On: TypeScript: Branded Types

[svieira]

Given a Branded / Nominal interface, they look about the same usage wise but the branded type disappears at runtime (whether that is a benefit or a detriment depends on your use case):

    type Email = Branded<string, 'email'>

    function Email(maybeEmail: string): Email {
      assertIsEmail(maybeEmail);
      return maybeEmail;
    }

    function assertIsEmail(value: string): asserts value is Email {
      if(!isEmail(value)) throw TypeError("Not an email");
      return value;
    }

    function isEmail(value: string): value is Email {
      return value.contains("@");
    }

Versus:

    class Email {
      #email: string;
      public constructor(maybeEmail: string) {
        if(!isEmail(maybeEmail)) throw TypeError("Not an email");
        this.#email = maybeEmail;
      }
      valueOf() {
        return this.#email;
      }
    }