bootstrap.js

    return (this.pinnedOffset = position.top - scrollTop)
  }

  Affix.prototype.checkPositionWithEventLoop = function () {
    setTimeout($.proxy(this.checkPosition, this), 1)
  }

  Affix.prototype.checkPosition = function () {
    if (!this.$element.is(':visible')) return

    var scrollHeight = $(document).height()
    var scrollTop    = this.$target.scrollTop()
    var position     = this.$element.offset()
    var offset       = this.options.offset
    var offsetTop    = offset.top
    var offsetBottom = offset.bottom

    if (typeof offset != 'object')         offsetBottom = offsetTop = offset
    if (typeof offsetTop == 'function')    offsetTop    = offset.top(this.$element)
    if (typeof offsetBottom == 'function') offsetBottom = offset.bottom(this.$element)

    var affix = this.unpin   != null && (scrollTop + this.unpin <= position.top) ? false :
                offsetBottom != null && (position.top + this.$element.height() >= scrollHeight - offsetBottom) ? 'bottom' :
                offsetTop    != null && (scrollTop <= offsetTop) ? 'top' : false

    if (this.affixed === affix) return
    if (this.unpin != null) this.$element.css('top', '')

    var affixType = 'affix' + (affix ? '-' + affix : '')
    var e         = $.Event(affixType + '.bs.affix')

    this.$element.trigger(e)

    if (e.isDefaultPrevented()) return

    this.affixed = affix
    this.unpin = affix == 'bottom' ? this.getPinnedOffset() : null

    this.$element
      .removeClass(Affix.RESET)
      .addClass(affixType)
      .trigger($.Event(affixType.replace('affix', 'affixed')))

    if (affix == 'bottom') {
      this.$element.offset({
        top: scrollHeight - this.$element.height() - offsetBottom
      })
    }
  }


  // AFFIX PLUGIN DEFINITION
  // =======================

  function Plugin(option) {
    return this.each(function () {
      var $this   = $(this)
      var data    = $this.data('bs.affix')
      var options = typeof option == 'object' && option

      if (!data) $this.data('bs.affix', (data = new Affix(this, options)))
      if (typeof option == 'string') data[option]()
    })
  }

  var old = $.fn.affix

  $.fn.affix             = Plugin
  $.fn.affix.Constructor = Affix


  // AFFIX NO CONFLICT
  // =================

  $.fn.affix.noConflict = function () {
    $.fn.affix = old
    return this
  }


  // AFFIX DATA-API
  // ==============

  $(window).on('load', function () {
    $('[data-spy="affix"]').each(function () {
      var $spy = $(this)
      var data = $spy.data()

      data.offset = data.offset || {}

      if (data.offsetBottom) data.offset.bottom = data.offsetBottom
      if (data.offsetTop)    data.offset.top    = data.offsetTop

      Plugin.call($spy, data)
    })
  })

}(jQuery);