无法tagname到div？？

// 封装一个代替getElementById()的方法

function byId(id){

return typeof(id)==="string"?document.getElementById(id):id;

}

// 全局变量

var index = 0,

timer = null,

pics = byId("banner").getElementsByTagName("div"),

len = pics.length;

function slideImg(){

var main = byId("main");

// 滑过清除定时器，离开继续

main.onmouseover = function(){

// 滑过清除定时器

}

main.onmouseout = function(){

timer = setInterval(function(){

index++; // len=3 0 1 2

if(index>=len){

index = 0;

}

},3000);

}

slideImg();

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>banner轮播</title>
<link rel="stylesheet" type="text/css" href="css/style.css">
</head>
<body>
<div class="main" id="main">
<!-- 图片轮播 -->
<div class="banner" id="banner">
<a href="">
<div class="banner-slide slide1 slide-active"></div>
</a>
<a href="">
<div class="banner-slide slide2"></div>
</a>
<a href="">
<div class="banner-slide slide3"></div>
</a>
</div>
<!-- 上一张、下一张按钮 -->
<a href="javascript:viod(0)" class="button prev"></a>
<a href="javascript:viod(0)" class="button next"></a>
<div class="dots">
<span class="active"></span>
<span></span>
<span></span>
</div>
</div>
<script type="text/javascript" src="js/script.js"></script>
</body>
</html>

  *{
  	margin: 0;
  	padding:0;
  }
 ul{
 	list-style: none;
 }

 body{
 	font-family: "微软雅黑";
 	color: #14191e;
 }

 .main{
 	width: 1200px;
 	height: 460px;
 	margin: 30px auto;
 	overflow: hidden;
 	position: relative;
 }

 .banner{
 	width: 1200px;
 	height: 460px;
 	position: relative;
 	overflow: hidden;
 }
 .banner-slide{
 	width: 1200px;
 	height: 460px;
 	position: absolute;
 	background-repeat: no-repeat;
 	display: none;
 }

 .slide-active{
 	display: block;
 }

 .slide1{
 	background-image:url(../img/bg1.jpg);

 }

  .slide2{
 	background-image:url(../img/bg2.jpg);
 }
  .slide3{
 	background-image:url(../img/bg3.jpg);
 }

.button{
	position: absolute;
	width: 40px;
	height: 80px;
	left:244px;
	top: 50%;
	margin-top: -40px;
	background: url(../img/arrow.png) no-repeat center center;
}

.button:hover{
	background-color: #333;
	opacity: 0.2;
	/*兼容其他浏览器*/
	filter: alpha(opacity=20);
}
.next{
	left: auto;
	right: 0;
}

.prev{
	transform: rotate(180deg);
}

.dots{
	position: absolute;
	right:20px;
	bottom: 24px;
	cursor: pointer;
}

.dots span{
	display: inline-block;
	width: 12px;
	height: 12px;
	background: rgba(7,17,27,0.4);
	border-radius: 50%;
	margin-left: 8px;
	border: 2px solid #fff;

}

.dots span.active{
	border: 2px solid rgba(7,17,27,0.4) ;
	background: #fff;
}

OlafChou 2017-07-29

源自：项目：JS实现轮播特效 3-3 轮播图之定时器设置

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