let只作用在当前块级作用域内
使用let或者const声明的变量，不能再被重新声明
let不存在`变量提升`

    console.log(dad);
    var dad = '我是爸爸!';//预定义undefined

    console.log(dad);
    let dad = '我是爸爸!';//报错

生成十个按钮 每个按点击的时候弹出1 - 10
var 方法：

    var i = 0;
    for (i = 1; i <= 10; i ++) {
      (function(i) {
        var btn = document.createElement('button');
        btn.innerText = i;
        btn.onclick = function() {
          alert(i)
        };
        document.body.appendChild(btn);
      })(i);
    }

let方法：

    for (let i = 1; i <= 10; i ++) {
      var btn = document.createElement('button');
      btn.innerText = i;
      btn.onclick = function() {
        alert(i)
      };
      document.body.appendChild(btn);
    }

ES6之前的作用域：全局作用域、函数作用域、eval作用域、ES6块级作用域

---

 

常量声明后不能被修改

  const NAME = '小明';
  NAME = '小红';//报错

q: 怎么防止常量为引用类型的时候能被修改的情况
Object.freeze()

    const xiaoming = {
      age: 14,
      name: '小明'
    };
    Object.freeze(xiaoming);
    console.log(xiaoming);
    xiaoming.age = 22;
    xiaoming.dd = 11;
    console.log(xiaoming);

    const ARR = [];
    Object.freeze(ARR);
    ARR.push(1);
    console.log(ARR);

Object的hasOwnProperty()方法返回一个布尔值，判断对象是否包含特定的自身（非继承）属性

    var obj1 = {
        a: 1,
        b: 2
    }

    var obj2 = Object.create(obj1);

    obj2.c = 3;
    obj2.d = 4;

    for (let i in obj2) {
        if (obj2.hasOwnProperty(i)) {
            document.body.innerHTML += (i + ': ' + obj2[i] + '<br>');
        }
    }

数组的解构赋值

    const arr = [1, 2, 3, 4];
    let [a, b, c, d] = arr;
    console.log(a);

更复杂的匹配规则

    const arr = ['a', 'b', ['c', 'd', ['e', 'f', 'g']]];
    const [ , b] = arr;
    const [ , , g] = ['e', 'f', 'g']
    const [ , , [ , , g]] = ['c', 'd', ['e', 'f', 'g']];
    const [ , , [ , , [ , , g]]] = arr;

扩展运算符  ...

    const arr1 = [1, 2, 3];
    const arr2 = ['a', 'b'];
    const arr3 = ['zz', 1];
    const arr4 = [...arr1, ...arr2, ...arr3];
    const arr = [1, 2, 3, 4, 5, 6];
    const [a, b, ...c] = arr;

默认值

    const arr = [1, null, undefined];
    const [a, b = 2, c, d = 'aaa'] = arr;

交换变量

    [a, b] = [b, a];

接收多个函数返回值

    function getUserInfo(id) {
      // .. ajax
      return [
        true,
        {
          name: '小明',
          gender: '女',
          id: id
        },
        '请求成功'
      ];
    };
    const [status, data, msg] = getUserInfo(123);

对象的解构赋值

    const obj = {
        saber: '阿尔托利亚',
        archer: '卫宫'
    };
    const { saber, archer1 } = obj;

稍微复杂的解构条件

    const player = {
        nickname: '感情的戏∫我没演技∆',
        master: '东海龙王',
        skill: [{
            skillName: '龙吟',
            mp: '100',
            time: 6000
        },{
            skillName: '龙卷雨击',
            mp: '400',
            time: 3000
        },{
            skillName: '龙腾',
            mp: '900',
            time: 60000
        }]
    };

    const { nickname } = player;
    const { master } = player;
    const { skill: [ skill1, { skillName }, { skillName: sklName } ] } = player;
    const { skill } = player;
    const [ skill1 ] = skill;

结合扩展运算符

    const obj = {
        saber: '阿尔托利亚',
        archer: '卫宫',
        lancer: '瑟坦达'
    };
    const { saber, ...oth } = obj;

对已经申明了的变量进行对象的解构赋值

    let age;
    const obj = {
        name: '小明',
        age: 22
    };
    ({ age } = obj);

默认值

    let girlfriend = {
        name: '小红',
        age: undefined,
    };
    let { name, age = 24, hobby = ['学习'] } = girlfriend;

字符串的结构赋值

    const str = 'I am the bone of my sword'; // 我是剑骨头
    const [ a, b ,c, ...oth ] = str;
    const [ ...spStr1 ] = str;

提取属性

    const { length, split } = str;

数值与布尔值的解构赋值

    const { valueOf: vo } = 1;
    const { toString: ts } = false;

函数参数的解构赋值

    function swap([x, y]) {
        return [y, x];
    };

    let arr = [1, 2];
    arr = swap(arr);

    function Computer({
        cpu,
        memory,
        software = ['ie6'],
        OS = 'windows 3.5'
    }) {

        console.log(cpu);
        console.log(memory);
        console.log(software);
        console.log(OS);

    };

    new Computer({
        memory: '128G',
        cpu: '80286',
        OS: 'windows 10'
    });

最后来一个高颜值弹框小案例~

按照国际惯例先放图

index.html

    <!DOCTYPE html>
    <html lang="en">
    <head>
      <meta charset="UTF-8">
      <meta name="viewport" content="width=device-width, initial-scale=1.0">
      <meta http-equiv="X-UA-Compatible" content="ie=edge">
      <title>Document</title>
      <link rel="stylesheet" href="./msg.css">
    </head>
    <body>
      <button>弹个框</button>
      <div id="t"></div>
      <script src="./msg.js"></script>
      <script>
        document.querySelector('button').addEventListener('click', function() {
          new $Msg({
            content: '真的要删除吗...',
            title: '确认删除',
            type: 'wrong',
            btnName: ['好的', '算了吧'],
            confirm: function(e) {
              console.log(this);
              console.log(e);
              new $Msg({
                content: '删除成功<span style="color: red">～</span>',
                type: 'success',
                footer: false,
                useHTML: true,
                header: false
              })
            },
            cancel: function(e) {
              document.querySelector('#t').innerHTML += '取消了，';
            }
          });
        });
      </script>
    </body>
    </html>

msg.css

    /* 弹出框最外层 */
    .msg__wrap {
      position: fixed;
      top: 50%;
      left: 50%;
      z-index: 10;
      transition: all .3s;
      transform: translate(-50%, -50%) scale(0, 0);
      max-width: 50%;
      
      background: #fff;
      box-shadow: 0 0 10px #eee;
      font-size: 10px;
    }

    /* 弹出框头部 */
    .msg__wrap .msg-header {
      padding: 10px 10px 0 10px;
      font-size: 1.8em;
    }

    .msg__wrap .msg-header .msg-header-close-button {
      float: right;
      cursor: pointer;
    }

    /* 弹出框中部 */
    .msg__wrap .msg-body {
      padding: 10px 10px 10px 10px;
      display: flex;
    }

    /* 图标 */
    .msg__wrap .msg-body .msg-body-icon{
      width: 80px;
    }

    .msg__wrap .msg-body .msg-body-icon div{
      width: 45px;
      height: 45px;
      margin: 0 auto;
      line-height: 45px;
      color: #fff;
      border-radius: 50% 50%;
      font-size: 2em;
    }

    .msg__wrap .msg-body .msg-body-icon .msg-body-icon-success{
      background: #32a323;
      text-align: center;
    }

    .msg__wrap .msg-body .msg-body-icon .msg-body-icon-success::after{
      content: "成";
    }

    .msg__wrap .msg-body .msg-body-icon .msg-body-icon-wrong{
      background: #ff8080;
      text-align: center;
    }

    .msg__wrap .msg-body .msg-body-icon .msg-body-icon-wrong::after{
      content: "误";
    }

    .msg__wrap .msg-body .msg-body-icon .msg-body-icon-info{
      background: #80b7ff;
      text-align: center;
    }

    .msg__wrap .msg-body .msg-body-icon .msg-body-icon-info::after{
      content: "注";
    }

    /* 内容 */
    .msg__wrap .msg-body .msg-body-content{
      min-width: 200px;
      font-size: 1.5em;
      word-break: break-all;
      display: flex;
      align-items: center;
      padding-left: 10px;
      box-sizing: border-box;
    }

    /* 弹出框底部 */
    .msg__wrap .msg-footer {
      padding: 0 10px 10px 10px;
      display: flex;
      flex-direction: row-reverse;
    }

    .msg__wrap .msg-footer .msg-footer-btn {
      width: 50px;
      height: 30px;
      border: 0 none;
      color: #fff;
      outline: none;
      font-size: 1em;
      border-radius: 2px;
      margin-left: 5px;
      cursor: pointer;
    }

    .msg__wrap .msg-footer .msg-footer-cancel-button{
      background-color: #ff3b3b;
    }

    .msg__wrap .msg-footer .msg-footer-cancel-button:active{
      background-color: #ff6f6f;
    }

    .msg__wrap .msg-footer .msg-footer-confirm-button{
      background-color: #4896f0;
    }

    .msg__wrap .msg-footer .msg-footer-confirm-button:active{
      background-color: #1d5fac;
    }

    /* 遮罩层 */
    .msg__overlay {
      position: fixed;
      top: 0;
      right: 0;
      bottom: 0;
      left: 0;
      z-index: 5;
      background-color: rgba(0, 0, 0, .4);
      transition: all .3s;
      opacity: 0;
    }

msg.js

    (function (window, document) {

      /**
       * 暴露出去的构造函数
       * @param {*} options 
       */
      let Msg = function (options) {
        this._init(options);
      }

      /**
       * 初始化传入的配置后创建元素 并显示
       * @param {*} param0 
       */
      Msg.prototype._init = function ({ 
        content = '', 
        type = 'info', 
        useHTML = false, 
        showIcon = true, 
        confirm = null, 
        cancel = null, 
        footer = true,
        header = true,
        title = '提示', 
        contentStyle = {}, 
        contentFontSize = '1.5em', 
        btnName = ['确定', '取消']
      }) {
        this.content = content;
        this.type = type;
        this.useHTML = useHTML;
        this.showIcon = showIcon;
        this.confirm = confirm;
        this.cancel = cancel;
        this.footer = footer;
        this.contentStyle = contentStyle;
        this.contentFontSize = contentFontSize;
        this.title = title;
        this.btnName = btnName;
        this.header = header;

        this._createElement();
        // 给dom上的按钮们和遮罩层绑定事件
        this._bind({ el: this._el, overlay: this._overlay });
        this._show({ el: this._el, overlay: this._overlay });
      }

      /**
       * 创建弹出框
       */
      Msg.prototype._createElement = function () {

        let wrap = document.createElement('div');
        wrap.className = 'msg__wrap';

        const [ confirmBtnName, cancelBtnName ] = this.btnName;

        // 判断是否显示图标
        const iconHTML = this.showIcon 
          ? '<div class="msg-body-icon">\
              <div class="msg-body-icon-' + this.type + '"></div>\
            </div>' 
          : '';

        // 判断是否显示弹出框底部按钮
        const footerHTML = this.footer 
          ? '<div class="msg-footer">\
              <button class="msg-footer-btn msg-footer-cancel-button">' + cancelBtnName + '</button>\
              <button class="msg-footer-btn msg-footer-confirm-button">' + confirmBtnName + '</button>\
            </div>' : '';

        const headerHTML = this.header
          ? '<div class="msg-header">\
              <span>' + this.title + '</span>\
              <span class="msg-header-close-button">×</span>\
            </div>'
          : '';

        // 拼成完整html
        const innerHTML = headerHTML +
                          '<div class="msg-body">' 
                            + iconHTML + 
                            '<div class="msg-body-content"></div>\
                          </div>'
                          + footerHTML;

        // 将容器内的html替换成弹出框内容
        wrap.innerHTML = innerHTML;

        // 生成合并自定义的内容样式
        const contentStyle = {
          fontSize: this.contentFontSize,
          ...this.contentStyle
        }

        // 获取内容dom
        let content = wrap.querySelector('.msg-body .msg-body-content');

        // 给内容容器加上自定义样式
        for (let key in contentStyle) {
          content.style[key] = contentStyle[key];
        }

        // 给弹出框内容赋值
        if (this.useHTML) {
          content.innerHTML = this.content;
        } else {
          content.innerText = this.content;
        }

        // 创建遮罩层
        let overlay = document.createElement('div');
        overlay.className = 'msg__overlay';

        // 把dom挂到当前实例上
        this._overlay = overlay;
        this._el = wrap;
      }

      /**
       * 显示弹出框
       * @param {*} param0 
       */
      Msg.prototype._show = function ({ el, overlay }) {

        // 把遮罩和弹出框插到页面中
        document.body.appendChild(el);
        document.body.appendChild(overlay);

        // 显示
        setTimeout(function () {
          el.style.transform = 'translate(-50%, -50%) scale(1, 1)';
          overlay.style.opacity = '1';
          console.log(1);
        });
        console.log(2);
      }

      /**
       * 绑定事件
       * @param {*} param0 
       */
      Msg.prototype._bind = function ({ el, overlay }) {
        // 保留this
        const _this = this;

        // 隐藏弹出框
        const hideMsg = function () {
          _this._el.style.transform = 'translate(-50%, -50%) scale(0, 0)';
          _this._overlay.style.opacity = '0';

          setTimeout(function () {
            document.body.removeChild(_this._el);
            document.body.removeChild(_this._overlay);
          }, 300);
        }

        // 取消事件
        const close = function (e) {
          _this.cancel && _this.cancel.call(_this, e);
          hideMsg();
        }

        // 确定事件
        const confirm = function (e) {
          _this.confirm && _this.confirm.call(_this, e);
          hideMsg();
        }

        overlay.addEventListener('click', close);

        if (this.header) {
          el.querySelector('.msg-header .msg-header-close-button').addEventListener('click', close);    
        }

        if (this.footer) {
          el.querySelector('.msg-footer .msg-footer-cancel-button').addEventListener('click', close);
          el.querySelector('.msg-footer .msg-footer-confirm-button').addEventListener('click', confirm);
        }
      }

      // 注册到全局对象
      window.$Msg = Msg;

    })(window, document);