JS Practice — Built-in Objects

Interface — Opaque/Brand Type

📖 Usage Example

type UserId = string & { readonly __brand: 'UserId' }
function createUserId(id: string): UserId {
  return id as UserId
}

A branded type uses an intersection with a phantom property to distinguish nominally equal types.

Brand / Opaque Types

A branded type adds a phantom property to distinguish types that share the same underlying representation.

Challenge: Implement createUserId(id) that casts a string to UserId.

type UserId = string & { __brand: 'UserId' }
createUserId('user-1')  // → 'user-1' (as UserId)

💡 Hints (1)

• At runtime, a branded type is just the underlying primitive. The brand only exists at the type level.