求帮忙！该怎么解决 _JavaScript

求帮忙！！！
<!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=gb2312" />
<title>无标题文档</title>
</head>

<body>
<script language="javascript">
var seq=0;
function type(){
var msg="I am very good";
var len=msg.length;
var output;
output=msg.substring(0,seq);
document.write(output);
if(seq<=len){
seq++;
setTimeout("type()",500)
}
else {
document.write("ok");
}
}
type();
</script>

</body>
</html>

怎么实现不了只出现了一个I

------解决方案--------------------

探讨

<script language="javascript">
var seq = 0;
function type() {
var msg = "I am very good";
var output = '';
document.getElementById('output').innerHTML = msg.substring(0, seq);
i……