jQuery纵向导航菜单效果实现方法

$( document ).ready( function(e){
  var $catCOnt= $( ".cat-cont" );  //二级菜单div
  var $catList = $( ".J_Cat" );    //一级菜单li
  $catList.on( "mouseenter", function(){
    var index = $( this ).index();
    var $curCatList = $( ".cat-cont-bd>li:eq(" + index + ")" );     //鼠标移上去对应的二级菜单的li
    $catList.removeClass( "selected selected-prev" );
    $( this ).addClass( "selected" ).prev().addClass( "selected-prev" );
    $catCont.show();
    $curCatList.css( "display", "list-item").siblings().css( "display", "none" );
    var viewHeight = $( window ).height();
    var catOffsetTop = $( this ).offset().top - $( window ).scrollTop();
    var catBottomGap = viewHeight - catOffsetTop;
    var catPositiOnTop= $( this ).position().top;
    var catCOntHeight= $catCont.height();
    if( catBottomGap >= catContHeight ) {
      $catCont.css( "top", catPositionTop );
    }
    if( catBottomGap = catContHeight ) {
      $catCont.css( "top", catPositionTop - ( catContHeight - catBottomGap ) - 20 );
    }
    if( catBottomGap