alert(document); // [object HTMLDocument]

Or:

alert(document.constructor.name); // HTMLDocument

So, document is an instance of HTMLDocument class.

What’s its place in the hierarchy?

Yeah, we could browse the specification, but it would be faster to figure out manually.

Let’s traverse the prototype chain via __proto__.

As we know, methods of a class are in the prototype of the constructor. For instance, HTMLDocument.prototype has methods for documents.

Also, there’s a reference to the constructor function inside the prototype:

alert(HTMLDocument.prototype.constructor === HTMLDocument); // true

To get a name of the class as a string, we can use constructor.name. Let’s do it for the whole document prototype chain, till class Node:

alert(HTMLDocument.prototype.constructor.name); // HTMLDocument
alert(HTMLDocument.prototype.__proto__.constructor.name); // Document
alert(HTMLDocument.prototype.__proto__.__proto__.constructor.name); // Node

That’s the hierarchy.

We also could examine the object using console.dir(document) and see these names by opening __proto__. The console takes them from constructor internally.