为什么用let不显示输出结果，用var却可以
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>无标题文档</title>
</head>
<script type="text/javascript">
var version = navigator.appVersion;
document.write("平台和版本信息："+version+"<br />");
</script>
<body>
</body>
</html>
