05-interfaces.ts

// ============================================
// 05. Interfaces
// ============================================

console.log("=== INTERFACES IN TYPESCRIPT ===\n");

// Basic Interface
interface User {  // User definition data type
  name: string;
  age: number;
  email: string;
}

const alice: User = {
  name: "Alice",
  age: 25,
  email: "alice@example.com"
};

interface User {  // User2 definition data type
  phone: string;
  address: string;
}

console.log("=== BASIC INTERFACE ===\n");
console.log("Interface defines the shape of an object:");
console.log(`  User: { name: string; age: number; email: string }`);
console.log(`  user object:`, alice);

// Optional Properties
console.log("\n=== OPTIONAL PROPERTIES ===\n");

interface UserOptional {
  name: string;
  age: number;
  email?: string;  // Optional
}

const user1: UserOptional = {
  name: "Alice",
  age: 25
  // email is optional, so we can omit it
};

const user2: UserOptional = {
  name: "Bob",
  age: 30,
  email: "bob@example.com"
};

console.log("Optional properties use '?' - they can be omitted:");
console.log(`  user1 (no email):`, user1);
console.log(`  user2 (with email):`, user2);

// Readonly Properties
console.log("\n=== READONLY PROPERTIES ===\n");

interface Point {
  readonly x: number;
  readonly y: number;
}

const point: Point = { x: 10, y: 20 };
// point.x = 30; // Error: Cannot assign to 'x' because it is a read-only property

console.log("Readonly properties cannot be reassigned after initialization:");
console.log(`  point:`, point);
console.log("  Note: point.x and point.y cannot be changed after creation");

// Function Types in Interfaces
console.log("\n=== FUNCTION TYPES IN INTERFACES ===\n");

interface Calculator {
  add(a: number, b: number): number;
  subtract(a: number, b: number): number;
}

const calc: Calculator = {
  add(a, b) {
    return a + b;
  },
  subtract(a, b) {
    return a - b;
  }
};

console.log("Interfaces can define function signatures:");
console.log(`  calc.add(5, 3) = ${calc.add(5, 3)}`);
console.log(`  calc.subtract(10, 4) = ${calc.subtract(10, 4)}`);

// Index Signatures
console.log("\n=== INDEX SIGNATURES ===\n");

interface Dictionary {
  [key: string]: string;
}

interface Dictionary2 {
  [key: number]: string;
}

const dict: Dictionary = {
  hello: "world",
  foo: "bar",
  // Can add any string key-value pairs
};

const dict2: Dictionary2 = {
  0: "world",
  1: "bar",
  // Can add any number key-value pairs
};

console.log("Index signatures allow dynamic properties:");
console.log(`  dict:`, dict);
console.log(`  dict.hello = "${dict.hello}"`);
console.log(`  dict.foo = "${dict.foo}"`);
console.log("  Can add any string key-value pairs");

console.log(`  dict2:`, dict2);
console.log(`  dict2[0] = "${dict2[0]}"`);
console.log(`  dict2[1] = "${dict2[1]}"`);
console.log("  Can add any number key-value pairs");

// Extending Interfaces
console.log("\n=== EXTENDING INTERFACES ===\n");

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

interface Dog extends Animal {
  breed: string;
  bark(): void;
}

const myDog: Dog = {
  name: "Buddy",
  age: 3,
  breed: "Golden Retriever",
  bark() {
    console.log("Woof!");
  }
};

console.log("Interfaces can extend other interfaces:");
console.log(`  Animal: { name: string; age: number }`);
console.log(`  Dog extends Animal: { name, age, breed: string, bark(): void }`);
console.log(`  myDog:`, { name: myDog.name, age: myDog.age, breed: myDog.breed });
console.log("  Calling myDog.bark():");
myDog.bark();

// Multiple Inheritance
console.log("\n=== MULTIPLE INHERITANCE ===\n");

interface Flyable {
  fly(): void;
}

interface Swimmable {
  swim(): void;
}

interface Duck extends Flyable, Swimmable {
  quack(): void;
}

const duck: Duck = {
  fly() {
    console.log("Flying...");
  },
  swim() {
    console.log("Swimming...");
  },
  quack() {
    console.log("Quack!");
  }
};

console.log("Interfaces can extend multiple interfaces:");
console.log(`  Flyable: { fly(): void }`);
console.log(`  Swimmable: { swim(): void }`);
console.log(`  Duck extends Flyable, Swimmable: { fly(), swim(), quack() }`);
console.log("\n  Demonstrating duck's abilities:");
duck.fly();
duck.swim();
duck.quack();

console.log("\n=== DEMO COMPLETE ===");

export { user, point, calc, myDog, duck };