//如果使用let，声明的变量仅在块级作用域内有效

        {

            var a = 1;

            let b = 2;

        }

        console.log(a);  // 1

        console.log(b);  // Uncaught ReferenceError: b is not defined

        var arr = [1,2,3]

        for (let i = 0, l = arr.length; i < l; ++i) {}

        console.log(i);

    //上面代码中，变量i是let声明的，当前的i只在本轮循环有效

        var arr = [];

        for (let i = 0, l = 10; i < l; ++i) {

            arr[i] = function() {

                console.log(i);

            }

        }

        arr[6]();  //?

    //不存在变量提升

        console.log(foo);

        console.log(bar);  //报错

        var foo = 2;

        let bar = 2;

    //块级作用域和 函数声明

    function f() { console.log('I am outside!'); }

    (function () {

      f();

    }());

    function f() { console.log('I am outside!'); }

    (function () {

      if (false) {

        function f() { console.log('I am inside!'); }

      }

      f();

    }());  

    //函数参数的解构赋值

    function add([x, y]){

      return x + y;

    }

    var result = add([1, 2]); // 3

    console.log(result);

        // es6 的 类的概念

        class Point {

          constructor(x, y) {

            this.x = x;

            this.y = y;

          }

          toString() {

            return '(' + this.x + ', ' + this.y + ')';

          }

        }        

        var point = new Point(1,2);

        console.log(point.toString());

        //可以写出 立即执行的 Class 类

        let person = new class {

          constructor(name) {

            this.name = name;

          }

          sayName() {

            return this.name;

          }

        }('张三');

        const result = person.sayName(); // "张三"

        console.log(result);