                        课程
                    
                        /前端开发
                        
                            /JavaScript
                        
                        /网页广告特效

我成功的实现了向下展开，但画面此时应该向上隐藏才对，但我的画面一起在抖动，请问这是怎么回事呢？

 <script>
        window.onload = function() {
            var oAd = document.getElementById('ad');
            var oAction = document.getElementById('action');

            var h = 0;
            var maxH = oAction.height;
            console.log(maxH);
            var timer;
            var step = 5; //变化程度都一样的
            function adDrop() {
                oAd.style.height = h + 'px';
                oAd.style.display = 'block';
                if( h < maxH){
                    h += step;
//                    oAd.style.height = h + 'px';
//                    oAd.style.display = 'block';
                     setInterval(adDrop, 500);
                }
                else {
//                    oAd.style.height = h + 'px';
//                    oAd.style.display = 'block';
                    setTimeout(adUp, 2000);
                }
            }

            function adUp() {
                if(h>0){  //有值时，已经判断过h 是》 = maxH
                    h -= step;
                    oAd.style.height = h + 'px';
                    oAd.style.display = 'none'
                    setInterval(adUp, 1);
                }

            }

            setTimeout(adDrop, 2000);



        }
    </script>
</head>

<body>
<div id="ad"><img src="img/1.jpg" alt="" id="action" ></div>
<div class="content">网页内容</div>


</body>