How can I get an object's absolute position on the page in Javascript?

I want to get an object's absolute x,y position on the page in Javascript. How can I do this?

I tried obj.offsetTop and obj.offsetLeft, but those only give the position relative to the parent element. I guess I could loop through and add the parent's offset, and its parent's offset, and so on until I get to the object with no parent, but it seems like there should be a better way. Googling didn't turn up much, and even SO site search isn't finding anything.

Also, I can't use jQuery.