this

inside

fn()

function refers to the global

window

object. That is why

firstName

and

lastName

attributes are not accessible using

this

keyword

let john = {
  firstName: 'John',
  lastName: 'Doe',
  driveCar() {
    function fn () {
      console.log(this.firstName + ' ' + this.lastName + ' is driving a car');  // "undefined undefined is driving a car"
      console.log(this);
    }
    fn(); // fn.call(this);
    console.log(this.firstName + ' ' + this.lastName + ' is driving a car'); 
    console.log(this)
  }
}

john.driveCar(); 

In order to fix the issue, one of the ways is to use

call

function and pass the object that we want

this

to refer to inside the

fn()

function

fn.call(this);

Inside

driveCar()

function

this

refers to the

john

object because it is called through

john

object

john.driveCar();