javascript for having a sidebar element follow you as you scroll -- behavior modeled after facebook sidebar

followScroll.js

var wrapperId = 'sidebarWrapperIdOnPage';

var
  $w = $(window),
  $wrap = $('#'+wrapperId),
  followerInitialOffset = $wrap.offset().top,
  marginTop = 0,
  $wrap, windowTop, windowBottom, followerHeight,
  followerTop, followerBottom, followerBottomPosition
;
$w.scroll(function() {
  windowHeight = $w.height();
  windowTop = $w.scrollTop();
  windowBottom = windowTop + windowHeight;
  followerHeight = $wrap.height();
  followerTop = $wrap.offset().top;
  followerBottom = followerTop + followerHeight;
  if (windowHeight > followerHeight || windowTop < followerTop) {
    marginTop = Math.max(windowTop - followerInitialOffset, 0);
  } else if (windowBottom > followerBottom) {
    marginTop = windowBottom - followerHeight - followerInitialOffset;
  }
  $wrap.css({'margin-top': marginTop});
});