TypeScript - Branded or Nominal Types

Prevent accidental mixing of structurally identical types

type UserId = string & { readonly __brand: 'UserId' };
type OrderId = string & { readonly __brand: 'OrderId' };
 
function getUser(id: UserId) { /* ... */ }
 
const userId = 'abc' as UserId;
const orderId = 'abc' as OrderId;
 
getUser(userId);  // Pass
getUser(orderId); // Type error

todo what is __brand specifically.