Interfaces, Types and Object Shapes

Describing an object

An interface defines the shape an object must have. Functions that take that interface only accept matching objects:

interface User {
  name: string;
  age: number;
}

function greet(u: User): string {
  return `Hello, ${u.name}`;
}

greet({ name: "Ada", age: 36 });   // OK
greet({ name: "Ada" });            // Error: age is missing

Optional and read-only members

interface Product {
  id: number;
  name: string;
  discount?: number;     // optional
  readonly sku: string;  // cannot be reassigned after creation
}

A ? makes a property optional; readonly prevents reassignment – great for identifiers.

Type aliases and unions

type gives a name to any type, including unions – a value that may be one of several types:

type ID = number | string;          // union
type Status = "active" | "banned";  // literal union

let userId: ID = 42;
userId = "u-42";                    // both allowed
let s: Status = "active";           // only these two strings

Literal unions are a lightweight, type-safe alternative to enums for a fixed set of values.

Interface vs type

Both describe object shapes and are largely interchangeable. Use an interface for object contracts you may extend or implement; use a type alias for unions, tuples and more complex compositions.

Key points

• Interfaces describe the required shape of objects.
• ? makes a property optional; readonly blocks reassignment.
• type names any type, including unions like number | string.
• Prefer interfaces for object contracts, type aliases for unions and compositions.