Interfaces

Interfaces define the shape of objects in TypeScript.

Basic Interface

interface User {
  id: number;
  name: string;
  email: string;
  age?: number;  // Optional property
}

const user: User = {
  id: 1,
  name: "Alice",
  email: "alice@example.com"
};

Interface Extension

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

interface Employee extends Person {
  employeeId: number;
  department: string;
}

const emp: Employee = {
  name: "Bob",
  age: 28,
  employeeId: 1001,
  department: "Engineering"
};

Function Interfaces

interface MathOperation {
  (x: number, y: number): number;
}

const add: MathOperation = (x, y) => x + y;
const multiply: MathOperation = (x, y) => x * y;

Readonly Properties

interface Config {
  readonly apiKey: string;
  readonly endpoint: string;
}

const config: Config = {
  apiKey: "secret",
  endpoint: "https://api.example.com"
};

// config.apiKey = "new";  // Error: readonly property