TypeScript Types — Advanced

// Type alias
type UserID = string | number;
type Status = "active" | "inactive" | "banned"; // literal union

type User = {
  id: UserID;
  name: string;
  email: string;
  status: Status;
  age?: number; // optional
  readonly createdAt: Date; // cannot change
};

// Intersection type
type Admin = User & {
  role: "admin";
  permissions: string[];
};

// Usage
const user: User = {
  id: "usr-123",
  name: "Zohaib",
  email: "z@test.com",
  status: "active",
  createdAt: new Date()
};

// Type guard
function isAdmin(user: User | Admin): user is Admin {
  return (user as Admin).role === "admin";
}

if (isAdmin(user)) {
  console.log(user.permissions); // TS knows it's Admin here
}

type User = { name: string; email: string; age: number; };

// Partial — sab optional
type UpdateUser = Partial<User>; // { name?: string; email?: string; age?: number; }

// Required — sab required
type FullUser = Required<User>;

// Pick — subset
type LoginData = Pick<User, "email">; // { email: string }

// Omit — exclude
type PublicUser = Omit<User, "email">; // { name, age }

// Record
type Scores = Record<string, number>;
const scores: Scores = { math: 90, english: 85 };

// Readonly
type ImmutableUser = Readonly<User>;
// const u: ImmutableUser = { name: "Ali", email: "a@b.com", age: 25 };
// u.name = "Ahmed"; // ❌ Error!