<script>
    var sub_classify = document.getElementsByClassName('sub_classify')[0];
    window.onscroll = function(){
       var mTop = sub_classify.offsetTop;
        //减去滚动条的高度
          var sTop = document.body.scrollTop;
          console.log('stop-->',sTop);
//        console.log('mtop-->',mTop);
        var result = mTop - sTop; 
         if(result<=50){
          sub_classify.style.position = "fixed";
          sub_classify.style.top = "50px"
        }
        if(sTop<365){
          sub_classify.removeAttribute("style");
        }
    }
  </script>