Difference between Document and document

Question

I had thought Document and document was always the same.

But the console shows, Document is a function in chrome,

while document is a object. Why?

Why is the former used?

I've searched SO, but I didn't get the answer and I googled, but still I didn't get my answer.

And how to use the Document since it is a function?

Answer 1

Document is the constructor for an HTML document in Chrome.

document is the instance of Document for the current window.