Javascript - creating method - object

Question

I am very new to javascript. I find some sample code for creating a object method.
<script>
function person(firstname,lastname,age,eyecolor){
this.firstname = firstname;
this.lastname = lastname;
this.age = age;
this.eyecolor = eyecolor;
this.changeName = changeName; //Why do I need this? What's it for?
function changeName(name){
this.lastname = name;
}
}
....
</script>
My question is: Why do I need to write this.changeName = changeName (also comment above). What's it for?
Thanks!

Answer 1

I think the key point is that javascript stores functions in variables. So
function changeName(name){
this.lastname = name;
}
is equivalent to
changeName = function () {
this.lastname = name;
}
Hence just writing function changeName(..)... in the constructor will create a function which is stored in a local variable not a class member variable. To turn it into a class member you need to do this.changeName = changeName (notice that this is exactly the same as for other variables).
Typically people add class functions like this:
function person(firstname,lastname,age,eyecolor){
this.firstname = firstname;
this.lastname = lastname;
this.age = age;
this.eyecolor = eyecolor;
}
person.changeName = function(name) {
this.lastname = name;
}

Answer 2

Because in your class you have a property called changeName and you are passing in a parameter called changeName, by using "this" it will be referenced to the class property and not the parameter that you have passed in.
You don't have to use this, you can do something like:
function person(_firstname, _lastname, _age, _eyecolor){
firstname = _firstname;
lastname = _lastname;
age = _age;
eyecolor = _eyecolor;
changeName = _changeName; //Why do I need this? What's it for?
function changeName(name){
this.lastname = name;
}
}
So basically by using this, it will just be able to tell apart from the local scope and class scope variables.