实现一个私有变量

1. 使用约定, 默认以_开头 ```javascript // 优点 // 简单、调试方便、兼容性好

// 缺点 // 外部可访问，命名冲突， class Demo { constructor() { this._private = “private” } getPrivate() { return this._private; } } const demo = new Demo(); console.log(demo.getPrivate()); //private console.log(demo._private);

2. 使用闭包
```javascript
/**
    实现1
  优点，没有命令冲突，外部不能访问和修改
  缺点，类构造器逻辑变复杂，方法存在类实例上，子类无法继承，增加性能开销
**/
class Demo {
    constructor() {
      this._private = "";
    _private = "private";
    this.getPrivate = function() {return _private};
  }
}
const demo = new Demo();
console.log(demo.getPrivate()); //private
console.log(demo._private); // undefined
/**
    实现2
  优点：无命名冲突，外部无法访问和修改
  缺点：写法叫复杂，增加性能开销
**/
const Demo = (function(){
    var _private = '';
  class Demo {
    constructor() {
      _private = 'private';
    }
    getPrivate() {
        return _private;
    }
  }
  return Demo;
})()
const demo = new Demo();
console.log(demo.getPrivate()); //private
console.log(demo._private); // undefined

1. 使用Symbol

/**
    优点，无命名冲突，外部无法访问和修改，无性能损失
  缺点，写法负责，兼容性一般
**/
const Demo = (function(){
    var _private = Symbol('private')
  class Demo {
    constructor() {
      this[_private] = 'private';
    }
    getPrivate() {
        return this[_private];
    }
  }
  return Demo;
})()
const demo = new Demo();
console.log(demo.getPrivate()); //private
console.log(demo._private); // undefined

1. 使用WeakMap

/**
    优点：无命名冲突，外部无法修改和访问
  缺点：写法复杂，兼容较差，有性能开销
**/
const Demo = (function(){
    var _private = new WeakMap() // 私有成员存储容器
  class Demo {
    constructor() {
      _private.set(this, 'private')
    }
    getPrivate() {
        return _private.get(this);
    }
  }
  return Demo;
})()
const demo = new Demo();
console.log(demo.getPrivate()); //private
console.log(demo._private); // undefined

1. ES未来提案

class Point {
    #x;
  #y; 
  constructor(x, y) {
      this.#x = x;
    this.#y = y;
  }
   equals(point) {
       return this.#x === point.#x && this.#y  === point.#y
   }
}

1. 使用typescript private关键字

class Point {
    private number x;
  private number y;
  constructor(x:number, y:number) {
      this.x = x;
    this.y = y;
  }
   equals(point:{x: number, y: number}) {
       return this.x === point.x && this.y  === point.y
   }
}

1. 参考ES6 系列之私有变量的实现