Question

$(document).ready(function(){

  function getDocumentHeight()
  {
     // What's the page height?
     var height = $('#mainbody').height();
     alert(height);
   ...
 getDocumentHeight();  }

jQuery keeps alerting null for the height of my #mainbody div. I don’t get why, it should at least be 500px or so.

Answer 1

If the content is being loaded dynmically, the elements may not have a height yet. Try moving this code to the bottom of the page, directly above and see if that resolves.

UPDATE:
Try placing your code in

$(window).load(function() {
  var height = $('#mainbody').height();
  alert(height);
});

outside of your

$(document).ready();