ES6

1. let

   1. 23
   2. 23

2. const

   1. we

   2. we

   3. us

3. 箭头函数 —— 替换匿名函数

   1. const myFunc = function() {
          const myVar = "value";
          return myVar;
      }
      

      1. 替换function为=>

         1. const myFunc = () => {
                const myVar = "value";
                return myVar;
            }
            

      2. 省略return语句和大括号

         1. const myFunc = () => 'value';
            

   2. 实现计算数组中正整数的平方

      1. 实现正整数的方法parseInt(string, radix)，

         • const squareList = (arr) => {
             "use strict";
             const squaredIntegers = arr.filter((elm) => elm > 0 && elm % parseInt(elm) === 0).map((elm) => elm*elm);
             return squaredIntegers;
           };
           

         • string不是字符串格式时，先转为字符串

         • radix默认为10(进制)

   3. Why?

4. 变长参数表和设置默认参数值

   1. 使用展开(spread)运算符to evaluate arrays in-place
      1.```js
      var arr = [6, 89, 3, 45];
      var maximus = Math.max.apply(null, arr); // returns 89

   • apply方法

     1. ...返回一个解包数组
        1. Math.max()需要的是多个参数，返回最大值
        2. const arr = [6, 89, 3, 45];
const max = Math.max(...arr); //解包数组

   • 展开运算符只在参数（如上）和数组字面量（如下）时work,

   • const arr1 = ['JAN', 'FEB', 'MAR', 'APR', 'MAY'];
     let arr2;
     (function() {
       "use strict";
       arr2 = [...arr1]; // 字面量
     })();
     console.log(arr2);
     

5. 解构赋值

   1. var voxel = {x: 3.6, y: 7.4, z: 6.54 };
      const { x, y, z } = voxel; // x = 3.6, y = 7.4, z = 6.54
      const { x : a, y : b, z : c } = voxel // a = 3.6, b = 7.4, c = 6.54
      

      1. 嵌套对象的解构赋值 --- 按属性名

         1. const a = {
              start: { x: 5, y: 6},
              end: { x: 6, y: -9 }
            };
            const { start : { x: startX, y: startY }} = a;
            

   2. 数组的解构赋值--- 按索引

      1. const [a, b,,, c] = [1, 2, 3, 4, 5, 6];
         console.log(a, b, c); // 1, 2, 5
         

      2. 用处： 交换赋值

         1. let a = 6, b = 8;
            [a, b] = [b, a];
            console.log(a);    // 8
            console.log(b);    // 6
            

      3. 剩余(rest)运算符给数组赋值

         1. const [a, b, ...arr] = [1, 2, 3, 4, 5, 7];
            console.log(a, b); // 1, 2
            console.log(arr); // [3, 4, 5, 7]
            

   3. 函数参数解构

      1. 23

      • const stats = {
          max: 56.78,
          standard_deviation: 4.34,
          median: 34.54,
          mode: 23.87,
          min: -0.75,
          average: 35.85
        };
        
        const half = (function() {
          "use strict";
        
          return function half(stats) {
            return (stats.max + stats.min) / 2.0;
          };
        
        })();
        

      • 函数改为如下两种

      • const half = (function() {
            'use strict';
        
            return function half({max, min}) {
                return (max + min) / 2.0;
            };
        })();
        
        const half = (function() {
            'use strict';
        
            return (({max, min}) => {
        
                return (max + min) / 2.0;
            });
        })();
        

6. 使用简单字段进行简明的对象字面量声明

   1. const getMousePosition = (x, y) => ({
        x: x,
        y: y
      });
      
      // 改为
      const getMousePosition = (x, y) => ({ x, y });
      

   2. 23

7. 编写简明的声明函数

   1. const person = {
        name: "Taylor",
        sayHello: function() {
          return `Hello! My name is ${this.name}.`;
        }
      };
      

   2. 去掉function 和 冒号

   3. const person = {
        name: "Taylor",
        sayHello() {
          return `Hello! My name is ${this.name}.`;
        }
      };
      

8. 使用class syntax定义构造函数

   1. class SpaceShuttle {
        constructor(targetPlanet){
          this.targetPlanet = targetPlanet;
        }
      }
      const zeus = new SpaceShuttle('Jupiter');
      

      • class is just a syntax，并不像其他的面向对象语言，声明一个完全的对象,所以还要向构造函数一样，用new来造一个对象
      • 添加一个构造函数

      • var SpaceShuttle = function(targetPlanet){
          this.targetPlanet = targetPlanet;
        }
        var zeus = new SpaceShuttle('Jupiter');
        

9. 使用setter和getter来控制对象的访问权限

   1. class Book {
        constructor(author) {
          this._author = author;
        }
        // getter
        get writer(){
          return this._author;
        }
        // setter
        set writer(updatedAuthor){
          this._author = updatedAuthor;
        }
      }
      const lol = new Book('anonymous');
      console.log(lol.writer);  // anonymous
      lol.writer = 'wut';
      console.log(lol.writer);  // wut
      

   2. get可以访问，set改变

10. import和export来复用代码

    1. const capitalizeString = (string) => {  
         return string.charAt(0).toUpperCase() + string.slice(1);  
       }  
       export { capitalizeString } // 函数export  
       export const foo = "bar"; // 变量export
       
       

    // 写在一行内
    export { capitalizeString, foo }

    import { function_name } from file_name // import的用法

    2.```js
      import * as 自命名 from 文件名;    //导入全部文件`
    3. 一个文件中只有一个函数或变量到处，`export default`
      1.`export default function (x, y) { return x + y; };\`  
    4. 23