js 实现弹性运动的简单应用----导航栏中弹性

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
<style>
    body,ul{margin:0;padding:0;}
    ul{width:400px;height:30px;position:relative;margin:20px auto;}
    li{width:98px;height:28px;border:1px solid black;float:left;line-height:28px;text-align:center;list-style:none;}
    .bg{width:100px;height:5px;background:red;position:absolute;left:0;bottom:0;overflow:hidden;border:none;}
</style>
<script>
    window.onload =function(){
        var oUl = document.getElementsByTagName('ul')[0];
        var arrLi = oUl.getElementsByTagName('li');
        var oBg = arrLi[arrLi.length-1];
        for(var i=0;i<arrLi.length-1;i++){
            arrLi[i].onmouseover = function(){
                elasticStartMove(oBg,this.offsetLeft);
            };
        }
        
    };

    var left = 0; /*必须放在外面，解决小数误差问题*/
    function elasticStartMove(obj,target){
        var speed = 0;
        clearInterval(obj.timer);
        obj.timer = setInterval(function(){
            speed += (target-obj.offsetLeft)/5;
            speed *= 0.75;
            left += speed;
            if(Math.abs(speed) < 1 && Math.abs(left-target) < 1){ /*停止的条件*/
                clearInterval(obj.timer);
                obj.style.left = target+'px';
            }
                
            obj.style.left=left+'px';
            document.title="speed:"+speed+" -  :"+Math.abs(left-target);
            
        },30);
    }
    /*
        弹性公式：
            速度 +=（目标值-oDiv.offsetLeft）/5
            速度*=0.7；
        不适用范围：对于有一些height不能取负数的不能使用    
    */
</script>
</head>

<body>
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    <li>首页</li>
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