无缝滚动

<div id="outer">
	<div id="inner">
		<div id="demo1"></div>
		<div id="demo2"></div>
	</div>
</div>
<input type="button" name="" id="" value="向左" />
<input type="button" name="" id="" value="向右" />

*{
	margin: 0;
	padding: 0;
}
img{
	width: 500px;
	height: 310px;
	float: left;
}
#outer{
	margin: 50px auto;
	width: 500px;
	height: 310px;
	border: 1px solid #000;
	overflow: hidden;
}
#inner{
	width: 6000px;
	height: 310px;
}
#inner div{
	float: left;
}

var imgArray=["https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1511167162133&di=fe27f741ca0c915199d4576a8ffac946&imgtype=0&src=http%3A%2F%2Fwww.ywga.gov.cn%2Fjwgk%2Fsyzc%2F201703%2FW020170331331041430290.jpg",
			"https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1511166484577&di=f000cde0483941d947a1628cc91b1c49&imgtype=0&src=http%3A%2F%2Fimg18.3lian.com%2Fd%2Ffile%2F201706%2F05%2F962ba7f592396f12cab97a9490031671.jpg",
			"https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1511167245165&di=b67c232671c9018f44c7bd2c158dc0a5&imgtype=0&src=http%3A%2F%2Fuploads.xuexila.com%2Fallimg%2F1703%2F1017-1F302110K6.jpg",
			"https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1511167262450&di=defa4f8b1f2cf03229473b93bd0b4713&imgtype=0&src=http%3A%2F%2Fww2.sinaimg.cn%2Flarge%2Fd75e3906jw1eiwxjge3zxj20t50iggqq.jpg",
			"https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1511167297748&di=5303510e2607fd41f607236ce043e475&imgtype=0&src=http%3A%2F%2Fimg2.zol.com.cn%2Fproduct%2F104%2F642%2FceqOwlYvIkPSk.jpg",
			"https://timgsa.baidu.com/timg?image&quality=80&size=b10000_10000&sec=1511157232&di=1c3c867d3eb753a7a098304afc0ed891&src=http://rescdn.qqmail.com/dyimg/20140807/74DA11875739.jpg"
];
var len=imgArray.length;
var dOuter=document.getElementById("outer");
var demo1=document.getElementById("demo1");
var demo2=document.getElementById("demo2"); 
var btn=document.getElementsByTagName("input");
var timer=null;
//页面初始化
for (var i=0;i<len;i++) {
	demo1.innerHTML+="<img src=‘"+imgArray[i]+"‘ />";
}
demo2.innerHTML=demo1.innerHTML;
dOuter.onOff=true;
//鼠标移入
dOuter.onmouseover=function(){
	clearInterval(timer);
}
//鼠标移出
dOuter.onmouseout=function(){
	if(this.onOff){
		clock();	
	}else{
		clock2();
	}
}
//鼠标点击按钮切换不同的运动方式
//向左运动
btn[0].onclick=function(){
	dOuter.onOff=true;
	clock();
}
btn[1].onclick=function(){
	dOuter.onOff=false;
	clock2();
}

setTimeout(clock,1500);
//向左运动
function clock(){
	clearInterval(timer);
	timer=setInterval(moveLft,10);
}


//向右运动
function clock2(){
	clearInterval(timer);
	timer=setInterval(moveRight,10);
}


//向左运动
function moveLft(){
	//容器向左滚动的距离
	dOuter.scrollLeft+=2;
	//判断临界值
	if(dOuter.scrollLeft>=demo1.offsetWidth){
		dOuter.scrollLeft=0;
	}
	//每一幅图停顿一次
	if(dOuter.scrollLeft%500==0){
		clearInterval(timer);
		timer=setTimeout(clock,1500);
	}
}


//向右运动
function moveRight(){
	dOuter.scrollLeft-=2;
	if(dOuter.scrollLeft<=0){
		dOuter.scrollLeft=demo1.offsetWidth;
	}
	if(dOuter.scrollLeft%500==0){
		clearInterval(timer);
		timer=setTimeout(clock2,1500)
	}
}