nodeName is the attribute you are looking for. For example:
var elt = document.getElementById('foo');
Note that nodeName returns the element name capitalized and without the angle brackets, which means that if you want to check if an element is an <div> element you could do it as follows:
elt.nodeName == "DIV"
While this would not give you the expected results:
elt.nodeName == "<div>"
Hope it helps!!