function log(x, y = 'World') {
  console.log(x, y);
}
log('Hello') // Hello World
log('Hello', 'China') // Hello China
log('Hello', '') // Hello

function foo({x, y = 5}) {
  console.log(x, y);
}
foo({}) // undefined, 5
foo({x: 1}) // 1, 5
foo({x: 1, y: 2}) // 1, 2
foo() // TypeError: Cannot read property 'x' of undefined

(function (a) {}).length // 1
(function (a = 5) {}).length // 0
(function (a, b, c = 5) {}).length // 2

function add(...values) {
  let sum = 0;
  for (var val of values) {
    sum += val;
  }
  return sum;
}
add(2, 5, 3) // 10

console.log(...[1, 2, 3])
// 1 2 3
console.log(1, ...[2, 3, 4], 5)
// 1 2 3 4 5
[...document.querySelectorAll('div')]
// [<div>, <div>, <div>]

var f = v => v;
//上面的箭头函数等同于：
var f = function(v) {
 return v;
};

var f = () => 5;
// 等同于
var f = function () { return 5 };
var sum = (num1, num2) => num1 + num2;
// 等同于
var sum = function(num1, num2) {
 return num1 + num2;
};

var sum = (num1, num2) => { return num1 + num2; }
//由于大括号被解释为代码块，所以如果箭头函数直接返回一个对象，必须在对象外面加上括号。
var getTempItem = id => ({ id: id, name: "Temp" });

const full = ({ first, last }) => first + ' ' + last;
// 等同于
function full(person) {
 return person.first + ' ' + person.last;
}

// 正常函数写法
[1,2,3].map(function (x) {
 return x * x;
});
// 箭头函数写法
[1,2,3].map(x => x * x);

const numbers = (...nums) => nums;
numbers(1, 2, 3, 4, 5)
// [1,2,3,4,5]

function clownsEverywhere(
  param1,
  param2
) { /* ... */ }

function clownsEverywhere(
  param1,
  param2,
) { /* ... */ }