underfined是var 变量没有赋值的情况,可用null覆盖
```内容
<!DOCTYPE html>
<html lang="en" xmlns:th="http://www.thymeleaf.org/">
<head>
<meta charset="UTF-8">
<title>Title</title>
<base th:href="${#servletContext.getContextPath+'/'}">
</head>
<body>
<script>
var username;
document.write(username+"<br>");
var username = null;
document.write(username+"<br>");
</script>
</body>
</html>
```
结果
undefined
null