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How to get the new size of a dynamically sized div?

I have a div that slides up from the bottom of my pagewhen a button is clicked. i do this using a css transition and changing the css "top" attribute of the div. works fine if the div size never changes. So for example if the div is 400px high, i just move it up 400px and it's now in position. cool.
BUT... what if the div has dynamically generated content and will be a different height every time? how can i figure out how much to move the div up in order to be 100% showing?
so in pseudo code i want something like
function movemydiv() {
var howMuchToMoveIt = ??? (somehow getting the dynamic containers height)
document.getelementbyId("mydiv").style.top = bottomOfScreen - howMuchToMoveIt
any tips on most straightforward way to do this??

You can use either clientHeight or offsetHeight to measure the height of your div.
Both clientHeight and offSetHeight include padding , but offsetHeight will also take into account borders and horizontal scrollbars (if rendered) - see MDN link.
So your js would be something like:
var howMuchToMoveIt = document.getElementById('mydiv').clientHeight;
The var will then contain the height of your element.
Hope this helps

What is the difference between offsetHeight and scrollHeight of an element in DOM?

In the DOM, what is the difference between an element’s offsetHeight and its scrollHeight? An image in explanation would be a great help.

HTMLElement.offsetHeight is a measurement which includes the element borders, the element vertical padding, the element horizontal scrollbar (if present, if rendered) and the element CSS height.
HTMLElement.scrollHeight is a measurement of the height of an element's content including content not visible on the screen due to overflow. The value returned by HTMLElement.scrollHeight WILL include the padding-top and padding-bottom, but will NOT include the element borders or the element horizontal scrollbar.
This page and this page are my sources.
The MDN documentation also provides images to demonstrate.

As #Csarsam said, offset height is the border-box height (I'm rewording). Scroll height, is the height of the scrollable content, which is generally composed of multiple elements. But scroll height it also defined on elements which does not scroll, hence does not have a scrollable content, in which case (I’ve checked but I have no reference to back it up) scroll height is its content height, that is, it does not include the margins and borders. But when the element is part of a scrollable content, its margin are taken into account to compute the scroll height of its parent.
Scroll height is defined on both scrollable content and non‑scrollable content, that’s what may confuse.
Update
Here is a reference which confirms what I’ve checked: https://developer.mozilla.org/en-US/docs/Web/API/Element/scrollHeight

Getting a div's "scrollWidth" when it has an absolute positioned child-div

I have a problem getting the width of a div's content (of the content, not the div itself).
The usual approach would be Javascript's scrollWidth property, I think.
The problem: within this div, another div is positioned absolute and has a negative right value (-350px). I can't change this (it's a menu, sliding in when you click a button, overlapping other elements. It needs to be positioned like that).
The scrollWidth returns the width of the outer div's content PLUS the negative right-value (in Chrome, didn't test other browsers).
Here's a short example:
/* ... */
http://jsfiddle.net/R4Cs5/12/
But I need the content's width that is accessible by scrollbars.
Any ideas?
Please use Jquery, no plain Javascript.
Thanks in advance.

I see that your jsfiddle doesn't import any jQuery library, while you wanted to use it. Anyway, with jQuery you can use .width to get an element's width see here: jsfiddle.

How come these two alerts showing the height and offsetheight of the same element don't display the same number?

the alerts are as follows:
alert(document.getElementById("col_st_scroll").offsetHeight);
alert(document.getElementById("col_st_scroll").style.height);
now, the col_st_scroll element is fully visible in the web browser, shouldn't that mean offsetHeight and style.height should returnt he same value?
problem solved. The element was resized by another function in the time it took to click ok ont he first alert.

This is from MDN:
Typically, an element's offsetHeight is a measurement which includes the element borders, the element vertical padding, the element horizontal scrollbar (if present, if rendered) and the element CSS height.
The style.height property represents the content height only (assuming you're on "strict" rendering mode, not "quirks").

Why document.body.scrollHeight can be greater than document.body.offsetHeight

I need help with this two elements properties.
According MDN element.scrollHeight "this a height of the scroll view of an element; it includes the element padding but not its margin", and element.offsetHeight "Typically, an element's offsetHeight is a measurement which includes the element borders, the element vertical padding, the element horizontal scrollbar (if present, if rendered) and the element CSS height."
I am trying to debug some javascript code and don't understand why document.body.scrollHeight can be greater than document.body.offsetHeight?
For example, my document.body.offsetHeight=909, but document.body.scrollHeight=1059 (body don't have any margins or paddings or borders), so we lost 150px somewhere. I inspected body and it height=909, and this is very confuse me. This reproduced in chrome and firefox.
Can you please help me in this question?

The offsetHeight property describes how far from the top of the current available space in the active window. The scrollHeight property is how far in pixels from the inside top of a contain to the inside bottom, which is different than clientHeight on a container set to a limited height with overflow:scroll css property.
I am using most of these in project I am working on at http://prettydiff.com/jsgui/