记录把方法添加到 JavaScript 对象不明白的地方

<!DOCTYPE html>
<html>
<body>
<script>
function person(firstname,lastname,age,eyecolor)
{
this.firstname=firstname;
this.lastname=lastname;
this.age=age;
this.eyecolor=eyecolor;
 
this.changeName=changeName;
function changeName(name)
{
this.lastname=name;
}
}
myMother=new person("Sally","Rally",48,"green");
myMother.changeName("Doe");
document.write(myMother.lastname);
</script>

</body>
</html>

来源：http://www.w3cschool.cc/js/js-objects.html

有一句不明白呀~为什么要this.changeName=changeName呢？是函数指针的意思么?