how do i get the y potition of an image in javaScript? (in relation to the document)

how do i get the y potition of an image in javaScript? (in relation to the
document)

I am trying to dynamically get the y position of an image in JavaScript.
In relation to the screen/document, that is. How can i do that? is there
anything like in AC3 where you can do: getPropery(obj,_x)?