获得 的高度 - 返回'undefined'

I have a problem getting the height of a (the one in the code below, with class="post_div"). And I want to return with a document.write whitin the HTML body - maybe later using it as height for other elements, that I want to be equal. I have tried every possible code for getting the height of such an element, but every time it returns 'undefined'. I have also tried setting height=auto for the DIV. I am almost sure my problem not has to do with the way I get the height, but have no other idea for what else it could be! Therefore I have choosen to bother you with the full structure of my coding. The JavaScript is placed in a separate document containing only the following statement:I have a problem getting the height of a (the o