作用域安全的构造函数

一般的构造函数是这个样子的。

function Person(name, age, job){            this.name = name;            this.age = age;            this.job = job;        }var person1 = new Person("Nicholas", 29, "Software Engineer");        alert(person1.name);     //"Nicholas"        alert(person1.age);      //29        alert(person1.job);      //"Software Engineer"var person2 = Person("Nicholas", 29, "Software Engineer");        alert(person2);         //undefined        alert(window.name);     //"Nicholas"        alert(window.age);      //29        alert(window.job);      //"Software Engineer"

这样就有一个弊端，如果没有使用 new 关键字，会把属性添加到window上。所以更好的做法应该是这样子的

function Person(name, age, job){           if (this instanceof Person){                this.name = name;                this.age = age;                this.job = job;            } else {                return new Person(name, age, job);            }        }

如上所示就可以避免这个问题，所以推荐使用。但是，这个也有一个要特别注意的地方，因为他侧通过检测是不是Person的实例，所以继承的时候
必须使用原型链或者寄生组合，如下。
1、错误

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

2.正确

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);        alert(rect.sides);   //2