brand_with_zod.ts

declare const BRAND: unique symbol

/**
 * Creates a new branded type by intersecting a given type with an object
 * containing a unique brand symbol.
 */
export type Brand<T, B> = T & { [BRAND]: B }

/** Creates a new branded type by intersecting a given type with an object containing a unique brand symbol. */
export function toBrandedType<T, B>(value: T, _brand: B): Brand<T, B> {
  return value as Brand<T, B>
}

type TodoId = Brand<string, 'TodoId'>

type Todo = Readonly<{
  id: TodoId
  title: string
  status: 'todo' | 'in_progress' | 'done'
}>

const TODO_SCHEMA = z
  .object({
    id: z.string(),
    title: z.string(),
    status: z.enum(['todo', 'in_progress', 'done']),
  })
  .transform<Todo>((parsed) => ({
    id: toBrandedType(parsed.id, 'TodoId'),
    title: parsed.title,
    status: parsed.status,
  }))