Branded Types for Validated Domain Values

Branded Types for Validated Domain Values: working example

Run this standalone example after compiling with TypeScript 5.4+ in strict mode, targeting ES2022. Expected result: The program prints Lookup user_42; plain strings require validation before use.

declare const userIdBrand: unique symbol;
type UserId = string & { readonly [userIdBrand]: true };
function userId(value: string): UserId {
  if (!/^user_[0-9]+$/.test(value)) throw new Error("Invalid user ID");
  return value as UserId;
}
function describe(id: UserId): string { return `Lookup ${id}`; }
console.log(describe(userId("user_42")));
// describe("user_42"); // Type error: plain strings are not branded.

1. Expected result: The program prints Lookup user_42; plain strings require validation before use.
2. The unique symbol marker distinguishes this domain type during checking.
3. No symbol property is attached at runtime; the factory verifies the chosen invariant before asserting the brand.