TypeScript Best Practices for Large-Scale Applications

TypeScript has become the language of choice for many developers building large-scale applications. Its static typing system helps catch errors early and provides better tooling support. However, using TypeScript effectively requires following certain best practices.

Type Everything (Almost)

One of the main benefits of TypeScript is its type system. To get the most out of it, you should type as much of your code as possible. This includes function parameters, return types, and variables.

// Bad
function processUser(user) {
  return {
    id: user.id,
    name: user.name,
    isActive: user.status === 'active'
  };
}

// Good
interface User {
  id: string;
  name: string;
  status: 'active' | 'inactive';
}

interface ProcessedUser {
  id: string;
  name: string;
  isActive: boolean;
}

function processUser(user: User): ProcessedUser {
  return {
    id: user.id,
    name: user.name,
    isActive: user.status === 'active'
  };
}

Use Strict Mode

Enable strict mode in your tsconfig.json to catch more potential issues:

{
  "compilerOptions": {
    "strict": true,
    // Other options...
  }
}

Leverage Union Types and Discriminated Unions

Union types are a powerful feature of TypeScript that allows a value to be one of several types. Discriminated unions take this a step further by adding a common property that TypeScript can use to narrow down the type.

// Union type
type Result = Success | Error;

// Discriminated union
interface Success {
  type: 'success';
  data: any;
}

interface Error {
  type: 'error';
  message: string;
}

function handleResult(result: Result) {
  if (result.type === 'success') {
    // TypeScript knows result is Success here
    console.log(result.data);
  } else {
    // TypeScript knows result is Error here
    console.error(result.message);
  }
}

Use Type Inference When Appropriate

While explicit typing is generally good, TypeScript's type inference is quite powerful. Use it when the types are obvious to avoid unnecessary verbosity.

// Unnecessary explicit typing
const numbers: number[] = [1, 2, 3].map((num: number): number => num * 2);

// Better - let TypeScript infer the types
const numbers = [1, 2, 3].map(num => num * 2);

Organize Types in Separate Files

For large applications, organize your types in separate files to improve maintainability. Consider creating a types directory with subdirectories for different domains of your application.

Use Utility Types

TypeScript provides several utility types that can help you transform existing types in useful ways:

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

// Create a type with all properties optional
type PartialUser = Partial;

// Create a type with only specified properties
type UserBasicInfo = Pick;

// Create a type excluding specified properties
type UserWithoutDates = Omit;

Conclusion

By following these best practices, you can leverage TypeScript effectively to build more maintainable and robust applications. Remember that the goal of TypeScript is to enhance your development experience and catch errors early, not to make your code more complex.