = strategyBuilder.buildStrategy(projectInfo);

console.log('Generated test strategy:', strategy);

const analyzer = new TestingStrategy.StrategyAnalyzer();
const analysis = analyzer.analyzeStrategy(strategy);

console.log('Strategy analysis:', analysis);
console.log(`Strategy score: ${analysis.score}/100`);