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function greeter(fn: (a: string) => void) {
  fn("Hello, World");
}
 
function printToConsole(s: string) {
  console.log(s);
}
 
greeter(printToConsole);


type DescribableFunction = {
  description: string;
  (someArg: number): boolean;
};
function doSomething(fn: DescribableFunction) {
  console.log(fn.description + " returned " + fn(6));
}


type SomeConstructor = {
  new (s: string): SomeObject;
};

function fn(ctor: SomeConstructor) {
  return new ctor("hello");
}


function firstElement(arr: any[]) {
  return arr[0];
}

// Generic 버전
function firstElement<Type>(arr: Type[]): Type | undefined {
  return arr[0];
}


function f(x?: number) {
  // ...
}
f(); // OK
f(10); // OK


function f(x = 10) {
  // ...
}

declare function f(x = 10): void;
// cut
// All OK
f(); // x가 10으로 됨
f(10);
f(undefined); // x가 10으로 됨


function multiply(n: number, ...m: number[]) {
  return m.map((x) => n * x);
}
// 'a' gets value [10, 20, 30, 40]
const a = multiply(10, 1, 2, 3, 4);


const arr1 = [1, 2, 3];
const arr2 = [4, 5, 6];
arr1.push(...arr2);


function sum({ a, b, c }) {
  console.log(a + b + c);
}
sum({ a: 10, b: 3, c: 9 });

// 원래는 받아서 안에서 풀어야 할 것을
function sum(obj) {
	console.log(obj.a + obj.b + obj.c);
}