TypeScript Best Practices for Large-Scale Applications

TypeScript has become the de facto standard for building large-scale JavaScript applications. But with great power comes great responsibility. Let's explore how to use TypeScript effectively in production applications.

Why TypeScript?

TypeScript provides:

• Catch errors early: Find bugs at compile time, not runtime
• Better IDE support: Autocomplete, refactoring, and navigation
• Self-documenting code: Types serve as inline documentation
• Safer refactoring: Confidence when making large changes

Essential Best Practices

1. Enable Strict Mode

Always use strict mode in your tsconfig.json:

{
  "compilerOptions": {
    "strict": true,
    "noUncheckedIndexedAccess": true,
    "noImplicitReturns": true,
    "noFallthroughCasesInSwitch": true
  }
}

2. Avoid any Like the Plague

Using any defeats the purpose of TypeScript. Instead:

// Bad
function processData(data: any) { ... }

// Good
function processData<T>(data: T) { ... }

// Better
interface UserData {
  id: string;
  name: string;
  email: string;
}
function processData(data: UserData) { ... }

3. Use Type Inference

Let TypeScript infer types when it's obvious:

// Unnecessary
const count: number = 42;

// Better
const count = 42; // TypeScript knows this is a number

4. Leverage Union Types

Union types are powerful for representing values that can be one of several types:

type Status = 'loading' | 'success' | 'error';

function handleStatus(status: Status) {
  switch (status) {
    case 'loading':
      // TypeScript knows status is 'loading'
      break;
    case 'success':
      // TypeScript knows status is 'success'
      break;
    case 'error':
      // TypeScript knows status is 'error'
      break;
  }
}

5. Use Utility Types

TypeScript provides powerful utility types:

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

// Pick only needed properties
type UserPreview = Pick<User, 'id' | 'name'>;

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

// Make all properties readonly
type ReadonlyUser = Readonly<User>;

// Omit properties
type UserWithoutAge = Omit<User, 'age'>;

Advanced Patterns

Type Guards

Create runtime type checks that TypeScript understands:

function isUser(value: unknown): value is User {
  return (
    typeof value === 'object' &&
    value !== null &&
    'id' in value &&
    'name' in value
  );
}

function processValue(value: unknown) {
  if (isUser(value)) {
    // TypeScript knows value is User here
    console.log(value.name);
  }
}

Discriminated Unions

Perfect for state management:

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

function handleState<T>(state: AsyncState<T>) {
  switch (state.status) {
    case 'idle':
      return 'Nothing loaded yet';
    case 'loading':
      return 'Loading...';
    case 'success':
      return state.data; // TypeScript knows data exists
    case 'error':
      return state.error.message; // TypeScript knows error exists
  }
}

Common Pitfalls to Avoid

1. Over-Engineering Types

Keep types simple and maintainable. Complex generic gymnastics often hurt more than they help.

2. Ignoring Errors with @ts-ignore

If you find yourself using @ts-ignore, there's usually a better solution. Fix the root cause instead.

3. Not Updating Types with Code Changes

Keep your types in sync with your implementation. Stale types are worse than no types.

Conclusion

TypeScript is a powerful tool, but it requires discipline and best practices to use effectively. Start strict, avoid any, leverage the type system, and your codebase will be more maintainable and less bug-prone.

Remember: TypeScript is there to help you, not fight you. If the types feel painful, you might be approaching the problem wrong. Keep it simple, keep it strict, and keep learning.