jquery: $(window).scrollTop() but no $(window).scrollBottom()

I would say that a scrollBottom as a direct opposite of scrollTop should be:

var scrollBottom = $(document).height() - $(window).height() - $(window).scrollTop();

Here is a small ugly test that works for me:

// SCROLLTESTER START //
$('<h1 id="st" style="position: fixed; right: 25px; bottom: 25px;"></h1>').insertAfter('body');

$(window).scroll(function () {
  var st = $(window).scrollTop();
  var scrollBottom = $(document).height() - $(window).height() - $(window).scrollTop();

  $('#st').replaceWith('<h1 id="st" style="position: fixed; right: 25px; bottom: 25px;">scrollTop: ' + st + '<br>scrollBottom: ' + scrollBottom + '</h1>');
});
// SCROLLTESTER END //

var scrollBottom = $(window).scrollTop() + $(window).height();

For the future, I've made scrollBottom into a jquery plugin, usable in the same way that scrollTop is (i.e. you can set a number and it will scroll that amount from the bottom of the page and return the number of pixels from the bottom of the page, or, return the number of pixels from the bottom of the page if no number is provided)

$.fn.scrollBottom = function(scroll){
  if(typeof scroll === 'number'){
    window.scrollTo(0,$(document).height() - $(window).height() - scroll);
    return $(document).height() - $(window).height() - scroll;
  } else {
    return $(document).height() - $(window).height() - $(window).scrollTop();
  }
}
//Basic Usage
$(window).scrollBottom(500);

var scrollBottom =
    $(document).height() - $(window).height() - $(window).scrollTop();

I think it is better to get bottom scroll.