Pure JS to realize the 3D display effect of rotating pictures_javascript skills

<style type="text/css">
#show{position:relative;margin:20px auto;width:800px;}
.item{position:absolute;height:40px;width:60px;background:#999999;border:1px solid #eeeeee;cursor:pointer;}</style>

input id="l" type="button" value="left" /> <input id="r" type="button" value="right" />
<div id="show">
<div class="item">1</div>
<div class="item">2</div>
<div class="item">3</div>
<div class="item">4</div>
<div class="item">5</div>
<div class="item">6</div>
<div class="item">7</div>
<div class="item">8</div>
<div class="item">9</div>
<div class="item">0</div>
<div class="item">a</div>
<div class="item">b</div>
<div class="item">1</div>
<div class="item">2</div>
<div class="item">3</div>
</div>

<script>
var len;
var showerObj;
var listObj;
var showerWidth=800;
var showerHeight=400;
var r;
var cR=0;
var ccR=0;
var timer=0;
window.onload=function(){
 showerObj=document.getElementById("show");
 listObj=showerObj.getElementsByTagName("div");
 len=listObj.length;
 r=Math.PI/180*360/len;
 for(var i=0;i<len;i++){
 var item=listObj[i];
 item.style.top=showerHeight/2+Math.sin(r*i)*showerWidth/2-20+"px";
 item.style.left=showerWidth/2+Math.cos(r*i)*showerWidth/2-30+"px";
 item.rotate=(r*i+2*Math.PI)%(2*Math.PI);
 item.onclick=function(){
  cR=Math.PI/2-this.rotate;
  timer || (timer=setInterval(rotate,10));
  
 }
 
 }
 var rX=showerObj.offsetLeft+showerWidth/2;
 var ry=showerObj.offsetTop+showerHeight/2;
 
 var rotate=function(){
 ccR=(ccR+2*Math.PI)%(2*Math.PI);
 if(cR-ccR<0) cR=cR+2*Math.PI;
 if(cR-ccR<Math.PI){
  ccR=ccR+(cR-ccR)/19;
 }else{
  ccR=ccR-(2*Math.PI+ccR-cR)/19;
 
 }
 
 if(Math.abs((cR+2*Math.PI)%(2*Math.PI)-(ccR+2*Math.PI)%(2*Math.PI))<Math.PI/720){
  ccR=cR;
  clearInterval(timer);
  timer=0;
 }
 
 for(var i=0;i<len;i++){
  var item=listObj[i];
  var w,h;
  var sinR=Math.sin(r*i+ccR);
  var cosR=Math.cos(r*i+ccR);
  w=60+0.6*60*sinR;
  h=(40+0.6*40*sinR);
  item.style.cssText +=";width:"+w+"px;height:"+h+"px;top:"+parseInt(showerHeight/2+sinR*showerWidth/2/3-w/2)+"px;left:"+parseInt(showerWidth/2+cosR*showerWidth/2-h/2)+"px;z-index:"+parseInt(showerHeight/2+sinR*showerWidth/2/3-w/2)+";";
 
 }
 }
 
 document.getElementById("l").onclick=function(){
 cR=(cR+r+2*Math.PI)%(2*Math.PI);
 timer || (timer=setInterval(rotate,10));
 }
 document.getElementById("r").onclick=function(){
 cR=(cR-r+2*Math.PI)%(2*Math.PI);
 timer || (timer=setInterval(rotate,10));
 }
 rotate();
}
</script>