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ECMAScript 2015 ES 6 (:star: let & const & var (定义方式 (let:…

- - - - let data =54 data += 6 console.log(data) //60
      - 同一个变量名只能被 声明一次
      - :star:每个ressign的value 构成独立的变量
    - - :star: const 定义变量名（常量）全大写
        
        定义只读变量
        
        demo:
        
        const data = 54; data += 65; //TypeError: Assignment to constant variable. console.log(data) ; //54
      - 对象（包括数组和函数）在使用const声明的时候依然是可变的。使用const来声明只会保证它的标识不会被重新赋值。
        
        demo:
        
        const s = [5,6,7] s[0] = 6 //s [6,6,7]
        
        由于const并不能保护变量，所以需要Object.freeze防止数据被修改
        
        demo:
        
        const obj = {name: "vino",age: 5} Object.freeze(obj) ojb.name = "test" //obj {name: "vino",age: 5}
- - - - const increment = (number, value = 1) => number + value increment(5,2 ) // 7 increment(5) //6
    - - demo
        
        const sum = (...args) => { return args.reduce((x,y) => {return x+y}) } sum(1,2,3) //6
      - These arguments are stored in an array that can be accessed later from inside the function.
    - - const arr1 =[5,7,6,"mamiya","pentax"] const arr2 =[...arr1]
  - - - Array 还有许多高阶函数
        
        array.every()
        
        arrary.find()
        
        array.findIndex()
        
        arrary.forEach()
        
        demo:
        
        const arr = [1,2,3,4] arr.forEach((element) => console.log(element))
    - - Array.map()
        
        注意：map()传入的参数是pow，即函数对象本身。
        
        demo
        
        function pow(x){ return x* x } let arr = [1,3,4,5] result = arr.map(pow) // [1,9,16,25]
      - 再看reduce的用法。Array的reduce()把一个函数作用在这个Array的[x1, x2, x3...]上，这个函数必须接收两个参数，reduce()把结果继续和序列的下一个元素做累积计算
        
        其效果就是：
        [x1, x2, x3, x4].reduce(f) = f(f(f(x1, x2), x3), x4)
        
        demo:
        
        let arr = [ 4,5,6] arr.reduce(function(x,,y){ return x +y}) //15
    - - 和map()不同的是，filter()把传入的函数依次作用于每个元素，然后根据返回值是true还是false决定保留还是丢弃该元素。
      - 回调函数
        
        demo:
        
        var arr = ["a","b","c"] var r = arr.filter(function (element,index,self) { console.log(element) // a, b,c console.log(index) // 0 ,1,2 console.log(self) //arr return true })
    - - 数字排序
        cosnt arr = [10, 2 ,40,5] arr.sort(function(x,y){ if(x>y){ return 1} if(x<y){ return -1} return 0 }) //[2,5,10,40]
- - - - class Book { constructor(author){ this._author = author } //getter get writer (){ return this._author } //setter set writer (updatedAuthor){ this._author = updatedAuthor } } const lol = new Book('anyone') console.log(lol.writer) //anyone lol.writer = "wot" console.log(lol.writer) //wot
- - - - tips:
        当使用export default去声明一个文件或者模块的返回值，你在每个文件或者模块中应当只默认导出一个值。特别地，你能将export deafult与var，let与const一起使用
      - export default function add(x,y) { return x+y }
- - - - const demo ={ name: "vino, age: 8 }
      - ${variable} 占位符
        
        const demo = {naem: "vino",age : 5} console.log ("this is demo name ${demo.name} !")