为什么第一个 我填5或者6都是对的呢？
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>string对象</title>
<script type="text/javascript">
var mystr="Hello World!"
document.write(mystr.substring(6) + "<br />");
document.write(mystr.substring(0,5) );
</script>
</head>
<body>
</body>
</html>
