What do you mean by "scroll area"?
Do you mean the part below the bottom of the viewport?
Do you mean white space at the bottom of a short document that doesn't fill the viewport?
BTW, the part that you see is called the "viewport", so keep that in mind when reading documentation.
What exactly are you trying to accomplish? Usually, when you want to get viewport width/height, there is a more modern solution in CSS that does not require any JS code.
flexbox and vh,vw units are the bees knees!