一下这段函数应该怎么理解function person(firstname,lastname){this.firstna

一下这段函数应该怎么理解
function person(firstname,lastname)
{
this.firstname=firstname;
this.lastname=lastname;
return this.lastname;
}
var myFather=new person("Steve","Jobs");
var myFather123=person("Steve","Jobs");
alert(myFather); //object Object
alert(myFather123); //jobs
为什么第一个alert出来的是 object Object 第二个是jobs 应该如何理解?