書上的解釋不太理解，求大神幫解釋下。為什么第一種沒有繼承到sides屬性// 創(chuàng)建作用域安全的構(gòu)造函數(shù)function Polygon(sides) {    if (this instanceof Polygon) {        console.log('this', this);        this.sides = sides;        this.getArea = function(){            return 0;        }    } else {        return new Polygon(sides);    }}// 非作用域安全的構(gòu)造函數(shù)function Rectangle(width, height) {    Polygon.call(this, 2);      this.width = width;    this.height = height;    this.getArea = function (){        return this.width * this.height;    }}let rect = new Rectangle(5,10);console.log(rect.sides);為什么rect沒有繼承到side屬性而通過原型就可以Rectangle.prototype = new Polygon();let rect = new Rectangle(2,4);console.log(rect.sides);// 2