TypeScript Interfaces

interface

An interface describes the shape of an object, much like an object type alias:

interface User {
  id: number;
  name: string;
  email?: string;
  greet(): string;
}

const ada: User = {
  id: 1,
  name: "Ada",
  greet() {
    return `Hi, I'm ${this.name}`;
  },
};
console.log(ada.greet());

Hi, I'm Ada

Extending interfaces

Interfaces can extend one or more others:

interface Entity {
  id: number;
  createdAt: string;
}
interface Post extends Entity {
  title: string;
  tags: string[];
}

const post: Post = { id: 7, createdAt: "2026-10-12", title: "Learning TS", tags: ["ts", "js"] };
console.log(`${post.title} (#${post.id}) tagged ${post.tags.join(", ")}`);

Learning TS (#7) tagged ts, js

interface or type?

Both work for object shapes. Differences:

• type can also name unions, tuples, primitives and mapped types; interface describes only objects (and functions/classes).
• Interfaces with the same name merge (declaration merging) — useful for extending library types.

A common convention: use interface for object shapes and public APIs, type for everything else. Consistency matters more than the choice.