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We can see which class it belongs by outputting it, like:
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
For built-in classes in all prototypes there's a constructor reference, and we can get constructor.name to see the name of the class. Let's do it for all objects in the document prototype chain:
alert(HTMLDocument.prototype.constructor.name); // HTMLDocument
alert(HTMLDocument.prototype.__proto__.constructor.name); // Document
alert(HTMLDocument.prototype.__proto__.__proto__.constructor.name); // Node
We also could examine the object using console.dir(document) and see these names by opening __proto__. The console takes them from constructor internally.