getOwnPropertyDescriptor returns undefine when it has a value

Question

I have been like one hour trying to solve with getOwnProperty returns undefined, I check my notes and cannot find the reason, could please some1 check or even better explain me why this behaviour??? Main goal in here is just overwrite a property on a method that extends another one.

Here is the jsfiddle

var Person = function(firstName, lastName) {
 this.firstName = firstName;
 this.lastName = lastName;
};

Object.defineProperties(Person.prototype, {
  sayHi : {
    value : function() {
     return "Hi there";
    },
    writable: true,
    enumerable : true
  },

  fullName : {
    get : function() {
     return this.firstName + " " + this.lastName;
    },
    configurable : true,
    enumerable : true
  }

 });

 var createEmployee = function(firstName, lastName, ocupation) {

    var employee = new Person(firstName, lastName);
    employee.ocupation = ocupation;

    /*over-w sayHi*/

    var sayHifn = employee.sayHi.bind(employee);

    employee.sayHi = function() {
      return sayHifn() + " my name is " + this.firstName;
    };

    /*over-w fullName*/

    var fullName = Object.getOwnPropertyDescriptor(employee, "fullName");
    var fullNameFn = fullName.get.bind(employee);

    Object.defineProperty(employee, 'fullName', {
     get : function() {
      return fullNameFn() + " this is o-w ";
     }

    });

    return employee;

  };

  var record = createEmployee('jhon', 'doe', 'eng');
  console.log(record);

Answer 1

The reason you're getting undefined is that the employee variable does not have an "own" property called fullName. That property belongs to Person.prototype.

Give this a try:

var fullName = Object.getOwnPropertyDescriptor(Person.prototype, "fullName");

If you change that line, the rest of your code should work as-is.

For reference (emphasis added):

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/getOwnPropertyDescriptor

The Object.getOwnPropertyDescriptor() method returns a property descriptor for an own property (that is, one directly present on an object, not present by dint of being along an object's prototype chain) of a given object.