Typed Functions

// Basic function types
function add(a: number, b: number): number {
  return a + b;
}

// Arrow function with types
const multiply = (a: number, b: number): number => a * b;

// Function type alias
type MathOperation = (a: number, b: number) => number;
const subtract: MathOperation = (a, b) => a - b;

// Optional and default parameters
function createUser(
  name: string,
  role: string = "viewer",
  age?: number
): { name: string; role: string; age?: number } {
  return { name, role, ...(age !== undefined && { age }) };
}

// Rest parameters
function sum(...numbers: number[]): number {
  return numbers.reduce((acc, n) => acc + n, 0);
}

// Function overloads
function formatDate(date: Date): string;
function formatDate(date: string): Date;
function formatDate(date: Date | string): string | Date {
  if (date instanceof Date) {
    return date.toISOString().split("T")[0];
  }
  return new Date(date);
}

const str = formatDate(new Date());    // TypeScript knows: string
const dateObj = formatDate("2025-01-01"); // TypeScript knows: Date

// Callback typing
function fetchData(
  url: string,
  onSuccess: (data: unknown) => void,
  onError?: (error: Error) => void
): void {
  // Implementation...
}

// this parameter (explicit this type)
interface Button {
  label: string;
  onClick(this: Button): void;
}