TypeScript Best Practices for Large-Scale Applications

TypeScript has become the de facto standard for building large-scale JavaScript applications. Here are the best practices I've learned from working on production applications.

1. Use Strict Mode

Always enable strict mode in your tsconfig.json:

{
  "compilerOptions": {
    "strict": true,
    "noUncheckedIndexedAccess": true,
    "noImplicitOverride": true
  }
}

This catches potential bugs at compile time rather than runtime.

2. Leverage Type Inference

TypeScript's type inference is powerful. Let it work for you:

// ❌ Redundant type annotation
const count: number = 5;

// ✅ Let TypeScript infer
const count = 5;

// ✅ Type annotation needed here
function multiply(a: number, b: number) {
  return a * b; // TypeScript infers return type as number
}

3. Use Union Types Instead of Enums

Union types are often more flexible than enums:

// ✅ Union type
type Status = 'pending' | 'approved' | 'rejected';

// Usage
function handleStatus(status: Status) {
  switch (status) {
    case 'pending':
      // TypeScript knows all possible values
      break;
  }
}

4. Create Reusable Utility Types

Build a library of utility types for your domain:

// Utility for making specific fields optional
type PartialBy<T, K extends keyof T> = Omit<T, K> & Partial<Pick<T, K>>;

// Utility for making specific fields required
type RequiredBy<T, K extends keyof T> = T & Required<Pick<T, K>>;

// Deep readonly utility
type DeepReadonly<T> = {
  readonly [P in keyof T]: T[P] extends object ? DeepReadonly<T[P]> : T[P];
};

5. Use Discriminated Unions for State Management

Discriminated unions help model complex states:

type LoadingState<T> = 
  | { status: 'idle' }
  | { status: 'loading' }
  | { status: 'success'; data: T }
  | { status: 'error'; error: Error };

function handleState<T>(state: LoadingState<T>) {
  switch (state.status) {
    case 'loading':
      return 'Loading...';
    case 'success':
      return state.data; // TypeScript knows data exists here
    case 'error':
      return state.error.message; // TypeScript knows error exists here
  }
}

6. Prefer Interfaces for Object Shapes

Use interfaces for object types that might be extended:

interface User {
  id: string;
  name: string;
  email: string;
}

// Can be extended
interface AdminUser extends User {
  role: 'admin';
  permissions: string[];
}

7. Use `as const` for Literal Types

Create deeply immutable objects:

const ROUTES = {
  home: '/',
  blog: '/blog',
  about: '/about',
} as const;

type Route = typeof ROUTES[keyof typeof ROUTES];
// Route is: '/' | '/blog' | '/about'

8. Implement Type Guards

Create custom type guards for runtime type checking:

interface Cat {
  meow: () => void;
}

interface Dog {
  bark: () => void;
}

function isCat(pet: Cat | Dog): pet is Cat {
  return 'meow' in pet;
}

function handlePet(pet: Cat | Dog) {
  if (isCat(pet)) {
    pet.meow(); // TypeScript knows it's a Cat
  } else {
    pet.bark(); // TypeScript knows it's a Dog
  }
}

9. Use Template Literal Types

Create powerful string manipulation types:

type HttpMethod = 'GET' | 'POST' | 'PUT' | 'DELETE';
type ApiEndpoint = 'users' | 'posts' | 'comments';

type ApiRoute = `/${ApiEndpoint}`;
// Result: '/users' | '/posts' | '/comments'

type ApiMethodRoute = `${HttpMethod} ${ApiRoute}`;
// Result: 'GET /users' | 'POST /users' | ...

10. Document Complex Types

Use JSDoc comments for better IDE support:

/**
 * Represents a paginated response from the API
 * @template T The type of items in the response
 */
interface PaginatedResponse<T> {
  /** The list of items for the current page */
  data: T[];
  /** Current page number (1-indexed) */
  page: number;
  /** Total number of items across all pages */
  total: number;
  /** Number of items per page */
  pageSize: number;
}

Conclusion

These TypeScript best practices will help you write more maintainable, type-safe code. Remember:

Enable strict mode
Let type inference work for you
Use discriminated unions for complex states
Create reusable utility types
Document your types

By following these patterns, you'll catch more bugs at compile time and create a better developer experience for your team.