Why getter returns old value

Question

I have the following code:

function User(fullName) {
    this.fullName = fullName;
    Object.defineProperties(this,
        {
            firstName: {
                get: function () {
                    return fullName.split(" ")[0];
                }
                ,
                set: function (fName) {
                    this.fullName = fName + " " + this.lastName;
                }
            },
            lastName: {
                get: function () {
                    return fullName.split(" ")[1];
                }
                ,
                set: function (lName) {
                    this.fullName = this.firstName + " " + lName;
                }
            }
        })

}

and following code to execute:

var vasya = new User("oldName oldSurname");

console.log(vasya.firstName); // 


vasya.firstName = "newName";
vasya.lastName = "newSurname"

console.log(vasya.fullName);

this output the newName OldSurname

if to change it a bit:

var vasya = new User("oldName oldSurname");

console.log(vasya.firstName); //
console.log(vasya.lastName); //

vasya.firstName = "newName";
vasya.lastName = "newSurname"

console.log(vasya.fullName);

it returns oldName newSurname

Please explain why now I see oldName insted of newName

Answer 1

You have to use "this" keyword when you reference fullNameValue or it will use the var that you passed as param

function User(fullName) {
    this.fullName = fullName;
    Object.defineProperties(this,
    {
        firstName: {
            get: function () {
                return this.fullName.split(" ")[0];
            }
            ,
            set: function (fName) {
                this.fullName = fName + " " + this.lastName;
            }
        },
        lastName: {
            get: function () {
                return this.fullName.split(" ")[1];
            }
            ,
            set: function (lName) {
                this.fullName = this.firstName + " " + lName;
            }
        }
    })

}

Answer 2

I played with this code and found out that it was naming conflict. this variant works properly

function User(fullNameValue) {
    this.fullName = fullNameValue; // renamed function argument
    Object.defineProperties(this,
        {
            firstName: {
                get: function () {
                    return this.fullName.split(" ")[0];//I use this here. without it I returned function argument
                }
                ,
                set: function (fName) {
                    this.fullName = fName + " " + this.lastName; 
                }
            },
            lastName: {
                get: function () {
                    return this.fullName.split(" ")[1];//I use this here. without it I 
                }
                ,
                set: function (lName) {
                    this.fullName = this.firstName + " " + lName;
                }
            }
        })

}