TypeScript Patterns I Actually Use

After five years of TypeScript in production, these are the patterns I reach for constantly.

Discriminated Unions for State Machines

Instead of separate loading, error, data booleans that can get into impossible states, use a discriminated union.

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

Now TypeScript enforces that you can’t access data when status is 'error'.

Branded Types

Use branded types to prevent confusion between values that are the same underlying type but semantically different.

type UserId = string & { readonly _brand: 'UserId' };
type SessionId = string & { readonly _brand: 'SessionId' };

satisfies Operator

The satisfies operator validates that a value matches a type without widening it. Perfect for configuration objects.