TypeScript Best Practices

Best practices ensure maintainable, scalable TypeScript code. This article covers organization, patterns, and production guidelines.

Introduction

Best practices provide:

Code quality
Maintainability
Scalability
Performance
Team collaboration

Understanding best practices helps you:

Write better code
Avoid common pitfalls
Build scalable systems
Improve team productivity
Reduce technical debt

Type Safety

Strict Mode

// ✅ Good: Enable strict mode
{
  "compilerOptions": {
    "strict": true,
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictBindCallApply": true,
    "strictPropertyInitialization": true,
    "noImplicitThis": true,
    "alwaysStrict": true,
    "noUnusedLocals": true,
    "noUnusedParameters": true,
    "noImplicitReturns": true,
    "noFallthroughCasesInSwitch": true
  }
}

// ✅ Good: Explicit types
const user: { name: string; age: number } = {
  name: 'John',
  age: 30
};

// ❌ Bad: Implicit any
const user: any = { name: 'John', age: 30 };

Null and Undefined Handling

// ✅ Good: Handle null/undefined
function processUser(user: User | null | undefined): void {
  if (!user) {
    console.log('No user provided');
    return;
  }

  console.log(user.name);
}

// ✅ Good: Use optional chaining
const name = user?.name;
const email = user?.contact?.email;

// ✅ Good: Use nullish coalescing
const port = config.port ?? 3000;
const timeout = config.timeout ?? 5000;

// ❌ Bad: Assume non-null
function processUser(user: User): void {
  console.log(user.name); // May crash if user is null
}

Code Organization

Project Structure

// ✅ Good: Organized project structure
/*
src/
├── types/
│   ├── index.ts
│   ├── user.ts
│   └── api.ts
├── interfaces/
│   ├── index.ts
│   └── IRepository.ts
├── classes/
│   ├── User.ts
│   └── UserService.ts
├── utils/
│   ├── index.ts
│   ├── validators.ts
│   └── helpers.ts
├── services/
│   ├── index.ts
│   ├── UserService.ts
│   └── ApiService.ts
├── middleware/
│   └── auth.ts
├── config/
│   └── index.ts
└── index.ts
*/

// ✅ Good: Barrel exports
// src/types/index.ts
export type { User } from './user';
export type { Post } from './post';
export type { ApiResponse } from './api';

// Usage
import type { User, Post } from '@/types';

Naming Conventions

// ✅ Good: Clear naming
interface IUserRepository {
  getUser(id: number): Promise<User>;
}

class UserService {
  private userRepository: IUserRepository;

  async getUserById(id: number): Promise<User> {
    return this.userRepository.getUser(id);
  }
}

// ✅ Good: Consistent naming
const MAX_RETRIES = 3;
const DEFAULT_TIMEOUT = 5000;
const API_BASE_URL = 'https://api.example.com';

// ❌ Bad: Inconsistent naming
const maxRetries = 3;
const default_timeout = 5000;
const apiBaseUrl = 'https://api.example.com';

Error Handling

Custom Error Classes

// ✅ Good: Custom error classes
class ApplicationError extends Error {
  constructor(
    message: string,
    public statusCode: number = 500,
    public details?: any
  ) {
    super(message);
    this.name = 'ApplicationError';
  }
}

class ValidationError extends ApplicationError {
  constructor(message: string, details?: any) {
    super(message, 400, details);
    this.name = 'ValidationError';
  }
}

class NotFoundError extends ApplicationError {
  constructor(resource: string) {
    super(`${resource} not found`, 404);
    this.name = 'NotFoundError';
  }
}

// ✅ Good: Error handling
async function getUser(id: number): Promise<User> {
  if (!id || id <= 0) {
    throw new ValidationError('Invalid user ID');
  }

  const user = await database.findUser(id);

  if (!user) {
    throw new NotFoundError('User');
  }

  return user;
}

Error Handling Patterns

// ✅ Good: Try-catch with type guards
async function handleRequest(req: Request): Promise<Response> {
  try {
    const data = await processRequest(req);
    return { status: 200, data };
  } catch (error) {
    if (error instanceof ValidationError) {
      return { status: 400, error: error.message };
    }

    if (error instanceof NotFoundError) {
      return { status: 404, error: error.message };
    }

    if (error instanceof Error) {
      return { status: 500, error: error.message };
    }

    return { status: 500, error: 'Unknown error' };
  }
}

// ✅ Good: Result type pattern
type Result<T, E = Error> = { ok: true; value: T } | { ok: false; error: E };

function createResult<T>(value: T): Result<T> {
  return { ok: true, value };
}

function createError<E>(error: E): Result<never, E> {
  return { ok: false, error };
}

async function getUser(id: number): Promise<Result<User>> {
  try {
    const user = await database.findUser(id);
    return createResult(user);
  } catch (error) {
    return createError(error);
  }
}

Testing

Type-Safe Testing

// ✅ Good: Type-safe tests
import { describe, it, expect } from '@jest/globals';

describe('UserService', () => {
  let userService: UserService;
  let mockRepository: jest.Mocked<IUserRepository>;

  beforeEach(() => {
    mockRepository = {
      getUser: jest.fn(),
      createUser: jest.fn(),
      updateUser: jest.fn(),
      deleteUser: jest.fn()
    };

    userService = new UserService(mockRepository);
  });

  it('should get user by id', async () => {
    const user: User = { id: 1, name: 'John', email: '[email protected]' };
    mockRepository.getUser.mockResolvedValue(user);

    const result = await userService.getUserById(1);

    expect(result).toEqual(user);
    expect(mockRepository.getUser).toHaveBeenCalledWith(1);
  });

  it('should throw NotFoundError when user not found', async () => {
    mockRepository.getUser.mockResolvedValue(null);

    await expect(userService.getUserById(999)).rejects.toThrow(NotFoundError);
  });
});

Performance

Optimization Patterns

// ✅ Good: Lazy loading
class DataService {
  private cache: Map<number, User> = new Map();

  async getUser(id: number): Promise<User> {
    if (this.cache.has(id)) {
      return this.cache.get(id)!;
    }

    const user = await this.fetchUser(id);
    this.cache.set(id, user);
    return user;
  }

  private async fetchUser(id: number): Promise<User> {
    // Fetch from API
    return { id, name: 'John', email: '[email protected]' };
  }
}

// ✅ Good: Memoization
function memoize<T extends (...args: any[]) => any>(fn: T): T {
  const cache = new Map();

  return ((...args: any[]) => {
    const key = JSON.stringify(args);

    if (cache.has(key)) {
      return cache.get(key);
    }

    const result = fn(...args);
    cache.set(key, result);
    return result;
  }) as T;
}

// ✅ Good: Batch operations
async function getUsersBatch(ids: number[]): Promise<User[]> {
  const uniqueIds = [...new Set(ids)];
  return Promise.all(uniqueIds.map(id => getUser(id)));
}

Dependency Injection

DI Pattern

// ✅ Good: Dependency injection
interface IUserRepository {
  getUser(id: number): Promise<User>;
}

interface IEmailService {
  sendEmail(to: string, subject: string, body: string): Promise<void>;
}

class UserService {
  constructor(
    private userRepository: IUserRepository,
    private emailService: IEmailService
  ) {}

  async createUser(data: CreateUserData): Promise<User> {
    const user = await this.userRepository.createUser(data);
    await this.emailService.sendEmail(
      user.email,
      'Welcome',
      'Welcome to our service'
    );
    return user;
  }
}

// ✅ Good: Container pattern
class Container {
  private services: Map<string, any> = new Map();

  register<T>(name: string, factory: () => T): void {
    this.services.set(name, factory);
  }

  get<T>(name: string): T {
    const factory = this.services.get(name);
    if (!factory) {
      throw new Error(`Service ${name} not found`);
    }
    return factory();
  }
}

const container = new Container();
container.register('userRepository', () => new UserRepository());
container.register('emailService', () => new EmailService());
container.register('userService', () => {
  return new UserService(
    container.get('userRepository'),
    container.get('emailService')
  );
});

Documentation

JSDoc Comments

// ✅ Good: Comprehensive JSDoc
/**
 * Fetches a user by ID
 * @param id - The user ID
 * @returns The user object
 * @throws {NotFoundError} If user not found
 * @example
 * const user = await getUser(1);
 */
async function getUser(id: number): Promise<User> {
  // Implementation
}

/**
 * Validates email format
 * @param email - Email to validate
 * @returns True if valid, false otherwise
 */
function isValidEmail(email: string): boolean {
  return /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(email);
}

/**
 * User service for managing users
 * @class
 */
class UserService {
  /**
   * Creates a new user
   * @param data - User data
   * @returns Created user
   */
  async createUser(data: CreateUserData): Promise<User> {
    // Implementation
  }
}

Production Patterns

Environment Configuration

// ✅ Good: Environment-based configuration
interface Config {
  nodeEnv: 'development' | 'production' | 'test';
  port: number;
  databaseUrl: string;
  apiKey: string;
  logLevel: 'debug' | 'info' | 'warn' | 'error';
}

function loadConfig(): Config {
  const nodeEnv = (process.env.NODE_ENV || 'development') as Config['nodeEnv'];

  const config: Config = {
    nodeEnv,
    port: parseInt(process.env.PORT || '3000', 10),
    databaseUrl: process.env.DATABASE_URL || 'postgresql://localhost/db',
    apiKey: process.env.API_KEY || '',
    logLevel: (process.env.LOG_LEVEL || 'info') as Config['logLevel']
  };

  if (!config.apiKey && nodeEnv === 'production') {
    throw new Error('API_KEY is required in production');
  }

  return config;
}

const config = loadConfig();

Logging

// ✅ Good: Structured logging
interface LogEntry {
  timestamp: string;
  level: 'debug' | 'info' | 'warn' | 'error';
  message: string;
  context?: Record<string, any>;
}

class Logger {
  private level: number;

  constructor(level: string = 'info') {
    this.level = { debug: 0, info: 1, warn: 2, error: 3 }[level] || 1;
  }

  private log(level: string, message: string, context?: any): void {
    const levelNum = { debug: 0, info: 1, warn: 2, error: 3 }[level] || 1;

    if (levelNum < this.level) return;

    const entry: LogEntry = {
      timestamp: new Date().toISOString(),
      level: level as any,
      message,
      context
    };

    console.log(JSON.stringify(entry));
  }

  debug(message: string, context?: any): void {
    this.log('debug', message, context);
  }

  info(message: string, context?: any): void {
    this.log('info', message, context);
  }

  warn(message: string, context?: any): void {
    this.log('warn', message, context);
  }

  error(message: string, context?: any): void {
    this.log('error', message, context);
  }
}

const logger = new Logger(config.logLevel);

Best Practices Summary

Enable strict mode
Use explicit types
Handle null/undefined
Organize code logically
Use custom error classes
Write type-safe tests
Implement dependency injection
Document with JSDoc
Use environment configuration
Implement structured logging

Common Mistakes

Using any:

// ❌ Bad
const value: any = 'string';

// ✅ Good
const value: string = 'string';
```javascript

Not handling errors:

// ❌ Bad
const user = await getUser(id);

// ✅ Good
try {
  const user = await getUser(id);
} catch (error) {
  // Handle error
}
```javascript

Tight coupling:

// ❌ Bad
class UserService {
  private repository = new UserRepository();
}

// ✅ Good
class UserService {
  constructor(private repository: IUserRepository) {}
}

Summary

Best practices ensure quality. Key takeaways:

Enable strict mode
Use explicit types
Handle errors properly
Organize code logically
Implement dependency injection
Write type-safe tests
Document code
Use environment configuration
Implement logging
Avoid common pitfalls

Next Steps

Review all TypeScript articles
Practice TypeScript patterns
Build TypeScript projects
Contribute to TypeScript community
Stay updated with TypeScript releases

TypeScript Best Practices

Introduction

Type Safety

Strict Mode

Null and Undefined Handling

Code Organization

Project Structure

Naming Conventions

Error Handling

Custom Error Classes

Error Handling Patterns

Testing

Type-Safe Testing

Performance

Optimization Patterns

Dependency Injection

DI Pattern

Documentation

JSDoc Comments

Production Patterns

Environment Configuration

Logging

Best Practices Summary

Common Mistakes

Summary

Next Steps

Resources

Comments

Share this article

👍 Was this article helpful?

Introduction

Type Safety

Strict Mode

Null and Undefined Handling

Code Organization

Project Structure

Naming Conventions

Error Handling

Custom Error Classes

Error Handling Patterns

Testing

Type-Safe Testing

Performance

Optimization Patterns

Dependency Injection

DI Pattern

Documentation

JSDoc Comments

Production Patterns

Environment Configuration

Logging

Best Practices Summary

Common Mistakes

Summary

Related Resources

Next Steps

Resources

Comments

Share this article

👍 Was this article helpful?