TypeScript Patterns That Scale: Lessons from Large Codebases

The Challenge of Scale

When a TypeScript codebase grows beyond 100K lines, you start feeling the pain of poor type design. Here are patterns that have saved us.

Discriminated Unions for State Management

Instead of optional fields scattered everywhere:

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

This forces exhaustive handling and eliminates impossible states.

Branded Types for Domain Safety

type UserId = string & { readonly __brand: 'UserId' };
type OrderId = string & { readonly __brand: 'OrderId' };

function getUser(id: UserId): Promise<User> { /* ... */ }

You can never accidentally pass an OrderId where a UserId is expected.

The Result Pattern

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

No more try-catch spaghetti. Errors become values you can compose and transform.

Module Boundaries with Barrel Exports

Each module exposes a clean public API through index.ts, hiding internal implementation details. This creates natural boundaries that prevent spaghetti imports.

Key Takeaways

• Use the type system as documentation
• Make illegal states unrepresentable
• Prefer composition over inheritance (yes, even in types)
• Invest in strict tsconfig settings early