TypeScript Best Practices for Large Applications

As applications grow in complexity, maintaining code quality becomes increasingly challenging. TypeScript provides powerful tools to help us build robust, maintainable applications, but knowing how to use them effectively is key.

1. Strict Configuration

Always start with a strict TypeScript configuration:

{
  "compilerOptions": {
    "strict": true,
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "noImplicitReturns": true,
    "noFallthroughCasesInSwitch": true,
    "noUncheckedIndexedAccess": true
  }
}

2. Use Utility Types Effectively

TypeScript’s utility types are incredibly powerful:

// Partial for optional updates
type UserUpdate = Partial<User>;

// Pick for selecting specific properties
type UserPreview = Pick<User, "id" | "name" | "email">;

// Omit for excluding properties
type CreateUser = Omit<User, "id" | "createdAt">;

// Record for key-value mappings
type UserRoles = Record<string, "admin" | "user" | "guest">;

3. Leverage Discriminated Unions

For handling different states or types:

type ApiResponse<T> =
  | { status: "loading" }
  | { status: "success"; data: T }
  | { status: "error"; error: string };

function handleResponse<T>(response: ApiResponse<T>) {
  switch (response.status) {
    case "loading":
      return "Loading...";
    case "success":
      return response.data; // TypeScript knows data exists
    case "error":
      return response.error; // TypeScript knows error exists
  }
}

4. Create Custom Type Guards

Type guards help narrow types safely:

function isUser(obj: unknown): obj is User {
  return (
    typeof obj === "object" &&
    obj !== null &&
    typeof (obj as User).id === "string" &&
    typeof (obj as User).name === "string"
  );
}

// Usage
if (isUser(data)) {
  // TypeScript knows data is User here
  console.log(data.name);
}

5. Use Generic Constraints

Make your generics more specific:

interface Identifiable {
  id: string;
}

function updateEntity<T extends Identifiable>(
  entity: T,
  updates: Partial<T>
): T {
  return { ...entity, ...updates };
}

6. Organize with Namespaces and Modules

For large applications, organize types:

// types/api.ts
export namespace API {
  export interface User {
    id: string;
    name: string;
    email: string;
  }

  export interface CreateUserRequest {
    name: string;
    email: string;
  }

  export type UserResponse = API.User | { error: string };
}

7. Use Template Literal Types

For creating dynamic types:

type EventName = "click" | "hover" | "focus";
type EventHandler<T extends EventName> = `on${Capitalize<T>}`;

// Results in: 'onClick' | 'onHover' | 'onFocus'

8. Implement Proper Error Handling

Create a robust error handling system:

class AppError extends Error {
  constructor(
    message: string,
    public code: string,
    public statusCode: number = 500
  ) {
    super(message);
    this.name = "AppError";
  }
}

type Result<T, E = AppError> =
  | { success: true; data: T }
  | { success: false; error: E };

9. Use Branded Types for Domain Modeling

Prevent mixing up similar types:

type UserId = string & { __brand: "UserId" };
type ProductId = string & { __brand: "ProductId" };

function getUserById(id: UserId): Promise<User> {
  // Implementation
}

// This prevents accidentally passing a ProductId where UserId is expected

10. Configuration and Environment Types

Strongly type your configuration:

interface Config {
  readonly database: {
    readonly host: string;
    readonly port: number;
    readonly name: string;
  };
  readonly api: {
    readonly baseUrl: string;
    readonly timeout: number;
  };
}

function createConfig(): Config {
  return {
    database: {
      host: process.env.DB_HOST || "localhost",
      port: parseInt(process.env.DB_PORT || "5432"),
      name: process.env.DB_NAME || "myapp",
    },
    api: {
      baseUrl: process.env.API_BASE_URL || "http://localhost:3000",
      timeout: parseInt(process.env.API_TIMEOUT || "5000"),
    },
  };
}

Conclusion

These TypeScript patterns and practices have helped me maintain code quality across multiple large-scale applications. The key is to start with strict settings and gradually adopt these patterns as your application grows.

Remember: TypeScript is not just about adding types – it’s about creating a more predictable, maintainable codebase that scales with your team and project requirements.

Have you encountered any TypeScript challenges in your projects? Share your experiences and solutions!