I'm getting the values undefined even though it's populated. I think I've already populated it but feels like something is going wrong with the object's instance. That'd be great if someone can explain that where I'm going wrong.

```js
function Student(fn, sn, a, d)
{
  this.firstName = fn;
  this.lastName = sn;
  this.age = a;
  this.degree = d;
  this.displayStudent = displayStudent;
}

function displayStudent()
{
  console.log(this.fn);
  console.log(this.sn);
  console.log(this.a);
  console.log(this.d);
}

var studentObj = new Student("d", "r", 20,
                         "bachelors of science");
studentObj.displayStudent();
```