Advanced Types

        
          // Union types
type StringOrNumber = string | number;

function formatValue(value: StringOrNumber): string {
  if (typeof value === "string") {
    return value.toUpperCase();
  }
  return value.toFixed(2);
}

// Discriminated unions
interface Square {
  kind: "square";
  size: number;
}

interface Rectangle {
  kind: "rectangle";
  width: number;
  height: number;
}

interface Circle {
  kind: "circle";
  radius: number;
}

type Shape = Square | Rectangle | Circle;

function getArea(shape: Shape): number {
  switch (shape.kind) {
    case "square":
      return shape.size ** 2;
    case "rectangle":
      return shape.width * shape.height;
    case "circle":
      return Math.PI * shape.radius ** 2;
  }
}

// Intersection types
type Name = { name: string };
type Age = { age: number };
type Person = Name & Age; // Must have both name and age

const person: Person = {
  name: "John",
  age: 25
};

// Type guards
function isString(value: unknown): value is string {
  return typeof value === "string";
}

function isShape(obj: any): obj is Shape {
  return obj && typeof obj.kind === "string";
}

// Using type guards
function processValue(value: unknown): string {
  if (isString(value)) {
    return value.toUpperCase(); // TypeScript knows value is string
  }
  return "Not a string";
}

// keyof operator
type PersonKeys = keyof Person; // "name" | "age"

// Indexed access types
type PersonName = Person["name"]; // string

// typeof operator for types
const config = {
  apiUrl: "https://api.example.com",
  timeout: 5000
};

type Config = typeof config;

// Mapped types with template literals
type EventHandlers<T extends string> = {
  [K in T as `on${Capitalize<K>}`]: () => void;
};

type ButtonEvents = EventHandlers<'click' | 'hover'>;
// Results in: { onClick: () => void; onHover: () => void; }

// Never type
function throwError(message: string): never {
  throw new Error(message);
}

// Exhaustive checking with never
function assertNever(value: never): never {
  throw new Error(`Unexpected value: ${value}`);
}

function handleShape(shape: Shape) {
  switch (shape.kind) {
    case "square":
      return shape.size;
    case "rectangle":
      return shape.width;
    case "circle":
      return shape.radius;
    default:
      return assertNever(shape); // Ensures all cases are handled
  }
}