Rest parameter

function a(x, y) {
  x = x || 0;
  y = y || 0;
  
  return x + y;
}
console.log(a()); //0
console.log(plus(1,2)); //3

function a(x= 0, y=0) {
  return x + y;
}
console.log(a());
console.log(a(1,2));

function a(...rest) {
console.log(Array.isArray(rest));
console.log(rest);
}
a(1,2,3,4,5)

var a = (x, y, z) => {
  console.log(x);
  console.log(y);
  console.log(z);
}
a(...[1,2,3])

// 기존 배열을 새로운 배열로 합칠경우
// ES5 
var arr = [1,2,3];
console.log(arr.concat([4,5,6])); // [1,2,3,4,5,6]
//ES6
const arr = [1, 2, 3];
console.log([...arr, 4, 5, 6]); // [1, 2, 3, 4, 5, 6]
//push
//ES5
var arr1 = [1, 2];
var arr2 = [3, 4];
Array.prototype.push.apply(arr1, arr2);
console.log(arr1); // [1, 2, 3, 4]
const arr1 = [1, 2];
const arr2 = [3, 4];
arr1.push(...arr2); // arr1.push(3, 4);
console.log(arr1); //[1, 2, 3, 4];
//copy
//ES5
var arr1 = [1, 2];
var arr1copy = arr1.slice();
arr1copy.push(3);
console.log(arr1copy); // [1, 2, 3];
console.log(arr1); // [1, 2];
//ES6
const arr1 = [1, 2, 3];
const arr1copy = [...arr1];
arr1copy.push(3);
console.log(arr1copy); // [1, 2, 3];
console.log(arr1); // [1, 2];