Variable

lastName

declared by the keyword

let

is not added to the global

window

object. That is why the variable is equal to

undefined

// Declaring variable using keyword "var"
var firstName = "Jet"

// Declaring variable using keyword "let"
let lastName = "Li"

// Declaring variable without using any keyword
carMark = "Tesla"

function driveCar() {
  console.log(this.firstName + ' ' + this.lastName + ' is driving a car '+this.carMark)
}

driveCar()

The result will be:

Jet undefined is driving a car Tesla

Because declaring variable with

var

keyword and without any declaration keyword are the same. Both add the variable to the global

window

object. After running the above code, global object

window

will have

firstName

and

lastName

attributes

window.firstName;  // "Jet"
window.lastName;  // undefined
window.carMark;  // "Tesla"