code: function backdrop ( callback ) {
      var that = this
        , animate = this.$element.hasClass('fade') ? 'fade' : ''
      if ( this.isShown && this.settings.backdrop ) {
        this.$backdrop = $('<div class="modal-backdrop ' + animate + '" />')
          .click($.proxy(this.hide, this))
          .appendTo(document.body)
  
        setTimeout(function () {
          that.$backdrop && that.$backdrop.addClass('in')
          $.support.transition && that.$backdrop.hasClass('fade') ?
            that.$backdrop.one(transitionEnd, callback) :
            callback()
        })
      } else if ( !this.isShown && this.$backdrop ) {
        this.$backdrop.removeClass('in')
  
        function removeElement() {
          that.$backdrop.remove()
          that.$backdrop = null
        }
  
        $.support.transition && this.$element.hasClass('fade')?
          this.$backdrop.one(transitionEnd, removeElement) :
          removeElement()
      } else if ( callback ) {
         callback()
      }
    }
docs: /* MODAL PRIVATE METHODS
  =====================