在窃取函数中用作用域安全的构造函数

function Polygon(sides) {
    if (this instanceof Polygon) {
        this.sides = sides;
        this.getArea = function() {
            return 0;
        };
    } else {
        return new Polygon(sides);
    }
}

function Rectangle(width, height) {
    Polygon.call(this, 2);
    this.width = width;
    this.height = height;
    this.getArea = function() {
        return this.width * this.height;
    };
}

var rect = new Rectangle(5, 10);
console.log(rect.sides);

 以上输出会报undefined

 

function Polygon(sides) {
    if (this instanceof Polygon) {
        this.sides = sides;
        this.getArea = function() {
            return 0;
        };
    } else {
        return new Polygon(sides);
    }
}

function Rectangle(width, height) {
    Polygon.call(this, 2);
    this.width = width;
    this.height = height;
    this.getArea = function() {
        return this.width * this.height;
    };
}

Rectangle.prototype = new Polygon();

var rect = new Rectangle(5, 10);
console.log(rect.sides);

 这样就正确了，使用原型链