js使用定时器实现轮播图效果

<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
		<style type="text/css">
			* {
				margin: 0;
				padding: 0;
			}

			img {
				vertical-align: top;
				width: 960px;
				height: 320px;
			}

			ul {
				width: 4800px;
				list-style: none;
				overflow: hidden;
				position: absolute;
				left: 0px;
			}

			li {
				float: left;
			}

			.wrap {
				position: relative;
				width: 960px;
				height: 320px;
				border: 1px solid #00498B;
				overflow: hidden;
				margin: 20px auto;
			}

			.menu {
				position: absolute;
				left: 50%;
				bottom: 0;
				transform: translate(-50%);
			}

			.menu>span {
				cursor: pointer;
				display: inline-block;
				width: 30px;
				height: 30px;
				background-color: #0000FF;
				font-size: 20px;
				text-align: center;
				line-height: 30px;
				color: #fff;
			}

			.menu .active {
				background: red;
			}
		</style>
	</head>
	<body>
		<div class="wrap">
			<ul>
				<li><img src="./images/l-1.jpg" alt="t"></li>
				<li><img src="./images/l-2.jpg" alt="t"></li>
				<li><img src="./images/l-3.jpg" alt="t"></li>
				<li><img src="./images/l-4.jpg" alt="t"></li>
			</ul>
			<div class="menu">
				<span class="active">1</span>
				<span>2</span>
				<span>3</span>
				<span>4</span>
			</div>
		</div>
	</body>
</html>

window.onload = function() {
	//获取元素
	var ul = document.querySelector('ul');
    var liS = document.querySelectorAll('li');
	var span = document.querySelectorAll('span');

	for (var i = 0; i < span.length; i++) {
		span[i].id = i;
		span[i].onclick = function() {
            //让点击的序号背景色变红
			for(var i=0; i<span.length; i++){
				span[i].className = '';
			}
            this.className = 'active';
             //切换到对应下标的图片  
             // ul定位公式：-序号下标*图片宽度
			//var left = this.id * (-960);
            var left = -this.id * liS[0].clientWidth;
			move(ul,'left',20,left);
			
		}
	}

	//执行任意元素某个方向的移动动画 用法：move(元素,'属性',步长,终点);
	// move(ul, 'left', 20, 900);
	var timer = null; //定时器id
	function move(ele, attr, step, end) {
		//清除定时器防止叠加
		clearInterval(timer);
		//获取起点 处理步长的正负
		/*判断正负
			 正： 当前值   目标值
					0        500
			 负： 当前值    目标值
					500        0
		*/
		step = end > parseInt(getStyle(ele, attr)) ? step : -step;
		//定时器
		timer = setInterval(function() {
			//1.在当前值的基础上 + step
			var speed = parseInt(getStyle(ele, attr)) + step;
			//3.到达目标点停止
			if ((speed >= end && step > 0) || (speed <= end && step < 0)) {
				//往前走用的判断 step：正的
				speed = end;
				clearInterval(timer);
			}
			//2.移动的是ele的attr
			ele.style[attr] = speed + 'px';
		}, 20);
	}
	//获取任意元素的非行内样式 用法：getStyle(元素,'属性');
	function getStyle(ele, attr) {
		if (ele.currentStyle) { //ie8-
			return ele.currentStyle[attr];
		} else { //标准浏览器
			return getComputedStyle(ele)[attr];
		}
	}
}