javascript 判断某元素的id是否存在

<script type="text/javascript">
   var oCarouselBox=document.getElementById("moveie");
if(oCarouselBox){
alert(99);
}else{
alert(100);

}

var oCarouselBox=document.getElementById("moveie2");
if(oCarouselBox==null){
alert(88);
}else{
alert(100);
}

  </script>
 
  <div id="moveie"></div>