跑马灯

//开始不跑，原因是函数必须放在调用函数之前！！！
<!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>
    <title>练习</title>
    <script type="text/javascript">
        onload = function () {
            var txt = document.getElementById("txt");
            var txt2 = txt.innerHTML.trim();
            // alert(txt.innerHTML.trim().charAt(0));
            //alert(txt.innerHTML.trim().substr(0,2));
　　　　　　　//函数放在前面！！
            var paoMaDeng = function () {
　　　　　　　　//两个获取字符的方法，charAt(0)和substr(0,1)
                var first = txt2.charAt(0);
                var rest = txt2.substr(1);
                txt2 = rest + first;
                txt.innerHTML = txt2;

            }
　　　　　　　　//调用放在后面！！！
            aaa = setInterval(paoMaDeng, 100);
        }

       
    </script>
</head>
<body>
    <p id="txt">
        北京欢迎你！</p>
        <input type = "button" value = "停止" id = "btn" onclick="clearInterval(aaa);"/>
</body>
</html>