私有数据

http://mp.weixin.qq.com/s/X8bbwYk3SAkDQVIe4CuGrg

1.在构造函数作用域中处理数据，还可以在构造函数中对函数进行定义，调用参数中的数据（私有变量）

class Countdown {
    constructor(counter, action) {
        Object.assign(this, {
            dec() {
                if (counter < 1) return;
                counter--;
                if (counter === 0) {
                    action();
                }
            }
        });
    }
}

> let c = new Countdown(2, () => console.log('DONE'));
> c.dec();
> c.dec();
DONE

优点：不会与实例属性的名字出现冲突

缺点：如果其他成员方法也要访问这个属性，就都要定义在构造函数中，这样就不太优雅了

2.使用前缀标记的方式

class Countdown {
    constructor(counter, action) {
        this._counter = counter;
        this._action = action;
    }
    dec() {
        if (this._counter < 1) return;
        this._counter--;
        if (this._counter === 0) {
            this._action();
        }
    }

缺点：数据不够安全

3.通过weakMap将私有数据作为附加数据，添加到实例上

let _counter = new WeakMap();
let _action = new WeakMap();
class Countdown {
    constructor(counter, action) {
        _counter.set(this, counter);
        _action.set(this, action);
    }
    dec() {
        let counter = _counter.get(this);
        if (counter < 1) return;
        counter--;
        _counter.set(this, counter);
        if (counter === 0) {
            _action.get(this)();
        }
    }
}

4.使用symbol

const _counter = Symbol('counter');
const _action = Symbol('action');

class Countdown {
    constructor(counter, action) {
        this[_counter] = counter;
        this[_action] = action;
    }
    dec() {
        if (this[_counter] < 1) return;
        this[_counter]--;
        if (this[_counter] === 0) {
            this[_action]();
        }
    }
}

symbol作为一个唯一的标识，但还是可以获取得到：

let c = new Countdown(2, () => console.log('DONE'));

console.log(Object.keys(c));
// []
console.log(Reflect.ownKeys(c));
// [Symbol(counter), Symbol(action)]

5.通过var创建私有成员

function myConstructor(message){  
        //公有属性  
    this.myMessage = message;  

        //私有属性  
    var separator = '_';  
    var myOwner = this;  

    //构造函数内私有方法  
    function alertMessage(){  
        alert(myOwner.myMessage);  
    }  
    alertMessage();  

    this.appendToMessage = function(string){  
        this.myMessage += separator + string;  
        alertMessage();  
    }  
}

6.多个实例共享的私有变量

const Person = function(){
    var data = 10;
    return class{
        dec(){
            data --;
        }
        showData(){
            console.log(data)
        }
    }
}();

var p = new Person();
var p1 = new Person();

p1.dec();
p.showData(); // 9