code: function (element, options) {
      this.options = $.extend({}, Affix.DEFAULTS, options)
      this.$window = $(window)
        .on('scroll.bs.affix.data-api', $.proxy(this.checkPosition, this))
        .on('click.bs.affix.data-api',  $.proxy(this.checkPositionWithEventLoop, this))
  
      this.$element     = $(element)
      this.affixed      =
      this.unpin        =
      this.pinnedOffset = null
  
      this.checkPosition()
    }
docs: AFFIX CLASS DEFINITION ======================