摘要：看代碼及注釋就懂了把代碼轉換為代碼解構賦值字符串反引號代替引號代替了正則表達式匹配數組展開符利用的實現對象對象屬性有屬性有值集合和添加元素，但是不會添加相同的元素，利用這個特性實現數組去重元素數量是否有指定元素刪除元

看代碼及注釋就懂了

$ npm install -g babel-cli

$ npm install babel-preset-env --save

$ babel ./src/es6.js -w --out-file ./dist/es5.js --presets env

const breakfast = () => ["cake", "coffee", "apple"]
let [dessert, drink, fruit] = breakfast()
console.info(dessert, drink, fruit) // "cake" "coffee" "apple"

const breakfast = () => {
    return {
        dessert: "cake",
        drink: "coffee",
        fruit: "apple"
    }
}
let {dessert, drink, fruit} = breakfast()
console.info(dessert, drink, fruit) // "cake" "coffee" "apple"

反引號代替引號：`some content ${variable} some content`

str.includes()、str.startsWidth()、str.endsWith() 代替了正則表達式匹配

// 利用 Array 的 concat() 實現
let breakfast = ["cake", "coffee", "apple"]
let food = ["rice", ...breakfast]
console.info(...breakfast, food) // "cake" "coffee" "apple", ["rice", "cake", "coffee", "apple"]

// 對象屬性
let food = {}
let drink = "hot drink"
food[drink] = "milk"
console.log(food["hot drink"]) // "milk"

let food = {}
let drink = "hot drink"
food[drink] = "milk"

let dessert = "cake"
food.fruit = "banane"

let breakfast = Object.assign({}, {dessert}, food) // {dessert} 有屬性有值
console.log(breakfast) // {dessert: "cake", hot drink: "milk", fruit: "banane"}

let fruit = new Set(["apple", "banane", "orange"])
/* 添加元素，但是不會添加相同的元素，利用這個特性實現數組去重：[...new Set(arr)] */
fruit.add("pear")
/* 元素數量 */
console.log(fruit.size) // 4
/* 是否有指定元素 */
console.log(fruit.has("apple")) // true
/* 刪除元素 */
fruit.delete("banana") 

console.log(fruit) // Set(4)?{"apple", "banane", "orange", "pear"}
/* 遍歷 */
fruit.forEach(item => {
    console.log(item)
})
/* 清空 */
fruit.clear()

console.log(fruit) // Set(0)?{}

let food = new Map()
let fruit = {}, cook = function(){}, dessert = "甜點"
food.set(fruit, "apple")
food.set(cook, "knife")
food.set(dessert, "cake")

console.log(food, food.size) // Map(3)?{{…} => "apple", ? => "knife", "甜點" => "cake"} 3

console.log(food.get(fruit)) // "apple"
console.log(food.get(dessert)) // "cake"

food.delete(dessert)
console.log(food.has(dessert)) // false

food.forEach((value, key) => {
    console.log(`${key} = ${value}`) // [object Object] = apple    function(){} = knife
})

food.clear()
console.log(food) // Map(0)?{}

function* calc(num){
    yield num+1
    yield num-2
    yield num*3
    yield num/4
    return num
}
let cal = calc(4)
console.info(
    cal.next(), // {value: 5, done: false}
    cal.next(), // {value: 2, done: false}
    cal.next(), // {value: 12, done: false}
    cal.next(), // {value: 1, done: false}
    cal.next(), // {value: 4, done: true} 遍歷完了后 done:true
    cal.next() // {value: undefined, done: true}
)

class Chef{
    constructor(food){
        this.food = food;
        this.dish = [];
    }
    get menu(){ // 不得有參數（Getter must not have any formal parameters.)
        console.log("getter 取值")
        console.log(this.dish)
        return this.dish
    }
    set menu(dish){ // 必須有一個參數（Setter must have exactly one formal parameter.）
        console.log("setter 存值")
        this.dish.push(dish)
    }
    cook(){
        console.log(this.food)
    }
}
let Gu = new Chef("vegetables");
Gu.cook() // "vegetables"
console.log(Gu.menu) // "get 取值" []

Gu.menu = "egg" // "setter 存值"
Gu.menu = "fish" // "setter 存值"

console.log(Gu.menu); // "getter 取值" ["egg", "fish"]

class Person {
    constructor(name, birth){
        this.name = name
        this.birth = birth
    }
    intro(){
        return `${this.name}的生日是${this.birth}`
    }
}

class One extends Person {
    constructor(name, birth){
        super(name, birth)
    }
}

let z = new Person("zz", "01-09")
let x = new One("xx", "01-09")

console.log(z.intro(), x.intro()) // "zz的生日是01-09" "xx的生日是01-09"
// 轉化為ES5的代碼大概就是
/* function Person(name, birth) {
    this.name = name;
    this.birth = birth;
}
Person.prototype.intro = function () {
    return this.name + "u7684u751Fu65E5u662F" + this.birth;
};
function One(name, birth) {
    Person.apply(this, arguments);
}
One.prototype = new Person();
var z = new Person("zz", "01-09");
var x = new One("xx", "01-09");

console.log(z.intro(), x.intro()); // "zz的生日是01-09" "xx的生日是01-09" */

回調地獄

function ajax(fn){
    setTimeout(()=>{
        console.log("執行業務")
        fn()
    },1000)
}
ajax(()=>{
    ajax(()=>{
        ajax(()=>{
            ajax(()=>{
                console.log("執行結束4")
            })
            console.log("執行結束3")
        })
        console.log("執行結束2")
    })
    console.log("執行結束1")
})
/*
    效果：
        1s： 執行業務 執行結束1
        2s:  執行業務 執行結束2
        3s： 執行業務 執行結束3
        4s:  執行業務 執行結束4
*/

Promise 這樣寫

function delay(word){
    return new Promise((reslove,reject) => {
        setTimeout(()=>{
            console.log("執行業務")
            reslove(word)
        },1000)
    })
}
delay("執行業務1")
    .then(word=>{
        console.log(word)
        return delay("執行業務2")
    })
    .then(word=>{
        console.log(word)
        return delay("執行業務3")
    })
    .then(word=>{
        console.log(word)
        return delay("執行業務4")
    })
    .then(word=>{
        console.log(word)
    })
    .catch()

async + await

function delay(word){
    return new Promise((reslove, reject) => {
        setTimeout(()=>{
            console.log("執行業務")
            reslove(word)
        },1000)
    })
}

const start = async () => {
    const word1 = await delay("執行業務1")
    console.log(word1)

    const word2 = await delay("執行業務2")
    console.log(word2)

    const word3 = await delay("執行業務3")
    console.log(word3)
    
    const word4 = await delay("執行業務4")
    console.log(word4)
}

start()