为类添加功能，但不会修改这个类。
    继承基本可以实现，也是最简单，但是类只能继承一个父类。如果要增加多个功能时，继承就不那么适合。
    这时装饰器就发挥作用。

    1. class Shape {}
    3. class Circle extends Shape {
    4.     constructor(radius = 0){
    5.       super();
    6.     this.radius = radius;
    7.   }
    9.   resize(factor){
    10.       this.radius *= factor;
    11.   }
    13.   toString(){
    14.       return `一个半径为 ${this.radius} 的圆形`;
    15.   }
    16. }

    装饰器

    1. class ColoredShape extends Shape {
    2.   constructor(shape, color){
    3.     super();
    4.     this.shape = shape;
    5.     this.color = color;
    6.   }
    8.   toString(){
    9.     return `${this.shape.toString()} 它有着 ${this.color} 的颜色`;
    10.   }
    11. }
    13. class TransparentShape extends Shape {
    14.   constructor(shape, transparency){
    15.     super();
    16.     this.shape = shape;
    17.     this.transparency = transparency;
    18.   }
    20.   toString(){
    21.     return `${this.shape.toString()} 它有着 ${this.transparency * 100}% 的透明度`;
    22.   }
    23. }
    25. let circle = new Circle(2);
    26. let redCircle = new ColoredShape(circle, 'red');
    27. redCircle.shape.resize(5);
    28. console.log(redCircle.toString());
    30. let redHalfCircle = new TransparentShape(redCircle, 0.5);
    31. redHalfCircle.shape.shape.resize(10);
    32. console.log(redHalfCircle.toString());

    调用 cirle 的 resize 时要多走几层。