javaScript中在全局作用域定义的函数实际上只能被某个对象调用是什么意思?

Question

  function Person(name, age, job) {        this.name = name;        this.age = age;        this.job = job;        this.sayName = sayName;      }        function sayName() {        alert(this.name);    }        var person1 = new Person("Nicholas", 19, "Teacher");    var person2 = new Person("Greg", 20, "Doctor");