nominaltypes05.ts

function parseUserId(userId: string): UserId {
  if (!userId.startsWith("__special__prefix")) {
    throw new Error("This is not a userId!");
  }
  return userId as UserId;
}

// The happy path
const user: User = { id: parseUserId("__special__prefix:abcd") };

// Fails at compile-time
const user: User = { id: "abcd" };

// Throws an error at runtime
const user: User = { id: parseUserId("abcd") };