Types & Interfaces

To describe the shape of objects, TypeScript gives you interface and type. Both work for object shapes; each has strengths.

Interfaces

interface User {
  id: number;
  name: string;
  email?: string;        // optional property
  readonly createdAt: Date; // can't be reassigned
}

const user: User = { id: 1, name: "Priya", createdAt: new Date() };

Interfaces can extend other interfaces and support declaration merging (re-opening to add members):

interface Admin extends User {
  role: "admin" | "superadmin";
}

Type aliases

type can describe object shapes too, but is more flexible — it can alias any type:

type ID = string | number;          // union
type Point = { x: number; y: number };
type Coords = Point & { z: number }; // intersection

type vs interface

Literal and union types

type Status = "idle" | "loading" | "success" | "error";

let state: Status = "loading"; // ✅
let bad: Status = "done";       // ❌ not assignable

Literal unions are perfect for modeling a fixed set of allowed values — far safer than plain string.