Interfaces in TypeScript

// Basic interface
interface User {
  name: string
  age: number
  email?: string  // Optional
  readonly id: number  // Readonly
}

// Extending interfaces
interface Admin extends User {
  role: string
  permissions: string[]
}

// Function in interface
interface Greetable {
  name: string
  greet(): void
}

// Index signatures
interface StringDictionary {
  [key: string]: string
}