鼠标滑动一定距离的左侧菜单置顶效果

/*鼠标滑动一定距离的菜单效果*/
$(document).scroll(function() {
    var top=$(this).scrollTop();
    console.log(top);

    if(top>90) {
        $(".sidebar").css("top","0");
    }
    if(top<90) {
        $(".sidebar").css("top","90px");
    }
})

.sidebar {
  font-size: 14px;
  cursor: pointer;
  position: fixed;
  top: 90px;
  bottom: 0;
  left: 0;
  z-index: 1;
  display: block;
  overflow-x: hidden;
  overflow-y: auto; 
  background: #E9F4F8;
  padding: 10px;
}