JavaScript实现鼠标拖拽Div左右滑动功能

<!DOCTYPE html>
<html lang="en">
<head>
 <meta charset="UTF-8">
 <title>Title</title>
 <script src="js/jquery.min.js"></script>
 <style>
  body{
   position: relative;
   margin:0;
   padding:0;
   width:100%;
   height: 1000px;
  }
  #box{
   height: 50px;
   width:200px;
   position: absolute;
   left:50%;
   top:50%;
   margin-left:-200px;
   margin-top:-200px;
   background: #CDCDCD;
  }
  #small-box{
   height: 50px;
   width:50px;
   position: absolute;
   left:0;
   top:0;
   background: #FF66CC;
   cursor:move ;
   opacity: 0.7;
  }
 </style>
</head>
<body>
 <div id="box">
  <div id="small-box"></div>
 </div>
<script>
 var box=$("#small-box");
 var body=$('body');
 var index=0;
 var x1;
 box.mousedown(function(){
  index=1;    //鼠标按下才能触发onmousemove方法
  var x=event.clientX;  //鼠标点击的坐标值，x
  var left= this.style.left;
  left=left.substr(0,left.length-2); //去掉px
  x1=parseInt(x-left);
 });
 box.mousemove(function(){
  if(index===1){
   this.style.left=event.clientX-x1+'px';
   if(this.style.left.substr(0,this.style.left.length-2)<0){
    this.style.left=0;
   };
   if(this.style.left.substr(0,this.style.left.length-2)>150){
    this.style.left='150px';
   };
  }
 });
 box.mouseup(function(){
  index=0;
 });
 body.mouseup(function(){
  index=0;
 });
</script>
</body>
</html>