Implementing custom scroll bar with native js

<div id="div">
 <div id="div1">
 <div id="div2"> </div>
 </div>
 <div id="div3">
 <div id="div4">
 <p>CSS3 Tutorial</p>
 <p>CSS3 Tutorial</p>
 <p>Introduction to CSS3</p>
 <p>CSS3 border</p>
 <p>CSS3 rounded corners</p>
 <p>CSS3 background</p>
 <p>CSS3 Gradients</p>
 <p>CSS3 text effects</p>
 <p>CSS3 fonts</p>
 <p>CSS3 2D Transformations</p>
 CSS3 3D Transformations
 <p>CSS3 Transitions</p>
 <p>CSS3 Animation</p>
 <p>CSS3 Multiple Columns</p>
 <p>CSS3 User Interface</p>
 <p>CSS3 images</p>
 <p>CSS3 Button</p>
 <p>CSS3 Pagination</p>
 <p>CSS3 box size</p>
 <p>CSS3 Flexible Box</p>
 <p>CSS3 Multimedia Queries</p>
 <p>CSS3 multimedia query example</p>
 </div>
 </div>
</div>

*{padding: 0; margin: 0;}
#div{top:200px;left:25%;width: 50%;height: 300px; position: absolute; 
}
#div1{width: 20px; height: 300px; position: relative; 
background: #CCCCCC; border-radius: 28px; float: right; cursor: pointer;}
#div1 #div2{left: -4px;width: 28px;height: 28px;border-radius: 50%; background: red;
position: absolute;}

#div3{width: 90%; height: 300px; border: 2px solid #CCCCCC;
position: relative; float: left; overflow: hidden;}
#div3 #div4{top:0;left:0;width: 100%; position: absolute; font-family: "Microsoft YaHei";
font-size: 19px; letter-spacing: 1px; padding: 3px 6px;}

window.onload = function(){
 var allDiv = document.getElementById('div');
 var oDiv = document.getElementById('div2');
 var aDiv = document.getElementById('div1');
 var textDiv1 = document.getElementById('div3');
 var textDiv2 = document.getElementById('div4');
 
 // The progress bar is dragged, and the content follows the motion event oDiv.onmousedown = function (ev) {
 var oEvent =ev||event;
 
 var disY =oEvent.clientY -oDiv.offsetTop;
 
 if (oDiv.setCapture) {
 oDiv.onmousemove =mouseMove;
 oDiv.onmouseup =mouseUp;
 
 oDiv.setCapture();
 }else{
 document.onmousemove =mouseMove;
 document.onmouseup =mouseUp;
 }
 
 function mouseMove(ev){
 var oEvent =ev||event; 
 var t =oEvent.clientY -disY; 
 var bottomLine = aDiv.offsetHeight - oDiv.offsetHeight; 
 
 
 if(t < 0){
 t =0;
 }else if(t >bottomLine){
 t = bottomLine;
 }
 
  var percent =t/272;
  
 oDiv.style.top = t+'px';
 textDiv2.style.top =-(textDiv2.offsetHeight-textDiv1.offsetHeight)*percent+'px';
 
 };
 
 function mouseUp(){
 this.onmousemove =null;
 this.onmouseup =null;
 
 if (oDiv.releaseCapture) {
 oDiv.releaseCapture();
 }
 };
 
 return false;
 };
 
 // Click the progress bar to start the timer. When the ball reaches the target at a constant speed, clear the timer aDiv.onmousedown=function(ev){
 var oEvent =ev||event; 
 var divY =oEvent.clientY-allDiv.offsetTop;
 var timer =null;var speed=10;
 
 
 clearInterval(timer)
 timer = setInterval(function(){
 var percent=oDiv.offsetTop/272;
 
 
 if(oDiv.offsetTop<divY-28){
 oDiv.style.top = oDiv.offsetTop + speed + 'px';
 textDiv2.style.top = -(textDiv2.offsetHeight-textDiv1.offsetHeight)*percent + 'px';
 }else if(oDiv.offsetTop>divY){
 oDiv.style.top = oDiv.offsetTop - speed + 'px';
 textDiv2.style.top = -(textDiv2.offsetHeight-textDiv1.offsetHeight)*percent + 'px';
 }else if(oDiv.offsetTop>260){
 oDiv.offsetTop = 272+'px';
 clearInterval(timer);
 }else if(oDiv.offsetTop<10){
 oDiv.offsetTop = 0+'px';
 clearInterval(timer);
 }else{
 clearInterval(timer);
 }
 
 
 },10);
 
 }
 
}