TypeScript Best Practices for Large Applications

As TypeScript applications grow in complexity, following established patterns becomes crucial for maintainability. Here are essential practices that will keep your codebase clean and scalable.

Strict Type Configuration

Start with strict TypeScript configuration:

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

Type-First Development

Define your types before implementation:

interface User {
  readonly id: string;
  name: string;
  email: string;
  roles: readonly Role[];
}

interface UserRepository {
  findById(id: string): Promise<User | null>;
  save(user: User): Promise<void>;
  findByEmail(email: string): Promise<User | null>;
}

Utility Types and Conditional Types

Leverage TypeScript’s powerful type system:

// Extract specific properties
type UserContact = Pick<User, 'email' | 'name'>;

// Make properties optional
type PartialUser = Partial<User>;

// Create discriminated unions
type ApiResponse<T> = 
  | { success: true; data: T }
  | { success: false; error: string };

Error Handling Patterns

Use discriminated unions for error handling:

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

function parseJson<T>(json: string): Result<T> {
  try {
    return { ok: true, value: JSON.parse(json) };
  } catch (error) {
    return { ok: false, error: error as Error };
  }
}

Generic Constraints

Use constraints to make generics more specific:

interface Identifiable {
  id: string;
}

function updateEntity<T extends Identifiable>(
  entity: T, 
  updates: Partial<Omit<T, 'id'>>
): T {
  return { ...entity, ...updates };
}

Branded Types

Create more specific types for better type safety:

type UserId = string & { readonly brand: unique symbol };
type Email = string & { readonly brand: unique symbol };

function createUserId(id: string): UserId {
  // Add validation here
  return id as UserId;
}

Module Organization

Structure your modules for clarity:

// types/user.ts
export interface User { /* ... */ }
export type UserRole = 'admin' | 'user' | 'guest';

// services/userService.ts
import type { User, UserRole } from '../types/user';

export class UserService {
  // Implementation
}

Testing with Types

Write tests that verify your types:

import { expectType } from 'tsd';

// Test that function returns correct type
const result = getUserById('123');
expectType<Promise<User | null>>(result);

Conclusion

These patterns will help you build more robust TypeScript applications. Remember: start strict, think in types, and leverage the type system to catch errors at compile time rather than runtime.