ES 语法

一、常量

    // ES5
    Object.defineProperty(window,"PI2",{
        value:123,
        writable:false,
    }); 
    console.log(window.PI2);
    window.PI2 = 333;
    console.log(window.PI2);

    // ES6
    const PI3 = "test";
    console.log(PI3);

二、作用域

    // ES5
    ((function () {
        const  foo = function () {
            return 2
        };
        console.log("foo()==2",foo() === 2);

        ((function () {
            const  foo = function () {
                return 1
            };
            console.log("foo()==1",foo() === 1);
        })());
    })());

    // ES6
    {
        const  foo = function () {
            return 3
        };
        console.log("foo()==3",foo() === 3);
        {
            const  foo = function () {
                return 4
            };
            console.log("foo()==4",foo() === 4);
        }
    }

三、箭头函数

this 为定义时的对象

    var events = [1,2,3,4,5];
    var odds = events.map(function (value) {
        return value * 2;
    });
    console.log(odds);

    var results = events.map(v => v + 1);
    console.log(results);


    {
        var  person = function () {
            this.name = "chenfan";
            this.age = 29;
            this.a = {
                name:"WHB",
                b:function () {
                    return this.name
                }
            }
        }

        console.log(new person().a.b());
    }

    {
        var  person = function () {
            this.name = "chenfan";
            this.age = 29;
            this.a = {
                name:"WHB",
                b: () => {
                    return this.name
                }
            }
        }

        console.log(new person().a.b());
    }

四、默认参数

    {
        function add(x, y) {
            if (x === undefined) {
                x = 1;
            }
            if (y === undefined) {
                y=2;
            }
            return x + y;
        }

        console.log(add(100));
    }

    {
        function add(x, y = 100) {
            return x + y;
        }

        console.log(add(100));
    }


    {
        function check() {
            throw new Error('fail');
        }

        function add(x = check()) {

        }

        add();
    }

    {
        function add() {
            var a = Array.prototype.slice.call(arguments);
            var sum = 0;
            a.forEach(function (item) {
                sum = item + 1;
            });
            return sum
        }
        console.log(add(1,2));
    }

    {
        function add(...a) {
            var sum = 0;
            a.forEach(item => {
                sum += item
            });
            return sum;
        }

        console.log(add(1,2));
    }

    {
        var arr1 = [1,2,3];
        var arr2 = [4,5,6];
        // ES5 连接
        var result = arr1.concat(arr2);
        console.log(result);
    }

    {
        var arr1 = [4,5,6];
        var arr2 = [
            1,2,3,...arr1
        ]
        console.log(arr2);
    }

五、数据保护

    {
        var person = function () {
            var data ={
                name:"chenfan",
                age:29
            };
            this.get = function (key) {
                return data[key];
            };
            this.set = function (key,value) {
                data[key] = value;
            }
        };

        var p = new person();
        p.set('name','11111');

        console.log(p.get('name'));
    }



    {
        var person = {

        };

        Object.defineProperty(person,'name',{
            value:"456",
            writable:false
        });

        person.name = "123";
        console.log(person.name);
    }

六、对象代理

    {
        let Person = {
            name:"888"
        };

        let person = new Proxy(Person,{
            get(target, key) {
                return target[key]
            },
            set(target,key, value) {
                if (key != "name") {
                    target[key] = value
                }
            }
        })

        console.log(person.name);
        person['name'] = 'wwww';
        console.log(person.name);
    }