var a = 2;
{
 let a = 3;
 console.log(a); // 3
}
console.log(a); // 2

{
 const ARR = [5,6];
 ARR.push(7);
 console.log(ARR); // [5,6,7]
 ARR = 10; // TypeError
}

var getPrice = function() {
 return 4.55;
};
 
// Implementation with Arrow Function
var getPrice = () => 4.55;

let arr = ['apple', 'banana', 'orange'];
 
let breakfast = arr.map(fruit => {
 return fruit + 's';
});
 
console.log(breakfast); // apples bananas oranges

function Person() {
 this.age = 0;
 
 setInterval(function growUp() {
 // 在非严格模式下，growUp() 函数的 this 指向 window 对象
 this.age++;
 }, 1000);
}
var person = new Person();

function Person() {
 var self = this;
 self.age = 0;

 setInterval(function growUp() {
 self.age++;
 }, 1000);
}

function Person(){
 this.age = 0;
 
 setInterval(() => {
 // |this| 指向 person 对象
 this.age++;
 }, 1000);
}
 
var person = new Person();

let getFinalPrice = (price, tax=0.7) => price + price * tax;
getFinalPrice(500); // 850

function foo(x,y,z) {
 console.log(x,y,z);
}
 
let arr = [1,2,3];
foo(...arr); // 1 2 3

function foo(...args) {
 console.log(args);
}
foo( 1, 2, 3, 4, 5); // [1, 2, 3, 4, 5]

function getCar(make, model, value) {
 return {
 // 简写变量
 make, // 等同于 make: make
 model, // 等同于 model: model
 value, // 等同于 value: value
 
 // 属性可以使用表达式计算值
 ['make' + make]: true,
 
 // 忽略 `function` 关键词简写对象函数
 depreciate() {
 this.value -= 2500;
 }
 };
}
 
let car = getCar('Barret', 'Lee', 40000);
 
// output: {
// make: 'Barret',
// model:'Lee',
// value: 40000,
// makeBarret: true,
// depreciate: function()
// }

let oValue = 0o10;
console.log(oValue); // 8
 
let bValue = 0b10; // 二进制使用 `0b` 或者 `0B`
console.log(bValue); // 2

function foo() {
 return [1,2,3];
}
let arr = foo(); // [1,2,3]
 
let [a, b, c] = foo();
console.log(a, b, c); // 1 2 3
 
function bar() {
 return {
 x: 4,
 y: 5,
 z: 6
 };
}
let {x: x, y: y, z: z} = bar();
console.log(x, y, z); // 4 5 6

var parent = {
 foo() {
 console.log("Hello from the Parent");
 }
}
 
var child = {
 foo() {
 super.foo();
 console.log("Hello from the Child");
 }
}
 
Object.setPrototypeOf(child, parent);
child.foo(); // Hello from the Parent
  // Hello from the Child

let user = 'Barret';
console.log(`Hi ${user}!`); // Hi Barret!

let nicknames = ['di', 'boo', 'punkeye'];
nicknames.size = 3;
for (let nickname of nicknames) {
 console.log(nickname);
}
Result: di, boo, punkeye

let nicknames = ['di', 'boo', 'punkeye'];
nicknames.size = 3;
for (let nickname in nicknames) {
 console.log(nickname);
}
Result: 0, 1, 2, size

var myMap = new Map();
 
var keyString = "a string",
 keyObj = {},
 keyFunc = function () {};
 
// 设置值
myMap.set(keyString, "value 与 'a string' 关联");
myMap.set(keyObj, "value 与 keyObj 关联");
myMap.set(keyFunc, "value 与 keyFunc 关联");
 
myMap.size; // 3
 
// 获取值
myMap.get(keyString); // "value 与 'a string' 关联"
myMap.get(keyObj); // "value 与 keyObj 关联"
myMap.get(keyFunc); // "value 与 keyFunc 关联"

let w = new WeakMap();
w.set('a', 'b'); 
// Uncaught TypeError: Invalid value used as weak map key
 
var o1 = {},
 o2 = function(){},
 o3 = window;
 
w.set(o1, 37);
w.set(o2, "azerty");
w.set(o3, undefined);
 
w.get(o3); // undefined, because that is the set value
 
w.has(o1); // true
w.delete(o1);
w.has(o1); // false

let mySet = new Set([1, 1, 2, 2, 3, 3]);
mySet.size; // 3
mySet.has(1); // true
mySet.add('strings');
mySet.add({ a: 1, b:2 });

mySet.forEach((item) => {
 console.log(item);
 // 1
 // 2
 // 3
 // 'strings'
 // Object { a: 1, b: 2 }
});
 
for (let value of mySet) {
 console.log(value);
 // 1
 // 2
 // 3
 // 'strings'
 // Object { a: 1, b: 2 }
}

var ws = new WeakSet();
var obj = {};
var foo = {};
 
ws.add(window);
ws.add(obj);
 
ws.has(window); // true
ws.has(foo); // false, foo 没有添加成功
 
ws.delete(window); // 从结合中删除 window 对象
ws.has(window); // false, window 对象已经被删除

class Task {
 constructor() {
 console.log("task instantiated!");
 }
 
 showId() {
 console.log(23);
 }
 
 static loadAll() {
 console.log("Loading all tasks..");
 }
}
 
console.log(typeof Task); // function
let task = new Task(); // "task instantiated!"
task.showId(); // 23
Task.loadAll(); // "Loading all tasks.."

class Car {
 constructor() {
 console.log("Creating a new car");
 }
}
 
class Porsche extends Car {
 constructor() {
 super();
 console.log("Creating Porsche");
 }
}
 
let c = new Porsche();
// Creating a new car
// Creating Porsche

var sym = Symbol( "some optional description" );
console.log(typeof sym); // symbol

var o = {
 val: 10,
 [ Symbol("random") ]: "I'm a symbol",
};
 
console.log(Object.getOwnPropertyNames(o)); // val

var arr = [11,12,13];
var itr = arr[Symbol.iterator]();
 
itr.next(); // { value: 11, done: false }
itr.next(); // { value: 12, done: false }
itr.next(); // { value: 13, done: false }
 
itr.next(); // { value: undefined, done: true }

function *infiniteNumbers() {
 var n = 1;
 while (true){
 yield n++;
 }
}
 
var numbers = infiniteNumbers(); // returns an iterable object
 
numbers.next(); // { value: 1, done: false }
numbers.next(); // { value: 2, done: false }
numbers.next(); // { value: 3, done: false }

var p = new Promise(function(resolve, reject) { 
 if (/* condition */) {
 // fulfilled successfully
 resolve(/* value */); 
 } else {
 // error, rejected
 reject(/* reason */); 
 }
});

p.then((val) => console.log("Promise Resolved", val),
 (err) => console.log("Promise Rejected", err));