Finding the Node's DOM Context

The DOM models an HTML document as a tree. The root of the tree is the document object. The node representing the root is document.documentElement. Internet Explorer supports a new relationship: the document context of a node. Each tag you define in an HTML file (each tag is modeled by a node on the DOM tree), belongs to a certain document object. This ownership relationship is given by the ownerDocument property. If we model the whole document by a single DOM tree, the ownerDocument property is a direct pointer from any node on the DOM tree to the root of the tree. The following expression should return the root of the DOM tree:

document.documentElement.ownerDocument

Let's print the node name (tag name), so it's easier to recognize which node it is. You should get HTML. For printing the root's properties, always go the root node by using the documentElement property. The printing above is actually done via:

alert(document.documentElement.ownerDocument.
  documentElement.nodeName);

Now, let's go deeper in the tree, to the document's first child (HEAD):

document.documentElement.firstChild

And then let's go directly back to the root of the DOM tree:

document.documentElement.firstChild.ownerDocument

As previously shown, let's print the node name. Again, we go to the root node by the documentElement property:

alert(document.documentElement.firstChild.
  ownerDocument.documentElement.nodeName);

You should get HTML.

Next: A Final Word