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");