设计安全的构造函数

JS

 function Person(name, age, job) {
    this.name = name;
    this.age = age;
    this.job = job;
}
var person = new Person("Nicholas", 34, 'software Engineer');
/*
在没有使用new运算符来调用构造函数的情况下。由于该this对象是在运行时绑定的，因此直接调用Person()会将该对象绑定到全局对象window上，这将导致错误属性意外增加到全局作用域上。这是由于this的晚绑定造成的，在这里this被解析成了window对象。
解决这个问题的方案是创建一个作用域安全的构造函数。首先确认this对象是否为正确的类型实例，如果不是，则创建新的实例并返回。
*/


function Person(name, age, job) {
    //检测this对象是否是Person的实例
    if(this instanceof Person) {
        this.name = name;
        this.age = age;
        this.job = job;
    } else {
        return new Person(name, age, job);
    }
}

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);
alert(rect.sides);//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;
    };
}
//使用原型链
Rectangle.prototype = new Polygon();
var rect = new Rectangle(5, 10);
alert(rect.sides);    //2